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The synthesis, properties and potential applications of cyclic polymers
Abstract
Unlike their more common linear counterparts, cyclic polymers have a ring-like structure and a lack of chain ends. Because of their topology, cyclic polymers exhibit a unique set of properties when compared with linear or branched macromolecules. For example, cyclic homopolymers exhibit a reduced hydrodynamic volume and a slower degradation profile compared with their linear analogues. Cyclic block copolymers self-assemble into compact nanostructures, as illustrated by their reduced domain spacing when cast into thin films and their reduced micellar size in solution. Although methods for preparing well-defined cyclic polymers have only been available since 1980, the extensive utilization of the cyclic topology in nature highlights the vital role that a cyclic architecture can play in imparting valuable physical properties, such as increased chemical stability or propensity towards self-assembly. This Review describes the major developments in the synthesis of cyclic polymers and provides an overview of their fundamental physical properties. In this context, preliminary studies exploring potential applications will be critically assessed and the remaining challenges for the field delineated. Cyclic polymers have a ring-like architecture and one of the most important consequences of this topology is the absence of any chain ends, which typically have a substantial impact on the physical properties of macromolecules. This Review Article discusses advances in the synthesis, purification and characterization of cyclic polymers and the potential applications they may prove useful for.
... S3). Given that cyclic structures have smaller hydrodynamic radii than their linear counterparts of the same chain length (32), and that the products are larger than monomers, SEC data strongly support the formation of cyclic dimers. Therefore, the observed mobility crossover is due to DNA topology, which is consistent with previous reports (32,33). ...
... Given that cyclic structures have smaller hydrodynamic radii than their linear counterparts of the same chain length (32), and that the products are larger than monomers, SEC data strongly support the formation of cyclic dimers. Therefore, the observed mobility crossover is due to DNA topology, which is consistent with previous reports (32,33). Compared to the linear structures, the cyclic topologies are hydrodynamically more mobile. ...
Molecular strain can be introduced to influence the outcome of chemical reactions. Once a thermodynamic product is formed, however, reversing the course of a strain-promoted reaction is challenging. Here, a reversible, strain-promoted polymerization in cyclic DNA is reported. The use of nonhybridizing, single-stranded spacers as short as a single nucleotide in length can promote DNA cyclization. Molecular strain is generated by duplexing the spacers, leading to ring opening and subsequent polymerization. Then, removal of the strain-generating duplexers triggers depolymerization and cyclic dimer recovery via enthalpy-driven cyclization and entropy-mediated ring contraction. This reversibility is retained even when a protein is conjugated to the DNA strands, and the architecture of the protein assemblies can be modulated between bivalent and polyvalent states. This work underscores the utility of using DNA not only as a programmable ligand for assembly but also as a route to access restorable bonds, thus providing a molecular basis for DNA-based materials with shape-memory, self-healing, and stimuli-responsive properties.
... Cyclic polymers have continued to attract much attention in polymer science due to their unique loop topology and still unmet synthetic challenges in preparing them in high molar mass and purity, and in a controlled fashion [1][2][3][4][5][6] . Without chain ends, cyclic polymers exhibit unique physical, rheological and thermal properties relative to their linear counterparts [7][8][9][10][11][12] . ...
... Notably, when performing depolymerization on a UHMM cyclic P3T(Me) 2 P film (1.40 g, M n = 2.23 MDa, Ð = 1.34) with 3 wt% NaOH at 210 °C under vacuum, pure (Me) 2 TPL was collected in a total yield of 95% ( Supplementary Fig. 42). Worth noting also is that, when the recovered (Me) 2 TPL was subjected to repolymerization through the t Bu-P 4 (12 h) revealed that the monomer recovery yield from the linear polymer (85%) is more than twice that from the cyclic counterpart (40%, Supplementary Fig. 44). This result is expected as the linear polymer has chain ends that allow for depolymerization via chain 'unzipping' initiated by the chain-end nucleophiles, whereas the depolymerization of cyclic polymer relies on spontaneous chain scission. ...
The selective synthesis of ultrahigh-molar-mass (UHMM, >2 million Da) cyclic polymers is challenging as an exceptional degree of spatiotemporal control is required to overcome the possible undesired reactions that can compete with the desired intramolecular cyclization. Here we present a counterintuitive synthetic methodology for cyclic polymers, represented here by polythioesters, which proceeds via superbase-mediated ring-opening polymerization of gem-dimethylated thiopropiolactone, followed by macromolecular cyclization triggered by protic quenching. This proton-triggered linear-to-cyclic topological transformation enables selective, linear polymer-like access to desired cyclic polythioesters, including those with UHMM surpassing 2 MDa. In addition, this method eliminates the need for stringent conditions such as high dilution to prevent or suppress linear polymer contaminants and presents the opposite scenario in which protic-free conditions are required to prevent cyclic polymer formation, which is capitalized to produce cyclic polymers on demand. Furthermore, such UHMM cyclic polythioester exhibits not only much enhanced thermostability and mechanical toughness, but it can also be quantitatively recycled back to monomer under mild conditions due to its gem-disubstitution.
... The characterization of cyclic polymer exhibits many challenges, including differentiating between cyclic and linear counterparts, determining the completion of cyclization, identifying product degradation, and so forth (Chan et al., 2009;Haque & Grayson, 2020;Hoskins & Grayson, 2009;Hoskins et al., 2011;Nasongkla et al., 2009). Several characterization techniques have been combined to determine the architecture, such as nuclear magnetic resonance (NMR), tandem mass spectrometry (MS/MS), matrixassisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), electrospray ionization mass spectrometry (ESI-MS), size-exclusion chromatography (SEC), silica gel column chromatography (SGCC), and so forth (Grayson & Fréchet, 2002;Haque & Grayson, 2020;Hoskins & Grayson, 2009;Nitsche et al., 2021). ...
... The characterization of cyclic polymer exhibits many challenges, including differentiating between cyclic and linear counterparts, determining the completion of cyclization, identifying product degradation, and so forth (Chan et al., 2009;Haque & Grayson, 2020;Hoskins & Grayson, 2009;Hoskins et al., 2011;Nasongkla et al., 2009). Several characterization techniques have been combined to determine the architecture, such as nuclear magnetic resonance (NMR), tandem mass spectrometry (MS/MS), matrixassisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), electrospray ionization mass spectrometry (ESI-MS), size-exclusion chromatography (SEC), silica gel column chromatography (SGCC), and so forth (Grayson & Fréchet, 2002;Haque & Grayson, 2020;Hoskins & Grayson, 2009;Nitsche et al., 2021). For the rest of this article, MALDI-TOF MS will be referred to as MALDI-MS. ...
Zwitterionic ring‐expansion polymerization (ZREP) is a polymerization method in which a cyclic monomer is converted into a cyclic polymer through a zwitterionic intermediate. In this review, we explored the ZREP of various cyclic polymers and how mass spectrometry assists in identifying the product architectures and understanding their intricate reaction mechanism. For the majority of polymers (from a few thousand to a few million Da) matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry is the most effective mass spectrometry technique to determine the true molecular weight (MW) of the resultant product, but only when the dispersity is low (approximately below 1.2). The key topics covered in this study were the ZREP of cyclic polyesters, cyclic polyamides, and cyclic ethers. In addition, this study also addresses a number of other preliminary topics, including the ZREP of cyclic polycarbonates, cyclic polysiloxanes, and cyclic poly(alkylene phosphates). The purity and efficiency of those syntheses largely depend on the catalyst. Among several catalysts, N‐heterocyclic carbenes have exhibited high efficiency in the synthesis of cyclic polyesters and polyamides, whereas tris(pentafluorophenyl)borane [B(C6F5)3] is the most optimal catalyst for cyclic polyether synthesis.
... In practice, the fabrication of active needles can be based on the chain structure of enzyme motors [74], catalytic nanomotors [75][76][77], or magnetic soft fibers [78,79]. After the formation of the knots, introducing specific bindings between the polymer tail and the anchoring point through controllable methods such as click chemistry or SpyTag-SpyCatcher chemistry is an effective means to permanently preserve the topology of the chain [80,81]. We believe the proposed knotting strategy provides a new technical routine which opens up many possibilities in macromolecular topology engineering [21,22]. ...
Macromolecules can gain special properties by adopting knotted conformations, but engineering knotted macromolecules is a challenging task. Here we surprisingly observed that knotting can be very effectively produced in active polymers. When one end of an actively reptative polymer is anchored, it can undergo continual self-knotting as a result of intermittent giant conformation fluctuations and the outward reptative motion. Once a knot is formed, it migrates to the anchored point due to a non-equilibrium ratchet effect. Moreover, when the active polymer is grafted on the end of a passive polymer, it can function as a self-propelling soft needle to either transfer its own knots to the passive polymer or directly braid knots on the passive polymer. We further show that these active needles can create inter-molecular bridging knots between two passive polymers. Our finding highlights the non-equilibrium effects in modifying the dynamic pathways of polymer systems, which have potential applications in macromolecular topology engineering, e.g., manipulating topological states of proteins and nucleic acids, as well as macromolecular braiding.
... Thermoplastic elastomers (TPEs) are an important kind of macromolecular material with excellent mechanical properties and low cost, which are used in many aspects of our lives, such as automotive, footwear, adhesives, textiles and biomedical applications [17,18]. In addition to the above-mentioned triblock copolymers, they also have many different topologies such as gradient, star, hyperbranched, dendritic, cyclic and graft [19][20][21][22][23]. Studies on these polymers not only facilitate the exploration of materials with new functionalities and properties, but also improve the ability to tune the properties through chemical design [24]. ...
Graft copolymers have unique application scenarios in the field of high-performance thermoplastic elastomers, resins and rubbers. β-myrcene (My) is a biomass monomer derived from renewable plant resources, and its homopolymer has a low glass transition temperature and high elasticity. In this work, a series of tapered copolymers P(My-co-AMS)k (k = 1, 2, 3) were first synthesized in cyclohexane by one-pot anionic polymerization of My and α-methyl styrene (AMS) using sec-BuLi as the initiator. PAMS chain would fracture when heated at high temperature and could endow the copolymer with thermal degradation property. The effect of the incorporation of AMS unit on the thermal stability and glass transition temperature of polymyrcene main chain was studied. Subsequently, the double bonds in the linear copolymers were partially epoxidized and hydroxylated into hydroxyl groups to obtain hydroxylated copolymer, which was finally used to initiate the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) to synthesize the graft copolymer with PCL as the side chain. All these copolymers before and after modifications were characterized by proton nuclear magnetic resonance (1H NMR), gel permeation chromatography (GPC), thermogravimetry analysis (TGA), and differential scanning calorimeter (DSC).
... This geometric information can be used to predict what size molecules can pass through the pores that CL creates in the host cell membrane. Additionally, when studying the kinetics of polymer loop formation or branching which often involves secondary polymerization interfaces 3,4 , it is informative to separate and quantify polymers by shape so that distinct reactions can be isolated. Topological analyses can be used to build kinetic models of looping and branching, to determine under what conditions polymer cyclization occurs, and to explore how conversion of linear to looped polymers can be controlled. ...
The fundamental molecules of life are polymers. Prominent examples include nucleic acids and proteins, both of which assume a vast array of mechanical properties and three-dimensional shapes. The persistence length represents a numerical value to classify the bending rigidity of individual polymers. The shape of a polymer, dictated by the topology of the polymer backbone - a line trace through the center of the polymer along the contour path – is also a critical metric. Common architectures include linear, ring-like or cyclic, and branched; combinations of these can also exist, as in complex polymer networks. Determination of persistence length and shape are largely informative to polymer function and stability in biological environments. Here we demonstrate PS Poly, a near-fully automated algorithm to obtain polymer persistence length and shape from single molecule images obtained in physiologically relevant fluid conditions via atomic force microscopy. The algorithm, which involves image reduction via skeletonization followed by end point and branch point detection via filtering, is capable of rapidly analyzing thousands of polymers with subpixel precision. Algorithm outputs were verified by analysis of deoxyribose nucleic acid, a very well characterized macromolecule. The utility of method was further demonstrated by application to a recently discovered polypeptide chain named candidalysin. This toxic protein segment polymerizes in solution and represents the first human fungal pathogen yet discovered. PS poly is a robust and general algorithm. It can be used to extract fundamental information about polymer backbone stiffness, shape, and more generally, polymerization mechanisms.
... In recent years, cyclic polymers with no chain ends have been attracting attention from polymer chemists because of their various unique physical properties, different from those of their corresponding linear counterparts, that is, exhibiting distinguished glass transition temperature, viscosity, diffusion behavior and hydrodynamic volume [1][2][3][4][5]. In addition, it has been reported that linear and cyclic poly(N-isopropylacrylamide) (PNIPAMs) show different aggregation behaviors [6][7][8]. ...
Polymers with cyclic topology have no terminal structure and, therefore, exhibit various unique physical and functional properties compared to those of linear analogs. In this paper, we report an innovative methodology for the synthesis of cyclic polymers via ring-expansion RAFT (RE-RAFT) polymerization of vinyl monomers using a cyclic trithiocarbonate derivative (CTTC) as a RAFT agent. RE-RAFT of tert-butyl acrylate (TBA) was performed to yield a mixture of polymers exhibiting a bimodal size exclusion chromatography (SEC) trace. Both the peak top molecular weights shifted to higher-molecular-weight regions as the monomer conversion increased. The structure of the resulting polymer mixture was examined by 1H NMR and MALDI-TOF-MS. Detailed studies indicated that the obtained polymer of higher molecular weight was one of the large-sized cyclic polymers generated by the fusion of smaller-sized cyclic polymers during the RE-RAFT polymerization process. This approach opens the door to the simple synthesis of well-controlled cyclic polymers with complex structures, such as alternating and multi-block repeat unit sequences.
... Despite the enormous interest in cyclic polymers and the great development of techniques associated with their synthesis [8][9][10][11], the cyclic structures of PG are relatively new and therefore, studies on their property-structure relationship is still in its infancy. The formation of cyclic PG in one-pot reaction of Gly with the Lewis acid, tris-(pentafluorophenyl)borane [B(C 6 F 5 ) 3 ], was published by some of us in 2014 [12], where we reported on the generation of cyclic polyethers by an electrophilic zwitterionic ring-opening polymerization (ZROP) mechanism, and later as an electrophilic zwitterionic ring expansion polymerization (ZREP) mechanism [13,14]. ...
The synthesis of polyglycidol (or polyglycerol) using tris-(pentafluorophenyl)borane [B(C 6 F 5) 3 ] as a catalyst produces a branched structure with a cyclic core by a mechanism of zwitterionic ring expansion polymerization. The solvent choice is limited since the polymerization does not occur in good solvents for polyglycidol (e.g. DMF, DMSO). In poor solvents for polyglycidol but good solvent for glycidol (e.g. toluene), the polymerization does occur but in a heterogeneous manner. The polymer precipitates during reaction forming two phases, the solution and precipitated phase. In the presence of water a competitive initiation mechanism consisting in the formation of hydronium ions by reaction between B(C 6 F 5) 3 and two water molecules, followed by the protonation of glycidol (Gly) epoxide is the responsible for the formation of analogous linear-core structures. In present study we evaluated the kinetic parameters of the initial stage of polymerization of Gly with B(C 6 F 5) 3 in the presence and absence of water by in situ 1 H NMR monitoring in toluene-d 8 phase. The results indicated first order kinetics with respect to Gly and B(C 6 F 5) 3 , zero order with respect to water, similar initial rate constants for the poly-merization initiated by B(C 6 F 5) 3 and H 3 O + and similar activation energies for the polymerization in the absence and presence of water. The decrease in intensity of the 19 F NMR signal relative to the initial value indicated that B(C 6 F 5) 3 goes to a precipitated phase just after the polymerization started due to a change in the solubility of the formed oligomeric active chains that carry the catalyst. In the precipitate, the reaction continued due to chain fusion events that take place increasing the molecular weight and producing a product with identical mass distribution as that of a polyglycidol produced under dry and solvent-free conditions. Density functional theory calculations supported the kinetic data by obtaining similar energy barriers and thermodynamic enthalpies for the reaction of Gly with B(C 6 F 5) 3 and H 3 O + .
... The targeted LC-MS analysis of specialized metabolites from fungi that are only partially represented in phylogenetic analyses represents a robust application of chemotaxonomy to resolve species. Fungi that produce cyclopeptides may be especially good candidates for chemotaxonomic profiling as many cyclopeptides are particularly resistant to degradation by oxidation, heating, or proteolytic cleavage (Haque and Grayson 2020). Chemotaxonomic profiling of stable metabolites also provides a framework for the analysis of fungal groups lacking genetic data for type specimens, whereby type specimens that afford only chemical data can be linked to samples for which both chemical and genetic data are available, if both types of data resolve species groups. ...
Molecular phylogenetic and chemical analyses, and morphological characterization of collections of North American Paraisaria specimens support the description of two new species and two new combinations for known species. P. cascadensis sp. nov. is a pathogen of Cyphoderris (Orthoptera) from the Pacific Northwest USA and P. pseudoheteropoda sp. nov. is a pathogen of cicadae (Hemiptera) from the Southeast USA. New combinations are made for Ophiocordyceps insignis and O. monticola based on morphological, ecological, and chemical study. A new cyclopeptide family proved indispensable in providing chemotaxonomic markers for resolving species in degraded herbarium specimens for which DNA sequencing is intractable. This approach enabled the critical linkage of a 142-year-old type specimen to a phylogenetic clade. The diversity of Paraisaria in North America and the utility of chemotaxonomy for the genus are discussed.
... Amphiphilic block copolymers containing hydrophobic and hydrophilic segments can self-assemble 1 into various nanostructures, including micelles, cylindrical micelles, vesicles, cubosomes, and hexosomes, which have advanced applications including drug delivery systems and self-healing materials. 2 In particular, cubosomes and hexosomes, 3 which comprise porous structures thereby affording high loading volumes and large surface areas, are promising for smart lipid nanoparticles. 4 The prevailing strategies for producing block copolymers focus on coupling different blocks with functionalized chain ends, 5−10 employing one of the blocks as a macroinitiator to polymerize comonomers, 11−17 or switching propagation centers to activate comonomers, 18−21 which are one-pot multistep approaches. 22 These multistep strategies have been extensively researched, 23−25 while the simultaneous copolymerization of comonomers, namely, the one-pot, one-step approach, is made possible by the development of using heterobifunctional initiators that activate different processes, e.g., ring-opening polymerization (ROP) and radical polymerization, 26,27 which can yield multiblock copolymers. ...
Amphiphilic block copolymers containing a cyclic topology exhibit distinct physical properties that can be exploited to obtain fascinating nanostructures for various applications. So far, these copolymers have been generally prepared via multistep approaches. Herein, we report the simultaneous zwitterionic copolymerization of δ-valerolactone (VL) and triethylene glycol carbonate (3EG) mediated by N-heterocyclic carbene to yield a cyclic amphiphilic block copoly(ester-b-carbonate) in a one-pot, one-step strategy. The copolymerization temperature has a considerable impact on the block length (L VL and L 3EG); thus, a block copolymer (L VL = 12.7 and L 3EG = 10.1) is obtained at a low temperature (−25°C), whereas a random copolymer is obtained at 50°C with a high overall conversion (≥95%). The copolymerization proceeds with a 150 times faster consumption of VL compared to that of 3EG with reactivity ratios (r) r VL ≫ r 3EG (r VL ≈ 5, r 3EG ≈ 0.055), determined using the error-in-variables-model method, demonstrating the VL block chain during copolymerization. Moreover, a theoretical study indicates that the obtained copolymer is not a kinetically controlled product but rather a thermodynamically controlled product. Scanning and transmission electron microscopies revealed that the copoly(ester-b-carbonate) self-assembled in a tetrahydrofuran/water solvent mixture (V THF /V H2O = 1:5) to form a cubic nanostructure.
Reactions between tungsten alkylidyne [tBuOCO]W≡CtBu(THF)2 1 and sulfur containing small molecules are reported. Complex 1 reacts with CS2 to produce intermediate η2 bound CS2 complex [O2C(tBuC═)W(η2-(S,C)-CS2)(THF)] 8. Heating complex 8 provides a mixture of a monomeric tungsten sulfido complex 9 and a dimeric complex 10 in a 4:1 ratio, respectively. Heating the mixture does not perturb the ratio. Addition of excess THF in a solution of 9 and 10 (4:1) converts 10 to 9 (>96%) with concomitant loss of (CS)x. Both 9 and 10 can be selectively crystallized from the mixture. An alternative synthesis of exclusively monomeric 9 involves the reaction between 1 and PhNCS. Demonstrating ring expansion metathesis polymerization (REMP), tethered tungsten alkylidene 8 polymerizes norbornene to produce cis-selective syndiotactic cyclic polynorbornene (c-poly(NBE)).
The recent success of gene therapy during the COVID‐19 pandemic has underscored the importance of effective and safe delivery systems. Complementing lipid‐based delivery systems, polymers present a promising alternative for gene delivery. Significant advances have been made in the recent past, with multiple clinical trials progressing beyond phase I and several companies actively working on polymeric delivery systems which provides assurance that polymeric carriers can soon achieve clinical translation. The massive advantage of structural tunability and vast chemical space of polymers is being actively leveraged to mitigate shortcomings of traditional polycationic polymers and improve the translatability of delivery systems. Tailored polymeric approaches for diverse nucleic acids and for specific subcellular targets are now being designed to improve therapeutic efficacy. This review describes the recent advances in polymer design for improved gene delivery by polyplexes and covalent polymer‐nucleic acid conjugates. The review also offers a brief note on novel computational techniques for improved polymer design. The review concludes with an overview of the current state of polymeric gene therapies in the clinic as well as future directions on their translation to the clinic.
Bioconjugation of polymers to proteins is a method to impart improved stability and pharmacokinetic properties to biologic systems. However, the precise effects of polymer architecture on the resulting bioconjugates are not well understood. Particularly, cyclic polymers are known to possess unique features such as a decreased hydrodynamic radius when compared to their linear counterparts of the same molecular weight, but have not yet been studied. Here, we report the first bioconjugation of a cyclic polymer, poly(ethylene glycol) (PEG), to a model protein, T4 lysozyme, containing a single engineered cysteine residue (V131C). We compare the stability and activity of this conjugate with those of a linear PEG-T4 lysozyme analogue of similar molecular weight. Furthermore, we used molecular dynamics (MD) simulations to determine the behavior of the polymer–protein conjugates in solution. We introduce cyclic polymer–protein conjugates as potential candidates for the improvement of biologic therapeutics.
The synthesis of cyclic polymers via zwitterionic ring expansion polymerization is limited to a few number of monomer and catalyst pairs. Herein we report the synthesis of cyclic poly( tert ‐butyl glycidyl ether) through the polymerization of tert ‐butyl glycidyl ether (tBGE) with B(C 6 F 5 ) 3 in different reaction conditions that include different solvents, monomer to initiator ratio, monomer concentration and temperature. We found that bimodal molecular weight distribution is formed in almost all reaction conditions caused by cyclization of short chains. Subsequent chain elongation leads to the formation of cycles of higher molecular weight, particularly in cyclohexane and under bulk conditions. The formation of non‐cyclic byproducts is common in all the systems investigated. Low molecular weight cyclic chains (M n = 0.7 kDa, Ð = 1.1) of high topological purity were successfully isolated by preparative gel permeation chromatography. By using a click scavenging protocol, the non‐cyclic byproducts were eliminated from the high molecular weight fraction (M n = 3 kDa, Ð = 1.3) generating pure cyclic chains. Mechanistic investigation using density functional theory calculations was performed on the formation of zwitterionic intermediates and the transfer reaction to the monomer, which notably affects chain growth by the attack of the glycidyl oxygen of the monomer on the growing chain.
Macromolecular architecture plays a pivotal role in endowing distinct properties to polymer nanomaterials. We introduce a synthetic approach to produce cyclic polystyrene-b-poly(acrylic acid) block copolymers featuring UV-cleavable motifs by combining...
The practically infinite chemical and morphological space of polymers makes them pervasive with applications in materials science but challenges the rational discovery of new materials with favorable properties. Polymer informatics aims to accelerate materials design through property prediction and large-scale virtual screening. In this study, a new method (Lieconv-Tg) has been developed to predict glass-transition temperature (Tg) values from repeating units of polymers based on Lieconv, which is equivariant with transformations from any specified Lie group. The introduction of equivariance allows the prediction of molecular properties from their 3D structures, independent of orientation and position. A total of 27,659 homopolymers with Tg values were collected from PolyInfo, and a standard data set containing 7166 polymers (named data set_Tg) was created for training a robust Lieconv-Tg model. Using the 3D coordinates as input, Lieconv-Tg performs better than Edge-Conditioned Convolution (ECC), and the mean absolute error (MAE) is significantly reduced by ∼6 from ∼30 to ∼24 on both the validation set and the test set, and the R² value for both the validation set and the test set can reach 0.90. Lieconv-Tg is thus used to screen promising candidates from a benchmark database named PI1M with 995,800 generated polymers. However, there are some implausible repeating units in PI1M. To get more reasonable candidates from PI1M, a new filtering method has been accomplished by utilizing Morgan fingerprints at the polymerization points (MF@PP) of repeating units in data set_Tg. The combination of a standard data set, Lieconv-Tg, and a more reasonable screening strategy provides new directions in materials design for polymers.
A rare asymmetric bicyclic polymer containing different length of conjugated polyacetylene segments was synthesized by metathesis cyclopolymerization‐mediated blocking‐cyclization technique. The size of each single ring differed from each other, and the unique cyclic polymer topology was controlled by adjusting the feed ratio of monofunctional monomer to catalyst. The topological difference between linear and bicyclic polymers was confirmed by several techniques, and the visualized morphology of asymmetric bicyclic polymer was directly observed without tedious post‐modification process. The photoelectric and thermal properties of polymers were investigated. This work expanded the pathway for the derivation of cyclic polymers, and such unique topological structure enriched the diversity of cyclic polymer classes.
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Polymers with different architectures, such as block, graft, star, and cyclic polymers, have been developed owing to recent advances in synthetic technology. Notably, minor changes in the architecture of amphiphilic...
A series of novel biodegradable homopolymers and copolymers of L-lactide (LA) and δ-valerolactone (VL) were polymerized at 165 °C using stannous octoate as a catalyst initiated with salicylic acid or benzyl alcohol via ring-expansion or ring-opening polymerization, respectively. The polymer chain topology was suggested to be either cyclic or linear depending on the initiator used. The feeding molar ratio of LA : VL was changed to investigate the chain microstructure, thermal properties, as well as degradability of copolymers. 1H-NMR revealed that incorporating 30, 50, and 70 mol% VL provided blocky, gradient, and random chain copolymers, respectively. Increasing VL content relative to the LA content decreased the Tg, Tm, and crystallinity of the copolymers. All copolymers displayed higher thermal stability than PLA homopolymers due to the presence of the VL comonomer. The cyclic polymers showed higher Tg, lower Tm, and lower crystallinity than their linear counterparts. Furthermore, it was found that the degradability of the copolymers can be controlled by adjusting the compositions of VL and LA and their chain architecture.
Cyclic polymers have unique physical properties and attracted extensive attention, while the precision synthesis of complex cyclic polymers was a challenging subject. Herein, by using a polymeric ladderphane with six...
Language models exhibit a profound aptitude for addressing multimodal and multidomain challenges, a competency that eludes the majority of off-the-shelf machine learning models. Consequently, language models hold great potential for...
Polyesters with both cyclic topology and chemical recyclability are attractive. Here, the alternating copolymerization of cyclic anhydride and o‐phthalaldehyde to synthesize a series of cyclic and recyclable polyesters are reported for the first time. Besides readily available monomers, the copolymerization is carried out at 25 °C, uses common Lewis/Brønsted acids as catalysts, and achieves high yields within 1 h. The resulting polyesters possess well‐defined alternating sequences, high‐purity cyclic topology, and tunable structures using distinct two monomer sets. Of interest, the copolymerization manifests obvious chemical reversibility as revealed by kinetic and thermodynamic studies, making the unprecedented polyesters easy to recycle to their distinct two monomers in a closed loop at high temperatures. This work furnishes a facile and efficient method to synthesize cyclic polyesters with closed‐loop recyclability.
Facile, room‐temperature coupling of brominated polymer chains can be accomplished in high yields (~80%) using a nickel‐based catalytic cycle that also includes magnesium and zinc. The extent of coupling ( X c ) was found to be similarly high when performed under vacuum or with a nitrogen purge, although its effectiveness varied significantly as a function of the solvent, reagents, and the structure of the polymer chain end. Kinetic studies show that even at room temperature, nearly all coupling occurs within the first ~2 h of the reaction when performed under nitrogen or vacuum, with accompanying color changes occurring in the catalytic system coinciding with catalytic activity. One step of the mechanistic cycle is proposed to occur via chain‐end radical intermediates inserted into the catalytic cycle, consistent with the addition of a radical trap essentially thwarting the dimerization by capturing the polymer radical and preventing its inclusion in the reaction. The presence of aryl bromide functional groups, however, does not impede the coupling reaction, demonstrating the catalyst's preference for the chain‐end alkyl bromides under our conditions. The impact and necessity of the other components were also explored, supporting the importance of the MgCl 2 serving as a Lewis acid to promote the reactivity of the CBr end groups.
Cyclic polymers have very unique structure and properties, and thus have drawn intense research attention. However, controlled synthesis of cyclic polymers with predictable molar mass and narrow distribution is still a challenging task. In this study, we developed a novel cyclic catalyst that initiates the ring-expansion polymerisation of isocyanides, producing a series of cyclic helical polymers with predictable molecular weight and low dispersity. Interestingly, the ring-expansion polymerization of the isocyanide macromonomers gives well-defined cyclic bottlebrush polymers. The cyclic topology was demonstrated using transmission electron microscopy.
We derive and parameterize effective interaction potentials between a multitude of different types of ring polymers and linear chains, varying the bending rigidity and solvent quality for the former species. We further develop and apply a density functional treatment for mixtures of both disconnected (chain–ring) and connected (chain-polycatenane) mixtures of the same, drawing coexistence binodals and exploring the ensuing response functions as well as the interface and wetting behavior of the mixtures. We show that worsening of the solvent quality for the rings brings about a stronger propensity for macroscopic phase separation in the linear-polycatenane mixtures, which is predominantly of the demixing type between phases of similar overall particle density. We formulate a simple criterion based on the effective interactions, allowing us to determine whether any specific linear-ring mixture will undergo a demixing phase separation.
To elucidate the usefulness of commercial Bu 2 SnO as catalyst for syntheses of linear and/or cyclic polylactides l ‐Lactide was polymerized in bulk with variation of the LA/Cat ratio, temperature and time. Based on a ring‐expansion polymerization (REP) mechanism cyclic polylactides (PLAs) with weight average molecular weights in the range of 200,000–300,000 are obtained at the highest temperature (180 °C). Polymerizations at 150 °C yielded crystalline, mainly cyclic polylactides which, after annealing, showed high melting temperatures (up to 197.5 °C) and high crystallinities (>80%). Polymerization at 120 °C confirmed the trend towards more linear chains with lower temperatures but yielded extended‐ring crystals showing a “saw‐tooth pattern” in the mass spectra. Oct 2 SnO gave similar results as Bu 2 SnO. Bu 2 SnS proved a sluggish polymerization catalyst, but a good transesterification catalyst in solid PLA.
We use molecular dynamics simulations to explore concentrated solutions of semiflexible polyelectrolyte ring polymers, akin to the DNA mini-circles, with counterions of different valences. We find that the assembly of rings into nanoscopic cylindrical stacks is a generic feature of the systems, but the morphology and dynamics of such a cluster can be steered by the counterion conditions. In general, a small addition of trivalent ions can stabilize the emergence of clusters due to the counterion condensation, which mitigates the repulsion between the like-charged rings. Stoichiometric addition of trivalent ions can even lead to phase separation of the polyelectrolyte ring phase due to the ion-bridging effects promoting otherwise entropically driven clustering. On the other hand, monovalent counterions cause the formation of stacks to be re-entrant with density. The clusters are stable within a certain window of concentration, while above the window the polyelectrolytes undergo an osmotic collapse, disfavoring ordering. The cluster phase exhibits characteristic cluster glass dynamics with arrest of collective degrees of freedom but not the self-ones. On the other hand, the collapsed phase shows arrest on both the collective and single level, suggesting an incipient glass-to-glass transition, from a cluster glass of ring clusters to a simple glass of rings.
The synthesis of well-defined cyclic polymers is crucial to exploring applications spanning engineering, energy, and biomedicine. These materials lack chain-ends and are therefore imbued with unique bulk properties. Despite recent advancements, the general methodology for controlled cyclic polymer synthesis via ring-expansion metathesis polymerization (REMP) remains challenging. Low initiator activity leads to high molar mass polymers at short reaction times that subsequently "evolve" to smaller polymeric products. In this work, we demonstrate that in situ addition of pyridine to the tethered ruthenium-benzylidene REMP initiator CB6 increases ancillary ligand lability to synthesize controlled and low dispersity cyclic poly(norbornene) on a short time scale without relying on molar mass evolution events.
In this study, we investigate structures and stabilities of the micelles of a cyclic amphiphile (c-PBA-b-PEO) composed of poly(-n-butyl acrylate) (PBA) and poly(ethylene oxide) (PEO) blocks and its linear diblock and triblock analogues (l-PBA-b-PEO and -l-PBA--b-PEO-b-PBA) by using synchrotron X-ray scattering and quantitative data analysis. The comprehensive scattering analysis gives details and insights to the micellar architecture through structural parameters. Furthermore, this analysis provides direct clues for structural stabilities in micelles, which can be used as a good guideline to design highly stable micelles. Interestingly, in water, all topological polymers are found to form ellipsoidal micelles rather than spherical micelles; more interestingly, the cyclic polymer and its linear triblock analog make oblate-ellipsoidal micelles while the linear diblock analog makes a prolate-ellipsoidal micelle. The analysis results collectively inform that the cyclic topology enables more compact micelle formation as well as provides a positive impact on the micellar structural integrity.
Ring-opening polymerization of α-amino acid N-carboxyanhydrides (NCAs) is a powerful synthetic methodology for generating well-defined functional polypeptides. However, conventional procedures require a compromise between obtaining controlled microstructures and employing the optimized polymerization conditions. Specifically, a versatile method to access sequenced cyclic polypeptides remains challenging due to the difficulty in site-specific cyclization. Here we describe a general and straightforward method for the synthesis of both linear and cyclic polypeptides using organocatalytic living polymerization of NCAs. The use of an air-stable organocatalyst, imidazolium hydrogen carbonate, allows for the rapid and controlled polymerization of a variety of NCAs, leading to high conversion within a few minutes under mild conditions. Linear and cyclic block copolypeptides are also accessible simply by controlling the type of initiators and the order of addition of NCA monomers. Ring-opening polymerisation of amino acid N-carboxyanhydrides is an established route to polypeptides, but controlling the product distribution can require careful optimisation. Here, simple variation of the choice of initiator provides a general route to linear or cyclic polypeptides and under mild conditions.
The light induced, catalyst-free ambient temperature preparation of macrocyclic aliphatic polyesters is pioneered. Based on the photo-induced Diels–Alder reaction of orthoquinodimethane and acrylate moieties, cyclic polyesters of high purity are readily synthesized. Considering the high tolerance to functional groups and the orthogonality of the ligation, the reported protocol can be easily transferred to a large range of polymers, complex topologies (tadpole, sun-shaped, jellyfish, etc.) and applications.
The melt rheology of highly-purified ring polystyrenes in a wide range of molecular weights (10K ≤ Mw ≤ 240K g/mol) was investigated. All the rings revealed no obvious rubbery plateau and faster terminal relaxation compared to the linear counterparts. The rheological data obtained were compared with some theoretical models such as the Rouse ring model and the lattice-animal model. Moreover, two rheological parameters, zero-shear viscosities η0 and the steady-state recoverable compliances Je, were estimated, and their Mw dependence was discussed. From these data analysis, it was found that relaxation mechanisms of ring chains can be divided into three categories depending on their Mw as follows: (i) Smaller rings (Mw ≤ 20K) exhibit faster terminal relaxation than the predicted Rouse rings. This behavior is related to the difference of the local chain conformation from linear chains. (ii) Rings with the moderate molecular weight (40K ≤ Mw ≤ 90K) exhibit dynamic moduli similar to the Rouse ring prediction. (iii) A larger ring (Mw > 90K) also shows deviant behavior from the Rouse chain because its relaxation time is much longer than the Rouse ring prediction and also the lattice-animal model, where some intermolecular interactions are considered to occur.
A unique method was developed for the preparation of cyclic polymers based on the combination of atom transfer radical polymerization (ATRP) and UV-induced strain promoted azide-alkyne cycloaddition (SPAAC) reaction. By virtue of a cyclopropenone-masked dibenzocyclooctyne functionalized ATRP initiator (I-1), well-defined telechelic polystyrene (PS) was synthesized to have a cyclopropenone-masked dibenzocyclooctyne at one polymer chain end and a bromo group at the other. The single electron transfer-nitroxide radical coupling reaction was then used to modify the bromo end group to azide, resulting in the corresponding linear PS precursor. Under UV irradiation on its highly diluted solution, the dibenzocyclooctyne end group was quantitatively released from cyclopropenone-masked dibenzocyclooctyne, which intramolecularly reacted with the azide end group in-situ to ring-close the linear PS precursor and produce the corresponding cyclic PS based on the SPAAC click reaction.
YOYO-1 is a green fluorescent dye which is widely used to image single DNA molecules in solution for biophysical studies. However, the question of whether the intercalation of YOYO-1 affects the mechanical properties of DNA is still not clearly answered. Investigators have put forth contradicting data on the changes in persistence length of DNA. Here, we use atomic force microscopy to systematically study the changes in the mechanical properties of DNA due to the intercalation of YOYO-1. We first measured the persistence length, contour length and the bending angle distribution of the DNA-YOYO-1 complex. We find that the persistence length of DNA remains unaffected with the intercalation of YOYO-1. However the contour length increases linearly with about 38% increase at full saturation of 1 YOYO-1 per 4 base pairs of DNA. Next we measured the change in topology of relaxed closed circular DNA after the intercalation of YOYO-1. We find that YOYO-1 introduces supercoiling in closed circular DNA. Our observations indicate that the intercalation of YOYO-1 results in the underwinding of DNA duplex, but does not significantly change the persistence length.
A new method of generating cyclic polyethers is reported. Glycidyl monomers react with B(C6F5)3 to generate cyclic polyethers in anhydrous conditions. In the presence of water, linear chains are formed. A zwitterionic ring-opening polymerization mechanism is postulated based on experimental evidence and DFT calculations. The obtained cyclic polyethers can be considered a new family of crown ethers, where peripheral functional groups such as phenyls, fluorinated aliphatic chains or hydroxyls decorate the rings.
A tandem mass spectrometry-based method is developed to determine the degree of purity achieved in the cyclization of a linear poly(L-Lactide) prepared by copper-catalyzed alkyne-azide cycloaddition. When proton nuclear magnetic resonance, size-exclusion chromatography, and single-stage mass spectrometry are unable to demonstrate the presence of residual linear polymer, the proposed ESI-tandem mass spectrometry methodology allows detecting starting materials traces (<5 %) based on radically different collision-induced dissociation (CID) behaviours. The technique is believed to be readily adaptable to numerous isomeric pairs of macromolecules presenting different CID characteristics.
A powerful ring-closure method was developed for the formation of cyclic polymers by combining reversible addition–fragmentation chain transfer polymerization (RAFT) and light-induced Diels–Alder click reaction. The outstanding features of this novel method were demonstrated from the following four aspects. This convenient and efficient technique could produce cyclic polymers in air at room temperature without any other catalyst or stimulus requirements other than a mild UV irradiation. The universality of this method was demonstrated by successfully producing five different types of cyclic homopolymers including polystyrene, poly(methyl methacrylate), poly(tert-butyl acrylate), poly(N,N-dimethylacrylamide), and poly(2-vinylpyridine) and two kinds of block copolymers of poly(methyl methacrylate)-block-polystyrene and poly(methyl methacrylate)-block-poly(tert-butyl acrylate). This is the first time to report a one-pot ring-closure method for the formation of cyclic polymers, in which the crude monomer polymerization solution was directly diluted as precursors and no more requirements were needed to purify and postfunctionalize the linear polymer intermediates. When combing with a batchwise operation, this novel light-induced ring-closure method could significantly improve the notorious disadvantage of low cyclic polymer yield accompanied by the ring-closure strategy.
We have measured the linear rheology of critically purified ring polyisoprenes, polystyrenes, and polyethyleneoxides of different molar masses. The ratio of the zero-shear viscosities of linear polymer melts η0,linear to their ring counterparts η0,ring at isofrictional conditions is discussed as a function of the number
of entanglements Z. In the unentangled regime η0,linear/η0,ring is virtually constant, consistent with the earlier data, atomistic simulations, and the theoretical expectation η0,linear/η0,ring = 2. In the entanglement regime, the Z-dependence of ring viscosity is much weaker than that of linear polymers, in qualitative agreement with predictions from scaling theory and simulations. The power-law extracted from the available
experimental data in the rather limited range 1 < Z < 20, η0,linear/η0,ring ∼ Z1.2±0.3, is
weaker than the scaling prediction (η0,linear/η0,ring ∼ Z1.6±0.3) and the simulations
(η0,linear/η0,ring ∼ Z2.0±0.3). Nevertheless, the present collection of state-of-the-art
experimental data unambiguously demonstrates that rings exhibit a universal trend clearly departing from that of their linear counterparts, and hence it represents a major step toward resolving a 30-year-old problem.
We present here a systematic investigation on the glass transition temperature (T g ) for a series of highly purified cyclic polystyrene (c-PS) samples with molecular weight (MW) varying from 3300 to 13,400. Compared with their linear counterparts, c-PS samples show much higher T g and exhibit weak T g -MW dependence. By blending linear polystyrenes with c-PS, the linear polymer contamination effect on the T g of c-PS was systematically investigated. It was found that the measured T g of the mixtures increases with the decrease of linear polymer content and the relationship is consistent with Fox equation. These results are helpful to understand the origin of previously reported different T g -MW relationship for c-PS.
Polypropylene (PP) comprises 25% of all polymer production globally. Fine-tuning the properties of PP via copolymerization, introducing additives, blending, controlling stereochemistry, and adjusting molecular weight (Mw) and dispersity (Ð) provide the numerous materials we interact with daily and aid in advancing quality of life. Manipulating polymer topology is an alternative method to alter the physical properties of polymers. For example, cyclic polymers exhibit different glass transition temperatures (Tg), densities, intrinsic viscosities, and diffusion and relaxation processes as compared to their linear analogs. Enabling access to cyclic examples of commodity polymers will provide a new dimension of control over physical properties and may lead to new applications and markets. This work provides the next step in realizing the large-scale production of commodity cyclic polymers.
Multicyclic polystyrene (PS) with hyperbranched structure was constructed in an efficient way. First, a seesaw-type PS was synthesized via atom transfer radical polymerization (ATRP) using a Y-shaped ATRP initiator containing one hydroxyl at center and bromine at each end. After azidation, the anthryl and hydroxyl groups were introduced to the ends of the polymer chain by click reaction with a trifunctional molecule bearing alkynyl, hydroxyl, and anthryl groups (alkynyl-OH-ant). By irradiation with 365 nm UV light in a highly dilute condition, cyclic polymer with three hydroxyl groups (c-PS-(OH)3) can be obtained; then it was converted to a cyclic polymer containing three azides (c-PS-(N3)3) by bromination of the hydroxyl groups and azidation. This "A3" cyclic macromonomer was then used to construct hyperbranched multicyclic polymers via self-accelerating click reaction with sym-dibenzo-1,5-cyclooctadiene-3,7-diyne (DBA). The properties of obtained polymer were characterized by NMR, FT-IR, MALDI-TOF MS, and TD-SEC. It was calculated from the MALLS results that there were about 35 "small rings" in the multicyclic polymer. Moreover, because of the photocleavage reaction of the anthracene dimer, this hyperbranched multicyclic polymer can be cleaved to long-chain hyperbranched PS by irradiation with 254 nm UV light.
We evaluate the use of tetrabutylammonium azide (N3NBu4) as an anionic ring-opening polymerization initiator for synthesizing azide-terminated linear poly(glycidyl phenyl ether) and then generating monocyclic structures with high purity. In particular, we perform a detailed study on the end-group fidelity of polymers obtained by initiation with N3NBu4 in the presence and absence of trisisobutylaluminum (iBu3Al) and evaluate the purity of the cyclic structures obtained via copper-catalyzed alkyne–azide cycloaddition “click” reaction. We demonstrate that in contrast to the polymerization initiated by N3NBu4 alone, the polymerization performed in the presence of iBu3Al allows the formation of polymers with high end-group fidelity (azide groups at the α-position) for Mn < 20 kDa. The cyclic purity is evaluated by SEC with triple detection and dielectric spectroscopy. The latter technique, although not conventional for such a purpose, is shown to be very convenient to ensure cyclic purity in polymers showing an end-to-end dielectric relaxation mode.
Ring closure click chemistry methods have been used to produce cyclic c-PLLA and c-PDLA of a number-average molecular weight close to 10 kg/mol. The effects of stereochemistry of the polymer chains and their topology on their structure, nucleation, and crystallization were studied in detail employing wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), polarized light optical microscopy (PLOM), and standard and advanced differential scanning calorimetry (DSC). The crystal structures of linear and cyclic PLAs are identical to each other, and no differences in superstructural morphology could be detected. Cyclic PLA chains are able to nucleate much faster and to produce a higher number of nuclei in comparison to linear analogues, either upon cooling from the melt or upon heating from the glassy state. In the samples prepared in this work, a small fraction of linear or higher molecular weight cycles were detected (according to SEC analyses). The presence of such “impurities” retards spherulitic growth rates of c-PLAs, making them nearly the same as those of l-PLAs. On the other hand, the overall crystallization rate determined by DSC was much larger for c-PLAs, as a consequence of the enhanced nucleation that occurs in cyclic chains. The equilibrium melting temperatures of cyclic chains were determined and found to be 5 °C higher in comparison with values for l-PLAs. This result is a consequence of the lower entropy of cyclic chains in the melt. Self-nucleation studies demonstrated that c-PLAs have a shorter crystalline memory than linear analogues, as a result of their lower entanglement density. Successive self-nucleation and annealing (SSA) experiments reveal the remarkable ability of cyclic molecules to thicken, even to the point of crystallization with extended collapsed ring conformations. In general terms, stereochemistry had less influence on the results obtained in comparison with the dominating effect of chain topology.
An efficient metal-free bimolecular homodifunctional ring-closure method was developed specifically for preparing well-defined cyclic polymers from unconjugated vinyl monomers. In this approach, well-defined homodifunctional linear polymers with azide terminals were prepared by using the reversible addition chain transfer polymerization/macromolecular design by interchange of xanthates (RAFT/MADIX) to polymerize unconjugated vinyl monomers in the presence of a diazide xanthate chain transfer agent. The self-accelerating double strain-promoted azide-alkyne cycloaddition (DSPAAC) reaction was then applied to ring-close the linear polymer precursors with sym-dibenzo-1,5-cyclooctadiene-3,7-diyne (DBA) as small linkers, leading to the formation of corresponding cyclic polymers. By virtue of the self-accelerating property of DSPAAC ring-closing reaction, this novel method smartly eliminated the requirement of equimolar amounts of the telechelic polymers and small linkers in the traditional bimolecular ring-closure methods for pure cyclic polymers. More importantly, the usage of excess DBA small linkers could significantly enhance the preparation efficiency of cyclic polymers. Moreover, the cyclic polymers resulted from this novel method could be conveniently cleaved back to linear polymers by a mild aminolysis of the S-C(S) bond within the cyclic polymer backbone.
A series of cyclic poly(tetrahydrofuran)s, poly(THF)s, having a pair of stereoisomeric forms of axially chiral, 2,2′-disubstituted-1,1′-binaphthyl (BiNap) unit at the opposite positions, CR-R, CR-S, and CR-R/S, have been newly synthesized through an electrostatic self-assembly and covalent fixation (ESA-CF) process by using a center-functionalized (kentro-telechelic) poly(THF) having an axially chiral BiNap unit and N-phenylpyrrolidinium salt end groups, carrying a dicarboxylate counteranion having a BiNap unit. The relevant cyclic and linear analogues, having one axially chiral unit, CR (CR-1 and CR-2) and LR, respectively, have also been prepared according to the ESA-CF protocol. The subsequent CD measurements of these cyclic and linear polymers having axially chiral units by lowering the temperature toward −10 °C in hexane revealed the noticeable reduction of the dihedral angle of the binaphthyl units exclusively for the cyclic CR-R as well as for the CR-1 and CR-R/S. The observed thermally induced cyclic topology effect on this chiroptical response is reasoned by the solvophobic interaction promoted for the topologically constrained, looped poly(THF) segments in the vicinity of the BiNap units.
The physical properties of cyclic polymers can be perturbed by the presence of architectural impurities, even in trace amounts. As a result, it is important to develop techniques for quantifying and improving the purity of cyclic polymer samples. The zwitterionic ring-expansion polymerization (ZREP) of glycidyl phenyl ether (GPE) with B(C6F5)(3) is a convenient, one-step method to generate cyclic polyethers in large amounts. However, the obtained cyclic samples are inherently contaminated by architectural impurities, which in this study are detected, identified, quantified, and successfully removed by a "click-scavenging" approach. The analytical techniques employed included matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), quantitative Fourier transform infrared spectroscopy (FTIR), and gel permeation chromatography/multiangle light scattering (GPC-MALS). The end-group functionalization of the architectural impurities was a particularly useful method in the identification of tadpole and linear polymers in cyclic poly(glycidyl phenyl ether) samples of M-n = 1.1 and 11.7 kg/mol. Moreover, differential scanning calorimetry (DSC) measurements demonstrated that the presence of architectural impurities cause only a small reduction (1.4-2.0 degrees C) of the glass transition temperature (T-g) of the cyclic polyether; an effect that is corroborated in this investigation by means of dielectric spectroscopy. The segmental and local dynamics of cyclic samples are shown to be only slightly modified by the presence of a small percentage of architectural impurities.
As the most straightforward synthetic strategy for cyclic polymers in theory, the traditional homodifunctional bimolecular ring-closure methods showed limited success for preparing pure cyclic polymers in practice even after several decades of development. A breakthrough was achieved in this paper to develop a successful homodifunctional bimolecular ring-closure method using a self-accelerating double strain-promoted azide-alkyne click reaction as the intermolecular and subsequent intramolecular coupling reactions. Because of the self-accelerating property of coupling reaction, this novel approach eliminated the usage of equimolar quantities between telechelic polymers and small molecule linkers, which was the prerequisite of traditional homodifunctional bimolecular ring-closure methods for pure cyclic polymers. More importantly, this approach could use an excess amount of small linkers to increase the intermolecular coupling reaction rate, further resulting in a significantly enhanced preparation efficiency of cyclic polymers.
Ring-driven physical properties of cyclic poly(vinyl ether)s were studied, such as thermal sensitivity in solution and glass transition temperature (Tg). The samples were precisely synthesized via ring-expansion cationic polymerization with a hemiacetal ester (HAE)-based cyclic compound as the initiator. To clarify the topology effects, linear polymers with similar molecular weights were also prepared via a conventional living cationic polymerization with the HAE-based acyclic initiator (i.e., an adduct of vinyl ether with acetic acid) for comparison. Cyclic poly(vinyl ether)s carrying bulky tricyclic alkane pendant exhibited higher Tgs than the linear counterparts of similar molecular weights. Interestingly, the Tg was not so decreased even as the molecular weight was lower, which was clearly different from linear polymers. The thermosensitivity of cyclic polymer was also studied with ethyl acetate solution of poly(dodecyl vinyl ether) showing upper critical solution temperature (UCST) at around 45 °C. The UCST behavior on cooling process was clearly different from for the linear counterpart, and the cyclic polymer showed duller sensitivity to temperature than the linear. These unique properties of cyclic polymers are likely attributed to the endless structures free from the terminal groups.
l-Lactides were polymerized in bulk at 120 or 160 °C with cyclic dibutyltin catalysts derived from 1,2-dimercaptoethane or 2-mercaptoethanol. Only linear chains having one benzyl ester and one OH-end group were obtained when benzyl alcohol was added. When l-lactides were polymerized with neat dibutyl-2-stanna-1,3-dithiolane, exclusively cyclic polylactides were formed even at 120 °C. The temperature, time and monomer/catalyst ratio (M/C) were varied. These results are best explained by a combination of ring-expansion polymerization and ring-extrusion of cyclic oligo- or polylactides with elimination of the cyclic catalyst. Neither syntheses of linear polylactides nor of cyclic lactides involved racemization up to 20 h at 160 °C.
We used differential scanning calorimetry and spectroscopic ellipsometry to measure the molecular weight (MW) dependence of bulk fragility (mbulk) and spectroscopic ellipsometry to measure the thickness dependences of the glass transition temperature (Tg) and fragility (m) in supported thin films of low MW cyclic or ring polymer. The effects of confinement on Tg and m of thin polymer films are important in a range of advanced technology applications, including nanoimprinting. It has previously been shown that nanoconfined films of high MW linear polystyrene (PS) exhibit major Tg- and m-confinement effects whereas films of low MW cyclic PS (c-PS) show at most a very weak Tg-confinement effect. In the absence of chain ends, c-PS exhibits very weak Tg,bulk- and mbulk-MW dependences compared to linear PS. Despite low MW c-PS having mbulk values similar to that of high MW linear PS, we found that low MW c-PS films show a very weak m-confinement effect because of a weak free-surface effect; e.g., m for a 27 nm thick film of 3.4 kg/mol c-PS is the same as mbulk within error. Overall, these results support a strong correlation between the susceptibility of fragility perturbation and the susceptibility of Tg perturbation caused by MW reduction, chain topology, and/or confinement.
The CuAAC cyclization of linear polymers has proved to be a versatile technique for preparing cyclic polymers with diverse functionality. However, these products often exhibited trace amounts of impurities. HPLC, SEC, and MALDI- TOF MS were utilized to quantify and characterize these trace impurities to ascertain their origin and optimize cyclic polymer purity.
Cyclic polymers have dramatically different physical properties compared with those of their equivalent linear counterparts. However, the exploration of cyclic polymers is limited because of the inherent challenges associated with their synthesis. Conjugated linear polyacetylenes are important materials for electrical conductivity, paramagnetic susceptibility, optical nonlinearity, photoconductivity, gas permeability, liquid crystallinity and chain helicity. However, their cyclic analogues are unknown, and therefore the ability to examine how a cyclic topology influences their properties is currently not possible. We have solved this challenge and now report a tungsten catalyst supported by a tetraanionic pincer ligand that can rapidly polymerize alkynes to form conjugated macrocycles in high yield. The catalyst works by tethering the ends of the polymer to the metal centre to overcome the inherent entropic penalty of cyclization. Gel-permeation chromatography, dynamic and static light scattering, viscometry and chemical tests are all consistent with theoretical predictions and provide unambiguous confirmation of a cyclic topology. Access to a wide variety of new cyclic polymers is now possible by simply choosing the appropriate alkyne monomer.
The architecture of polycations plays an important role in both gene transfection efficiency and cytotoxicity. In this work, a new polymer, sunflower poly(2-dimethyl amino)ethyl methacrylate) (pDMAEMA), is prepared by atom transfer radical polymerization and employed as nucleic acid carriers compared to linear pDMAEMA homopolymer and comb pDMAEMA. The sunflower pDMAEMAs show higher IC50 , greater buffering capacity, and stronger binding capacity toward plasmid DNA than their linear and comb counterparts. In vitro transfection studies demonstrate that sunflower pDMAEMAs exhibit high transfection efficiency as well as relatively low cytotoxicity in complete growth medium. In vivo gene delivery by intraventricular injection to the brain shows that sunflower polymer delivers plasmid DNA more effectively than comb polymer. This study provides a new insight into the relationship between polymeric architecture and gene delivery capability, and as well as a useful means to design potent vectors for successful gene delivery.
The tungsten alkylidyne [tBuOCO]W≡C(tBu)(THF)2 (1) reacts with CO2, leading to complete cleavage of one C=O bond, followed by migratory insertion to generate the tungsten-oxo alkylidene 2. Complex 2 is the first catalyst to polymerize norbornene via Ring Expansion Metathesis Polymerization (REMP) to yield highly cis-syndiotactic cyclic polynorbornene.
Important, yet unexplored effects of chemically distinct initiator fragments incorporated at chain ends in linear polymer are investigated in depth. Polystyrene (PS) samples of a wide range of molecular weight (MW) were synthesized by conventional free radical polymerization and controlled radical polymerization using seven different initiators and compared with anionically polymerized PS. The initiator fragments incorporated during polymerization have major consequences on the glass transition temperature (Tg) and dynamic fragility of low MW PS. For example, with ∼4 kg/mol PS, the Tg onset value and fragility can be tuned from ∼334 K and ∼65, respectively, with dodecanethiol and hydrogen atom chain ends to ∼367 K and ∼130, respectively, with cyanopentanoic acid chain ends. A similar high Tg and high fragility were measured with isobutyric acid/SG1 nitroxide chain ends. These remarkable effects, with a greater than 30 K difference in Tg and a factor of 2 difference in fragility, indicate that chain ends in low MW PS homopolymer play an "outsize" role in comparison to comonomer units in perturbing properties that are sensitive to the density of chain ends. The Tg results also provide further direct evidence against any correlation between the MW at which the Tg-MW dependence saturates and entanglement MW. Instead, the perturbation of Tg by the combined effects of a reduction in MW (increase in chain-end density) and chain-end structure correlates one-to-one within error with the perturbation of fragility. These results suggest that the susceptibility of fragility to be perturbed is key to the susceptibility of Tg to be perturbed.
The endless molecular topology endows cyclic polymers with fascinating physical properties and applications. As a powerful strategy for the preparation of well-defined cyclic polymers, the current ring-closure methods have an inherent disadvantage of low production efficiency using the classic batch reaction condition due to the requirement of ultralow reaction concentration for ring-closing linear polymer precursors. Assisted by the continuous-flow technique, we developed an efficient and practical way to ingeniously solve this essential problem and successfully produce the cyclic polymer in large scale by a light-induced ring-closure method for the first time. In addition, due to the large surface-to-volume ratio of flow reactor, the continuous-flow technique provides the light-induced ring-closing reaction with more uniform light irradiation and transmission, resulting in the significantly increased reaction efficiency compared to that from the classic batch reaction condition.
Ellipsometry measurements as a function of cooling rate are used to study nanoscale confinement effects on dynamic fragility (kinetic fragility), m, in supported films of freely deposited, linear polymer. Polymers include neat polystyrene (PS), neat polycarbonate (PC), and PS + 2 wt % 1,10-bis(1-pyrene)decane (BPD) as small-molecule diluent; in each case, the substrate/polymer interface lacks significant attractive interactions. In terms of both the length scale at which confinement effects become evident and the percentage reduction in m from its bulk value, the magnitude of the m-confinement effect increases with increasing bulk polymer system m. Additionally, for films of linear polymer lacking significant attractive interactions with the substrate surface, m-confinement effects are evident at larger onset thicknesses than those commonly reported in the literature for the glass transition temperature (Tg)-confinement effect. Evans et al. [ Macromolecules 2013, 46, 6091 ] found that the Tg-confinement effect in related films exhibits a universal nature as a function of scaled thickness. Fragility-confinement effects of films of freely deposited, linear polymer chains exhibit a similar universal nature as a function of scaled thickness using shift factors consistent with those used by Evans et al. However, when PS is confined in a dense brush with one end of each chain covalently attached to the substrate surface, both m and Tg are independent of brush thickness. The strong correlation of fragility-confinement and Tg-confinement effects has important implications for understanding the fundamental natures of both the Tg-confinement effect and the glass transition itself.
Cyclic and linear, isoregic and aregic, and isotactic and atactic poly(glycidyl phenyl ether) (PGPE) with molecular weights up to Mw = 5.5 kg/mol are synthesized by ring-opening polymerization of glycidyl phenyl ether. Initiation with tetrabutylammonium fluoride leads to isoregic linear polymers with ∼95% regular linkages, and initiation with B(C6F5)3 and B(C6F5)3/water leads to aregic cyclic and linear polymers, respectively, with ∼50% regular linkages as quantified by 13C NMR. Local, segmental, and chain dynamics in PGPE is investigated by broadband dielectric spectroscopy (10-2-106 Hz). The β-relaxation for linear PGPE is separated into two contributions arising from the motions of side groups and end groups with activation energies of 35.4 and 23.8 kJ/mol, respectively. The β-relaxation process for cyclic PGPE shows the same activation energy as that shown by the side-group contribution in linear PGPE, indicating that topology does not play a key role on the side-group local dynamics. Moreover, cyclic PGPE samples show higher calorimetric and dynamic glass transition temperatures as well as lower dynamic fragility compared to linear chains. Unexpectedly from topological considerations, cyclic PGPE shows low frequency dielectric contributions that can be attributed to short wavelength internal ring motions and that are detectable by dielectric relaxation due to the aregic nature of the rings.
High purity cyclic PS (c-PS) samples with number-average molecular weight (MW) of 3.4 and 9.1 kg/mol were synthesized via atom transfer radical polymerization and "click" chemistry with narrow MW distribution. Bulk glass transition temperature (Tg) measured by differential scanning calorimetry exhibited a much weaker MW dependence for c-PS relative to its linear precursor and anionically polymerized linear PS (A-PS). Using ellipsometry and fluorescence spectroscopy, major differences were observed in the Tg-confinement effect in c-PS films supported on silicon substrates compared to A-PS. Whereas a large Tg reduction with confinement is commonly observed for A-PS supported on silica, within error, no confinement effect is seen in c-PS/3.4k films on Si/SiOx substrates down to 21 nm thickness. Although the c-PS linking group contains nitrogen and oxygen atoms potentially able to undergo hydrogen bonding, Tg is invariant with confinement for c-PS/3.4k or slightly reduced for c-PS/9.1k regardless of the level of substrate-surface hydroxyl groups. Ellipsometry indicates that the near elimination of the Tg-confinement effect in c-PS originates mainly from a very weak perturbation to Tg near the free surface (in comparison to linear PS) rather than a strong perturbation at the polymer-substrate interface. We hypothesize that unlike linear polymers, the packing efficiency of cyclic PS segments, i.e., cyclic PS fragility, is not significantly perturbed by the free surface, which in turn results in at most a very weak Tg perturbation at the free surface and an invariance of average Tg across the film with confinement.
To improve the stability of pharmaceutical micelles and make them long-circulating, we have prepared new polymeric micelles from diacyllipid (phosphatidyl ethanol, PE) derivatives of polyethylene glycol (PEG-PE). PEG-PE micelles are extremely stable (very low CMC value) and can easily incorporate poorly soluble pharmaceuticals including both low-molecular-weight compounds and proteins, as well as diagnostic agents. Comparative studies on micellar and liposomal carriers demonstrated that, under certain circumstances, micelles proved to be superior carriers. Thus, the lipophilic cationic drug dequalinium cannot be loaded into liposomes, while it can be easily solubilized by PEG-PE micelles forming stable mixed micelles. The small size of micelles permits their easier penetration into tumors with small cutoff size. Protein-loaded micelles accumulate in subcutaneous Lewis lung carcinoma in mice better than larger protein-loaded liposomes.
The delivery of genetic material to cells offers the potential to treat many genetic diseases. Cationic polymers, specifically poly(ethylene imine) (PEI), are promising gene delivery vectors due to their inherent ability to condense genetic materials and successfully affect its transfection. However, PEI and many other cationic polymers also exhibit high cytotoxicity. In order to systematically study the effect of polymer architecture on gene delivery efficiency and cell cytotoxicity, a set of cyclic PEIs were prepared for the first time and compared to a set of linear PEIs of the exact same molecular weight. Subsequent in vitro transfection studies determined a higher transfection efficiency for each cyclic PEI sample when compared to its linear PEI analog in addition to reduced toxicity relative to the branched PEI "gold standard" control. These results highlight the critical role that the architecture of PEI can play in both optimizing transfection and reducing cell toxicity.
The vital role of chain ends in the crystallization of linear polymers can be understood by unraveling the mechanisms of crystallization of cyclic polymers. A commercial chip-calorimeter Flash DSC1 was employed to compare the nucleation and overall crystallization rates between linear and cyclic poly(epsilon-caprolactones) (PCL) with similar molar masses of around 2 kg/mol. In the high temperature region, faster overall crystallization of cyclic PCL relative to linear PCL is consistent with previously reported results employing conventional DSC. In the low temperature region, the cyclic PCL exhibits a lower onset temperature of homogenous nucleation when compared to linear PCL analogs. This result was attributed to a higher mobility of free chain ends in the linear PCL as compared to cyclic PCL. A simplified data-treatment method on the nucleation half-time has been proposed.
Complex polymer structures, including a spiro tricyclic and first generation dendritic structures, were constructed from cyclic polymer building blocks. We described a new method to produce monocyclic polymers with hydroxyl groups equally spaced along the polymer backbone. A key synthetic feature was carrying out the CuAAC reaction of telechelic polymer chains in the presence of a bromine group through modulating the Cu(I) activity toward the "click" reaction over radical formation. This allowed the precise control over the location of the OH groups. Azidation of the bromine groups and cyclization using a modified feed approach resulted in multifunctional monocyclics in high amounts and high purity of greater than 99% after fractionation. Conversion of the OH groups to either azide or alkyne functionality produced the central core macromolecule from which the more complex topologies were built. All four complex topologies, including a spiro tricyclic, and dendritic structures consisting of a G1 pentacyclic, G1 tertacyclic, and a G1 heptacyclic were produced in high amounts with good "click" efficiencies.
Polymeric micelles, self-assembling nano-constructs of amphiphilic copolymers, are widely considered as convenient nano-carriers for a variety of applications, such as diagnostic imaging, and drug and gene delivery. They have demonstrated a variety of favorable properties including biocompatibility, longevity, high stability in vitro and in vivo, capacity to effectively solubilize a variety of poorly soluble drugs, changing the release profile of the incorporated pharmaceutical agents, and the ability to accumulate in the target zone based on the enhanced permeability and retention effect. Moreover, additional functions can be imparted to the micelle-based delivery systems by engineering their surface for specific applications. Various targeting ligands can be attached for cell or intracellular accumulation at a site of interest. Also, the chelation or incorporation of imaging moieties into the micelle structure enables in vivo biodistribution studies. Moreover, pH-, thermo-, ultrasound-, enzyme- and light-sensitive block-copolymers allow for controlled micelle dissociation and triggered drug release in response to the pathological environment-specific stimuli and/or externally applied signals. The combination of these approaches can further improve specificity and efficacy of micelle-based drug delivery to promote the development of smart multifunctional micelles. For further resources related to this article, please visit the WIREs website. Conflict of interest: The authors have declared no conflicts of interest for this article.
© 2015 Wiley Periodicals, Inc.
Cyclic peptides are found in a diverse range of organisms and are characterized by their stability and role in defense. Why is only one class of cyclic peptides found in mammals? Possibly we have not looked hard enough for them, or the technologies needed to identify them are not fully developed. We also do not yet understand their intriguing biosynthesis from two separate gene products. Addressing these challenges will require the application of chemical tools and insights from other classes of cyclic peptides. Herein, we highlight recent developments in the characterization of theta defensins and describe the important role that chemistry has played in delineating their modes of action. Furthermore, we emphasize the potential of theta defensins as antimicrobial agents and scaffolds for peptide drug design.
Matching macrocyclic and linear polystyrenes (PS) were synthesized by the initiation of styrene with 2,7‐dimethyl‐3,6‐diphenyloctane dianion lithium salt followed by high dilution coupling with 1,4‐bis(bromomethyl)benzene or protonation. Liquid chromatography at the critical condition shows the presence of less than 4% of linear PS impurities in the fractionated cycles. SEC studies confirm that the ratios of apparent MWs of cyclic and linear PS increase from about 0.7 to more than 0.9 as MWs decrease. Fluorescence studies show that the monomer emissions at 285 nm strongly increase with decreasing MW whereas those of the linear polymers are not significantly affected. This may be due in part to the increased rigidity of the smaller cycles that decreases the rate of radiation‐less deactivation.
Dependence of 〈G〉 on number average MW.
magnified image Dependence of 〈G〉 on number average MW.
Cyclic polymers have intriguing physical properties, including those found in biological membranes for greater temperature, salt and acid stability. Although, many unique and complex synthetic cyclic structures have been prepared, there are no reports of ABC miktoarm stars constructed of three cyclic polymers with very different chemical compositions. We report such a structure in one pot at 25 °C by modulating the copper catalyst activity using combinations of solvents and ligands.
While amphiphilic block copolymers have demonstrated their utility for a range of practical applications, the behavior of cyclic block copolymers remains largely unexplored due to limited synthetic access. To investigate their micelle formation, biocompatible cyclic amphiphilic poly(ethylene glycol)-polycaprolactone, c-(PEG-PCL), was synthesized by a combination of ring-opening polymerization (ROP) and click chemistry. In addition, exactly analogous linear block copolymers have been prepared as a control sample to elucidate the role of polymer architecture in their self-assembly and acid-catalyzed degradation.
A triply-fused tetracyclic macromolecular K3,3 graph has been constructed through electrostatic self-assembly of a uniform-size dendritic polymer precursor having six cyclic ammonium salt end groups carrying two units of a trifunctional carboxylate counteranions, and subsequent covalent conversion by the ring-opening reaction of cyclic ammonium salt groups at an elevated temperature under dilution. The K3,3 graph product was isolated from the two constitutional isomers by means of a recycling SEC technique, as the hydrodynamic volume of the triply-fused tetracyclic product is remarkably contracted in comparison with another isomer having a ladder form in solution.
Disulfide-rich cyclic peptides have exciting potential as leads or frameworks in drug discovery; however, their use is faced with some synthetic challenges, mainly associated with construction of the circular backbone and formation of the correct disulfides. Here we describe a simple and efficient Fmoc solid-phase peptide synthesis (SPPS)-based method for synthesizing disulfide-rich cyclic peptides. This approach involves SPPS on 2-chlorotrityl resin, cyclization of the partially protected peptide in solution, cleavage of the side-chain protecting groups, and oxidization of cysteines to yield the desired product. We illustrate this method with the synthesis of peptides from three different classes of cyclic cystine knot motif-containing cyclotides: Möbius (M), trypsin inhibitor (T), and bracelet (B). We show that the method is broadly applicable to peptide engineering, illustrated by the synthesis of two mutants and three grafted analogues of kalata B1. The method reduces the use of highly caustic and toxic reagents and is better suited for high-throughput synthesis than previously reported methods for producing disulfide-rich cyclic peptides, thus offering great potential to facilitate pharmaceutical optimization of these scaffolds.
The segmental relaxation properties of a low molecular weight (4.6 kg/mol), cyclic polystyrene (PS) were characterized. The sample was obtained by fractionation using HPLC at the chromatographic critical condition, which yields a ring uncontaminated by its linear precursor. Both the glass temperature and the temperature dependence of the segmental relaxation times for the ring PS were equivalent to the high molecular weight limiting values for the linear polymer. These results are interpreted by considering the configurational mobility of a polymer lacking chain ends.
A cyclic polymer is one of the ideal model polymers with which to investigate the effects of chain topology on the physical properties of polymers. In this review, I summarize my recent work demonstrating that the chain structure of cyclic polymers can be directly observed by atomic force microscopy (AFM) and that the mutual diffusion of cyclic polymers is faster than that of corresponding linear ones. For direct evidence of the cyclic structure, isolated molecules of cyclic poly(sodium styrenesulfonate), which was derived from cyclic polystyrene, were observed by AFM. The mutual diffusion of cyclic polystyrene/cyclic deuterated polystyrene was investigated as a function of temperature and molecular weight by dynamic secondary ion mass spectroscopy. For the molecular weight of 113k, the mutual diffusion coefficient of cyclic polystyrene, DC, was approximately twofold larger than that of the corresponding linear polymer, DL, at all temperatures. Under an iso-free volume condition, the DC value was larger than the DL value for all the molecular weights. These results clearly show that the chain topology strongly affects the molecular motion of the whole chain.
The architecture of polycation gene carriers has been shown to affect both their transfection efficiency and cytotoxicity. This work reports the synthesis of cyclic polycations and their use for gene transfer to mammalian cells. Cyclic poly((2-dimethylamino) ethylmethacrylate) (pDMAEMA) homopolymers of various molecular weights were synthesized by “intrachain”click cyclization of α-alkyne-ω-azide heterodifunctional linear precursors prepared by atom transfer radical polymerization (ATRP). Polymers were characterized by size exclusion chromatography and FT-IR analyses to confirm efficient cyclization and products with low polydispersity. Cyclic polymers formed more compact particles with plasmid DNA compared to linear analogues. Cellular uptake, membrane disruption, and nucleic acid delivery efficiency were determined for all polymers. In general, cyclic polymers complexed and delivered nucleic acids with efficiencies similar to their linear counterparts. Notably, cyclic polymers were less cytotoxic than linear polymers due to reduced membrane disruption and are therefore promising alternative structures for biological applications.
The synthesis and characterization of model cyclic diblock copolymers of styrene (St) or perdeuterated styrene (St-d(8)) and butadiene (Bd) are presented. Since conventional methods of characterization cannot separate completely the cyclic copolymer from its linear precursor, differences in the micellar behavior were used as a method for investigation of their purity. For this purpose, in addition to the cyclic and linear triblock copolymers, two linear diblocks with similar compositions, and molecular weight equal or half of the cyclic diblocks, were also synthesized. The synthetic approach of the cyclics involved the reaction of (1,3-phenylene)bis(3-methyl-1-phenylpentylidene)dilithium initiator with butadiene in the presence of sec-BuOLi, followed by polymerization of St (or St-d8). The cyclization of the resulting a,co-difunctional triblock copolymer was performed by using bis(dimethylchlorosilyl)ethane, under high dilution conditions. The copolymers were characterized by size exclusion chromatography, membrane osmometry, NMR and UV spectrometry, and viscometry. The micelles formed in the selective solvents n-decane (for PBd) and dimethylformamide (for PS-d(8)) were characterized by small-angle neutron scattering and dynamic light scattering. It was found that the aggregation number of the cyclic copolymers was the smallest among the different macromolecular architectures. Moreover, the SANS data for the triblocks in n-decane indicated the presence of 37% dangling chains which did not appear in the data for the corresponding cyclic copolymers. Considering that 5% of dangling chains is possible to be detected, it proves that the cyclic copolymers are at least 87% pure. A scaling model was used in order to justify the difference in the aggregation numbers between the four different copolymers.
High purity homo-arm and mikto-arm poly(ethylene glycol) (PEG) stars are successfully prepared by the combination of epoxide ring-openings and azide-alkyne click reactions. First, monohydroxy-PEG was modified via epoxide chemistry to bear one hydroxyl and one azide functionality at the same end. An alkyne-functionalized PEG chain was then coupled to the azide. Subsequently, the remaining hydroxyl could be reactivated to an azide again and again to enable stepwise addition of alkyne-functionalized polymer arms. The use of efficient reactions for this iterative route provides star polymers with an exact number of arms, and a tailorable degree of polymerization for each arm. Detailed characterization confirms the high purity of multi-arm polyethylene glycol products.