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Synthesis and characterization of poly(ϵ‐caprolactone)–poly(L‐lactide) diblock copolymers with an organic amino calcium catalyst
Journal of Applied Polymer Science
Abstract
The quasiliving characteristics of the ring-opening polymerization of ϵ-caprolactone (CL) catalyzed by an organic amino calcium were demonstrated. Taking advantage of this feature, we synthesized a series of poly(ϵ-caprolactone) (PCL)–poly(L-lactide) (PLA) diblock copolymers with the sequential addition of the monomers CL and L-lactide. The block structure was confirmed by 1H-NMR, 13C-NMR, and gel permeation chromatography analysis. The crystalline structure of the copolymers was investigated by differential scanning calorimetry and wide-angle X-ray diffraction analysis. When the molecular weight of the PLA block was high enough, phase separation took place in the block copolymer to form PCL and PLA domains, respectively. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 2654–2660, 2006
... It is well known that amphiphilic block copolymers containing poly (ethylene oxide) (PEO) show the unique and outstanding properties, such as biocompatibility, lack of immunogenicity and easy clearance from the human body, it has caused widely concern in the field of medicine. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] PEO is widely used not only for its hydrophilicity and biocompatibility, but also for some properties of POE present in the materials modified by POE. [6] Among the investigated amphiphilic block copolymers, the biodegradable copolymers are of special interest. ...
... [6] Among the investigated amphiphilic block copolymers, the biodegradable copolymers are of special interest. The biodegradable polymers, such as poly (ε-caprolactone) (PCL) [12][13][14][15][16][17][18] have been used as important biomaterials for a wide variety of drug delivery carriers because of their biocompatibility and biodegradability. The biodegradable polymers PCL modified by PEO have attracted more attention in order to meet the increasing demands for better performances and some specific applications. ...
A series of amphiphilic graft copolymers PEO-g-PCL with different poly (ε-caprolactone) (PCL) molecular weight were successfully synthesized by a combination of anionic ring-opening polymerization (AROP) and coordination-insertion ring-opening polymerization. The linear PEO was produced by AROP of ethylene oxide (EO) and ethoxyethyl glycidyl ether initiated by 2-(2-methoxyethoxy) ethoxide potassium, and the hydroxyl groups on the backbone were deprotected after hydrolysis. The ring-opening polymerization of CL was initiated using the linear poly (ethylene oxide) (PEO) with hydroxyl group on repeated monomer as macroinitiator and Sn(Oct)2 as catalyst, then amphiphilic graft copolymers PEO-g-PCL were obtained. By changing the ratio of monomer and macroinitiator, a series of PEO-g-PCL with well-defined structure, molecular weight control, and narrow molecular weight distribution were prepared. The expected intermediates and final products were confirmed by 1H NMR and GPC analyzes. In addition, these amphiphilic graft copolymers could form spherical aggregates in aqueous solution by self-assemble, which were characterized by transmission electron microscopy, and the critical micelle concentration values of graft copolymers PEO-g-PCL were also examined in this article.
... [50,51] Copolymerization of LA by different methods with suitable comonomers and catalysts resulted in novel diblock, triblock and multiblock copolymers with distinct properties as that of the respective homopolymers is given in Table 1. [19,23,[52][53][54][55][56][57][58][59] Apart from the synthetic methods, the structure of the block copolymer is also an important parameter, which determines the properties of the polymer. [60] The difference in properties of diblock, triblock and multiblock copolymers of PLA is due to this structural difference and the different types of comonomers added. ...
Poly(lactic acid) (PLA) is an important biomaterial with its easy availability and wide spread acceptability in biomedical field. The most appreciable characteristic nature of PLA is its versatility itself, even though certain drawbacks of PLA like hydrophobicity, lack of reactive functional groups, poor toughness and slow biodegradation rates limits its uses in various sectors. Therefore in the past few decades research interests have grown to overcome these limitations. Many researchers have reported different physical and chemical modifications for augmenting the inherent properties of PLA so that its applications can be widened. Generally, methods such as copolymerization, blending, incorporation of chemicals or additives, plasma treatment were executed in order to enhance its property. Out of these, copolymerization method is the one favored by most researchers. Moreover PLA copolymers can also be used in biomedical field too. Therefore, extensive research works were conducted on copolymerization method. However, to the best of our knowledge, an overview focusing mainly on the properties of PLA modified by copolymerization technique was not yet reviewed till date. This review article summarizes the chemical modifications of PLA through copolymerization technique along with the characterization methods used for the confirmation of copolymer formation and various enhanced properties and their applications in biomedical field are discussed.
... However, due to their hydrophobic surface, their cell recognition signals are low [3,4]. Poly(L-lactide) (PLLA) is a semicrystalline thermoplastic which has been widely used in biomedical purposes, such as bone fixation and surgical sutures because of its good bioresorbability and biocompatibility, however, brittleness has limited its applications [5,6]. Poly(ε-caprolactone) (PCL) is relatively non-toxic and possesses sufficient mechanical strength, biocompatibility, and thermal stability for scaffolding applications. ...
The halogenated β‐diketiminato magnesium butyl complexes LXMgBu(THF) (X = Cl (1), Br (2)) and allyl complexes LXMg(allyl)(THF) (X = Cl (3), Br (4)) (LCl = ((2,6‐Cl2C6H3)N═C(Me)CH═C(Me)N(2,6‐Cl2C6H3)), LBr = ((2,6‐Br2C6H3)N═C(Me)CH═C(Me)N(2,6‐Br2C6H3)) were prepared and characterized. The treatment of butyl complex 2 with (S)‐1‐phenylethanol gave the corresponding alkoxide complex [LBrMgOCHMePh]2 (5), the dimeric structure of which was confirmed by single‐crystal x‐ray diffraction. When bulky diphenylmethanol was employed to react with Complex 2, THF‐ligated monomeric complex LBrMgOCHPh2(THF) (6) was afforded. The catalytic performances of these magnesium complexes for ring‐opening polymerization of ε‐caprolactone (ε‐CL), δ‐valerolactone (δ‐VL), and γ‐butyrolactone (γ‐BL) were evaluated. β‐Diketiminato ligands bearing alkyl groups (Me, Et, and iPr) on the N‐aryl moieties have been widely investigated in the organometallic chemistry and the ring‐opening polymerization (ROP) of cyclic esters, while halogenated β‐diketiminate ligands have been far less unexplored. In this contribution, a series of halogenated β‐diketiminato magnesium complexes with various initiating groups have been synthesized and characterized, among them a dimeric alkoxide complex was confirmed with single‐crystal x‐ray diffraction. These complexes could efficiently initiate the ROP of ε‐CL and δ‐VL at ambient temperature, where the alkoxide complexes (MgO bond) exert better control than the hydrocarbyl complexes (MgC bond). Moreover, these complexes could catalyze the ROP of nonstrained γ‐BL at −40 °C.
The confined crystallization of poly(ϵ‐caprolactone) (PCL) block in poly(ϵ‐caprolactone)–poly(l‐lactide) (PCL‐PLLA) copolymers was investigated using differential scanning calorimetry, polarized optical microscopy, scanning electronic microscopy and atomic force microscopy. To study the effect of crystallization and molecular chain motion state of PLLA blocks in PCL‐PLLA copolymers on PCL crystallization morphology, high‐temperature annealing (180 °C) and low‐temperature annealing (80 °C) were applied to treat the samples. It was found that the crystallization morphology of PCL block in PCL‐PLLA copolymers is not only related to the ratio of block components, but also related to the thermal history. After annealing PCL‐PLLA copolymers at 180 °C, the molten PCL blocks are rejected from the front of PLLA crystal growth into the amorphous regions, which will lead to PCL and PLLA blocks exhibiting obvious fractionated crystallization and forming various morphologies depending on the length of PLLA segment. On the contrary, PCL blocks more easily form banded spherulites after PCL‐PLLA copolymers are annealed at 80 °C because the preexisting PLLA crystal template and the dangling amorphous PLLA chains on PCL segments more easily cause unequal stresses at opposite fold surfaces of PCL lamellae during the growth process. Also, it was found that the growth rate of banded spherulites is less than that of classical spherulites and the growth rate of banded spherulites decreases with decreasing band spacing. © 2019 Society of Chemical Industry
Poly(L-lactide-co- ε-caprolactone) P(LLA-co-CL) copolymers with various L-lactide/ε-caprolactone(L- LA/ε-CL) mole ratios were prepared by the ring opening copolymerization of L-LA and ε-CL using nontoxic magnesium octoate as a catalyst in bulk. The time-conversion curves of L-LA and ε-CL copolymerization were recorded by1 H-NMR. The results showed significantly more rapid polymerization of L-LA relative to ε-CL. The carbonyl carbon signals in13 C-NMR are most sensitive to the sequence distributions of the lactidyl and caproyl units and used for calculating their average block length (LLLe and L ce). Two modes of transesterification occurred during the copolymerization process and played an important role in the redistribution of comonomer sequences. With polymerization progress, LLLe decreased sharply and Lce increased at first and then decreased gradually, as a result, the run number (R) increased gradually and the extent of randomness enhanced. The CLC sequence formed by the second mode of transesterification was observed at the end of reaction. Then, the reactivity ratios of L-LA and e-CL were measured to be rLA= 23 and r cl - 0.22, respectively, using the Fineman-Ross method. The results(rLA > 1 and +++ rcl < 1) imply that L-lactide monomer added preferentially into the copolymer chain end in the first step of the reaction, resulting in the formation of long blocks of lactidyl unit in the polymer chain. The copolymer composition has a profound influence on the values of LLLeand Lce. Both LLLeand Lce increased as the relative proportions of their respective monomers increased. The transesterification coefficient of the second mode of lactidyl units (T II [CLC]) increased as the feed mole fraction of e-CL increased. In terms of the overall feed compositions, the LLLr values calculated from the reactivity ratio exceeded the LLLe values determined from the product; however, the Lcr values were identical or shorter than the Lce values. Therefore, the repeat unit sequence distribution of the copolymers was varied more or less toward a random sequence distribution by variation in the comonomer feed ratio and reaction time. The thermal properties and crystallization of the copolymers were investigated by DSC and XRD. The results showed that a close relationship between copolymer crystallinity and the length of the copolymer sequences. All copolymers of L-LA and e-CL exhibited a single Tgs determined from the second heating scan. The experimental Tgs were in agreement with the calculated Tgs values from Fox equation. It can be concluded that the copolymers are random, or comprise a mixture of compatible block copolymers.
A catalytic heavier alkaline earth chemistry somewhat reminiscent of d0 transition metal catalysis has emerged in the first decade of the twenty-first century. As complexes of these elements are effectively redox inactive, catalytic reactivity is dependent on the viability of sequential σ-bond metathesis and/or polarized insertion steps. This chapter reviews this reactivity with an emphasis upon the types of processes which have been reported as well as the fundamental insights into alkaline earth reactivity that have been deduced from catalytic and stoichiometric mechanistic studies.
The reduction of 1,4-diphenyl-1,4-bis(diphenylphosphanyl)buta-1,3-diene (1) (1,4-diphenyl- NUPHOS) with potassium in THF yields bis(THF)potassium 2,5-diphenylphospholide (2) which crystallizes with a chain structure. The metathesis reaction of 2 with the iodides of calcium, strontium, and barium leads to the formation of [bis(THF)calcium bis(2,5-diphenylphospholide)] (3), [bis(THF)strontium bis(2,5-diphenylphospholide)] (4), and [bis(THF)barium bis(2,5-diphenylphospholide)] (5). The reaction of M{P(H)SiiPr
Two aluminium and magnesium heterobimetallic complexes of [(LAl)MgOBn]2 (2) and [(LAl)Mg(OC2H4OCH3)]2 (3), in which L = N1,N1,N2,N2-tetrakis(2-hydroxy-3,5-dimethylbenzyl)-1,2-ethanediamine, have been synthesized and characterized by NMR spectroscopy, X-ray crystallography, and elemental analysis. The magnesium alkoxide of complex 2 supported by the bulky aluminium tetraphenolates, as a rare helical heterobimetallic initiator, can effectively initiate the ring-opening polymerization (ROP) of L-lactide (L-LA) leading to polymers with good molecular weight control and narrow molecular weight distributions. Kinetic studies have shown the overall rate expression is –d[lactide]/dt = k[lactide][complex] for the ROP of L-LA. Furthermore, complex 2 shows modest isotactic selectivity in the ROP of rac-lactide.
Catalyst systems obtained from the inexpensive, ligand-free and commercially available MgnBu2 and alcohols were used for the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) under mild conditions. The catalytic system of MgnBu2/Ph2CHOH showed very high activity as compared with the system of MgnBu2/Ph3COH, which applied a more bulky methanol derivative. Interestingly, in the presence of excess amount of alcohol (Ph2CHOH), i.e., varying the OH-to-Mg ratio in a wide range from 10:1 to 800:1, the system MgnBu2/Ph2CHOH still remained highly active, and produced up to 800 polycaprolactone (PCL) chains per Mg center, suggesting a “living immortal” polymerization mode. The molecular weights of the obtained PCLs are accurately controlled when the ratio of [CL]0/[Mg]0 changed from 500 to 8000 together with narrow molecular weight distributions. Moreover, diblock PCL-b-PLA copolymers with narrow PDIs have been facilely achieved. Furthermore, the allyl- and propargyl-functionalized diphenylmethanols could be employed as the chain transfer agents (CTA) in this system for in situ construction of allyl- and propargyl-functionalized PCLs that were facilely modified further to PCLs with multiple functionality as building blocks for amphiphilic and topological microstructured PCLs via coupling and click reactions.
The N,O-bidentate pyridyl functionalized alkoxy ligands 2-(6-methyl-2-pyridinyl)-1,1-dimethyl-1-ethanol (L1–H) and 2-(6-methyl-2-pyridinyl)-1,1-diphenyl-1-ethanol (L2–H) have been prepared by treatment of acetone and benzophenone with monolithiated 2,6-lutidine. Deprotonolysis of the ligands L1–H and L2–H with 1 equiv of MgnBu2 and ZnEt2 in toluene by releasing butane and ethane, respectively, gave the corresponding dimeric metal-monoalkyl complexes [L1MgnBu]2 (1), [L2MgnBu]2 (2), [L1ZnEt]2 (3), and [L2ZnEt]2 (4). Complexes 1–4 were characterized by 1H and 13C NMR spectroscopy analysis, and the molecular structures of 1, 3, and 4 were further confirmed by X-ray diffraction analysis. The investigation of the catalytic behavior of these complexes toward ε-caprolactone (ε-CL) and l-lactide (l-LA) polymerizations showed that the Mg-based complexes gave higher activity than those attached to zinc metal, probably owing to the greater ionic character of the magnesium metal. Remarkably, the magnesium complex 2 exhibited a striking “immortal” nature in the presence of primary alcohols where up to 500 PCL chains grew from each Mg active center when benzyl alcohol was employed, while, in particular, in the presence of triethanolamine, complex 2 also displayed an immortal mode for the polymerization of l-LA.
The crystallization kinetics and morphology of biodegradable and double crystalline poly(l-lactide)-b-poly(ε-caprolactone) diblock copolymers (PLLA-b-PCL) was studied in a wide composition range by differential scanning calorimetry (DSC) and polarized light optical microscopy (PLOM). The two blocks were found to be partially miscible according to the variations of their thermal transitions with composition. PLOM results showed that PLLA crystallizes in a wide composition range with a spherulitic superstructural morphology. Only when the PLLA content is as low as 10 wt % are axialites formed. These results were in good agreement with the overall crystallization kinetics obtained by DSC isothermal experiments and analyzed by the Avrami equation. Both overall crystallization rates and spherulitic growth rates of the PLLA block decrease with PCL content because PCL acts as a diluent for the PLLA block in view of their miscibility. Reorganization processes revealed as double melting peaks for the PLLA block (not observed for the PLLA homopolymer) were observed during heating scans performed after isothermal crystallization. The reorganization ability of the PLLA block was found to increase with PCL content, a fact that quantified the perturbation caused by molten PCL block chains during the isothermal crystallization of the PLLA block. The PCL block crystallizes within previously formed PLLA spherulites or axialites. Despite the partial miscibility, unexpected and novel fractionated crystallization of the PCL occurs at contents of PCL between 40 and 19 wt %. For the lowest PCL content (i.e., 19%), a homogeneous nucleation process was detected, as indicated by the large supercooling needed for crystallization and by the first-order crystallization kinetics obtained (i.e., Avrami index close to 1). Because of the partial miscibility, the glass-transition temperature of the PLLA block (Tg,PLLA) decreases with PCL addition, so at PCL contents lower than 40 wt %, the Tg,PLLA values are close to or higher than the crystallization temperature of the PCL block. Therefore, PCL fractionated crystallization is induced by hard confinement of the PLLA amorphous and crystalline regions. This is the first time that a homogeneous nucleation process has been documented for a crystallizable component in a miscible or weakly segregated diblock copolymer. The PCL block can also be nucleated by previously formed PLLA crystals depending on the crystallization degree of the PLLA, which was varied by self-nucleation experiments. The crystallization rate of PCL strongly decreased with increasing PLLA content.
The preparation, single crystal and molecular structures of the compounds (BDI)Mg(OCMe2COOEt)(L) are described where BDI = 2-[(2,6-diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-ene and L = pyridine (py) and 4-dimethylaminopyridine (DMAP). In the solid-state the molecules have five coordinate Mg(2+) ions that may be considered as derived from a trigonal bipyramid where the chelating BDI and OCMe2COOEt ligands span equatorial-axial sites. IR spectroscopic data obtained in CH2Cl2 indicate that the chelating nature of the OCMe2COOEt ligand is maintained in solution and variable temperature NMR studies reveal that the ligands py or DMAP undergo a rapid reversible dissociation in toluene-d8 and especially in THF-d8. This leads to a facile exchange involving coordinated and free L upon the addition of L and evidence is presented to support the view that this occurs exclusively by a dissociative interchange mechanism. Zinc forms a related complex (BDI)Zn(OCMe2COOEt) lacking additional coordination of L but maintaining the chelating nature of the OCMe2COOEt ligand and undergoing reversible coordination of L in the presence of added ligands L. The magnesium and zinc complexes will initiate and sustain the living polymerization of lactide (L- or rac-LA) but not the ring-opening polymerization of ε-caprolactone.
The compounds LMgBun(THF) and L′MgBun(THF) where L = 1,5,9-trimesityldipyrromethene and L′ = 2-[(2,6-diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-ene have been prepared from reactions between MgBun2 and the protonated ligands LH and L′H, respectively. Single crystal X-ray crystallographic studies reveal that in each compound the Mg2+ ion is in a distorted tetrahedral environment and further that the ligand L is more sterically demanding with respect to access to the Mg–Bun group. Related alkyl complexes (R = Me, Et, Prn, Pri, n-hexyl and CH2CH2Ph) were prepared from reactions involving LLi(THF) or L′Li(THF) and the appropriate RMgX (X = Cl or Br) and characterized by 1H NMR spectroscopy. Those where R = CH2CH2X (X = alkyl or phenyl) have characteristic α-CH2 proton resonances which are the part of an AA′XX′ pattern. Reactions with alcohols ROH (1 equiv.) give the kinetic products LMg(OR)(THF) and L′Mg(OR)(THF) which are isolable and kinetically persistent when R = a bulky group such as But and the structure of LMg(OBut)(THF) is reported and compared with that of the known compound L′Mg(OBut)(THF). With less bulky groups the compounds are labile toward ligand scrambling and the compound L′MgBun(THF) reacts with alcohols (2 equiv.) to give L′H and Mg(OR)2. Reactions with amines and carbon dioxide are described which indicate the greater reactivity of the Mg–Bun group relative to both the L′ and L ligand. With PhCHO, Ph2CO and cyclohexanone both LMgBun(THF) and L′MgBun(THF) react via β-hydrogen atom transfer to generate the appropriate alkoxide with the elimination of 1-butene. Similarly L-lactide reacts by β-H transfer to give poly-L-lactide (P-L-LA) and 1-butene while rac-lactide yields atactic polylactide in toluene–dichloromethane solutions but in the presence of ≥10 equiv. of THF, heterotactic PLA is formed. The ring-opening polymerization of ε-caprolactone also is initiated by β-H transfer and is about ten times faster than for lactide; kp(LA) = 10.7 M−1 s−1vs. kp(CL) = 110 M−1 s−1. Interestingly, the presence of lactide completely suppresses the polymerization of ε-caprolactone.
The morphology of solution grown single crystals of a series of double crystalline diblock copolymers derived from l-lactide and ɛ-caprolactone has been investigated by means of transmission electron microscopy. The copolymers had a variable composition with a poly(l-lactide) weight percentage that ranged between 81 and 10%. All samples had a low polydispersity index (1.4–1.1) and a similar number average molecular weight (20,000–35,000g/mol).Bulk crystallization and melting behaviour of diblock copolymers were evaluated by DSC and the results demonstrated the double crystalline nature of the samples. Fractionated crystallization clearly occurred in copolymers having an intermediate composition.Isothermal crystallizations were performed in dilute n-hexanol solutions at temperatures that ranged between 80 and 50°C. Crystal morphologies were dependent on the crystallization temperature and even on the composition. Thus, the inability of poly(ɛ-caprolactone) (PCL) blocks to crystallize between 80 and 70°C rendered lozenge, truncated and spindle-shaped crystals associated to the poly(l-lactide) (PLLA) block. These usually had thicker edges due to PLLA overgrowths that mainly took place in their periphery. However, an overgrowth of irregular PCL crystals during subsequent cooling and crystallization at room temperature was also detected. Complex morphologies constituted by lamellar crystals of both PCL and PLLA blocks were developed at intermediate temperatures (70–65°C), whereas elongated hexagonal morphologies mainly associated to the PCL block were detected at the lowest crystallization temperature. In general, electron diffraction patterns showed for all samples’ reflections associated to both poly(ɛ-caprolactone) and poly(l-lactide) (α-form) crystals. The relative intensity between the two types of reflections varied according to the copolymer composition.
The aim of this paper is to present a synthesis method of poly(ε-caprolactone) by the ring opening polymerization of ε-caprolactone catalyzed with nontoxic magnesium lactate. The results indicated that Mg(Lac)2 is a catalyst with moderate activity for the polymerization of ε-caprolactone. Effects of the catalyst and reaction temperatures on the microstructure of the ε-caprolactone and L-lactide copolymers were investigated by means of 13C-NMR spectroscopy. An increase in the reaction temperature enhances the role of transesterification and the extent of randomness.
Copolymerizations of l-lactide (l-LA) and ɛ-caprolactone (ɛ-CL) were conducted in bulk using magnesium octoate as a catalyst. The reactivity ratios of l-LA and ɛ-CL were determined to be rL=23 and rC=0.22, respectively. The sequence analyses of the copolymers were performed on 13C NMR. An increase in the reaction temperature enhances the role of transesterification and the extent of randomness.
Summary Poly(ε-caprolactone)-poly(L-lactide) (PCL-PLLA) block copolymers were synthesized via melt
or solution sequential copolymerization of ε-caprolactone (ε-CL) and L-lactide (L-LA) using
nontoxic dibutylmagnesium as initiator. The formation of block structure was confirmed by 1H-,
13C NMR, GPC, and FT-IR, it can be concluded that the block copolymers PCL-PLLA
have been successfully synthesized by both melt and solution sequential copolymerization methods. Two
melting endothermic peaks (Tm) during heating and two crystallization exothermal
peaks (Tc) during cooling were observed in DSC curves. XRD patterns of the copolymers
were approximately the superposition of both the PCL and PLLA homopolymers. The results indicated the
coexistence of both PCL and PLLA crystalline microdomains, and the microphase separation took place
in the block copolymers.
Biodegradable copolymers of poly(lactic acid)-block-poly(ε-caprolactone) (PLA-b-PCL) were successfully prepared by two steps. In the first step, lactic acid monomer is oligomerized to low molecular weight prepolymer and copolymerized with the (ε-caprolactone) diol to prepolymer, and then the molecular weight is raised by joining prepolymer chains together using 1,6-hexamethylene diisocyanate (HDI) as the chain extender. The polymer was carefully characterized by using 1H-NMR analysis, gel permeation chromatography (GPC), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR). The results of 1H-NMR and TGA indicate PLA-b-PCL prepolymer with number average molecular weights (Mn) of 4000–6000 were obtained. When PCL-diols are 10 wt%, copolymer is better for chain extension reaction to obtain the polymer with high molecular weight. After chain extension, the weight average molecular weight can reach 250,000 g/mol, as determined by GPC, when the molar ratio of –NCO to –OH was 3:1. DSC curve showed that the degree of crystallization of PLA–PCL copolymer was low, even became amorphous after chain extended reaction. The product exhibits superior mechanical properties with elongation at break above 297% that is much higher than that of PLA chain extended products. Copyright © 2009 John Wiley & Sons, Ltd.
The alkaline earth metal complex calcium bis(hexamethyldisilazide), Ca(HMDS)2, has proven to be a useful reagent to carry out the regio- and stereoselective enolization reactions of ketones. The reactions give almost quantitative conversions to the corresponding silyl enol ethers at 0 °C using THF as solvent medium. Excellent kinetic selectivities (up to >99%) are found in the reactions of Ca(HMDS)2 with a series of unsymmetrical ketones, and this base system displays high Z-selectivity (up to 96%) for stereoselective enolization reactions. Six new enolate-containing crystal structures have also been elucidated. The amidocalcium enolates adopt monomeric, dimeric, tetranuclear, and charge-separated constitutions. In addition, the unexpected preparation of a hexanuclear complex composed of amide, enolate, and enolized aldolate units was discovered. NMR spectroscopic studies of the amidocalcium enolate systems reveal that a common dynamic equilibrium between multiple species is established in pyridine-d5. The solution species have been identified as amidocalcium enolates, bisenolates, bisamides, and charge-separated complexes. These studies demonstrate the structural diversity and complexity underlying these apparently straightforward calcium-mediated deprotonation reactions.
Triethanolamine (TEA) reacted with 3 equiv of yttrium alkyl complex 1 bearing O,N,N,O-tetradentate Salan ligand to give a trinuclear yttrium alkoxide counterpart 2 via metathesis reaction. Complex 2 initiated the ring-opening polymerization of rac-lactide in a livingness mode under mild conditions. Remarkably, 2 was so tolerant of the protic TEA that under various TEA-to-yttrium molar ratios ranging from 1:3 up to 81:1, the polymerization performed smoothly, suggesting an immortal characteristic. The resultant polylactides had variable molecular weights (Mn = 1.36 × 104to 32.54 × 104) but narrow molecular weight distributions (PDI = 1.02−1.09), and, especially, pure heterotactic (Pr = 0.99), hydroxyl-functionalized and star-shaped architecture, which were fully characterized by NMR spectroscopy and MALDI−TOF mass spectrum analyses and confirmed further by calculating the contraction factor value g′ (g′ = 0.96) via Mark−Houwink equations. Detailed investigation of the “immortal” polymerization gave a unique kinetics law of −d[LA]/dt = kp[LA][TEA]−0.352.
BACKGROUND: Poly(lactic acid) (PLA) has received much attention as a biodegradable polymer. But the physical properties of PLA, such as brittleness, limit its wider applicability. Copolymerization polymers ing soft blocks provides a useful method to improve the mechanical properties of PLA.
RESULTS: Poly(lactic acid)-block-(polycarbonate diol) (PLA-PCD) copolymers were synthesized using a two-step process with polycondensation and chain extension reactions. The effect of polycarbonate diol content on the prepolymer properties was studied. The chain-extended products were characterized using 1H NMR, F and. The effect of the NCO/OH ratio on the properties of the chain-extended products, including the mechanical properties and thermal properties, was studied. eight-average molecular weight of the product can reach 210 000 g mol−1 when the molar ratio of NCO to OH is 3:1.
CONCLUSION: The PLA-PCD copolymers obtained can crystallize, and the crystallinity decreases with chain-extension reaction. The products exhibit superior mechanical properties with elongation at break above 230%, which is much higher than that of PLA chain-extended products. The products have a good potential for packaging applications. Copyright
The reaction of [(thf)4Ca(PPh2)2] (1) with diisopropyl– and dicyclohexylcarbodiimides yields the phospha(III)guanidinates [(thf)2Ca{RNC(PPh2)NR}2] with R = isopropyl (2) and cyclohexyl (3). The metathesis reaction of K{RNC(PPh2)NR} with anhydrous CaI2 also allows the synthesis of these phospha(III)guanidinate complexes 2 and 3. For 2 a cis arrangement is observed whereas 3 crystallizes as trans isomer. The phospha(III)guanidinates act as bidentate chelate bases with an average Ca–N distance of 242.5 pm. The C–P bond length between the PPh2 fragment and the 1,3–diazaallyl unit is with values above 190 pm very large. The complexes 2 and 3 show a moderate catalytic activity in hydrophosphanylation reactions of dialkylcarbodiimides with diphenylphosphane.
Using an in situ-generated calcium-based initiating species derived from pentaerythritol, the bulk synthesis of well-defined 4-arm star poly(L-lactide) oligomers has been studied in detail. The substitution of the traditional initiator, stannous octoate with calcium hydride allowed the synthesis of oligomers that had both low PDIs and a comparable number of polymeric arms (3.7 – 3.9) to oligomers of similar molecular weight. Investigations into the degree of control observed during the course of the polymerization found that the insolubility of pentaerythritol in molten L-lactide resulted in an uncontrolled polymerization only when the feed mole ratio of L-lactide to pentaerythritol was 13. At feed ratios of 40 and greater, a pseudo-living polymerization was observed. As part of this study, in situ FT-Raman spectroscopy was demonstrated to be a suitable method to monitor the kinetics of the ring-opening polymerization (ROP) of lactide. The advantages of using this technique rather than FT-IR-ATR and 1H NMR for monitoring L-lactide consumption during polymerization are discussed.
Biodegradable block copolymers have been obtained as promising biomaterials because their hydrophilicity, mechanical and physical properties can be manipulated by combining different chemical structures or adjusting the ratio of the constituting blocks. This article reviews recent literature on the crystallization and morphology of biodegradable block copolymers with at least one crystallizable component. Emphasis has been placed on novel double crystalline diblock copolymers. These properties are important to define the final optical and mechanical performance as well as the rate of biodegradation and drug release kinetics. Additionally, block copolymers with biostable components such as polyethylene and poly(ethylene oxide) are also considered. The characterization of these systems by Differential Scanning Calorimetry, Transmission Electron Microscopy, Small-angle and Wide-angle X-ray Scattering and Polarized Light Optical Microscopy are considered in detail. The effects that each block has on the location of the thermal transitions and on the nucleation and crystallization kinetics of the other blocks are also discussed. The crystallization kinetics of each block can be dramatically affected by the presence of the other, and the magnitude of the effect is a function of the segregation strength. Complicated morphology formation and competition effects during the crystallization of two different crystalline blocks are also highlighted. Other many interesting effects have been found for either miscible or immiscible biodegradable block copolymers; amongst them, homogeneous nucleation, sequential or coincident crystallization, fractionated crystallization and fractionated melting can be mentioned. Also different superstructural morphologies such as double concentric spherulites with peculiar changes in their birefringence patterns have been reported for miscible or weakly segregated diblock copolymers as well as distinct nanoscale microdomains for strongly segregated systems.
Immiscible binary blends of poly(l,l-lactide) (PLLA), and poly(ε-caprolactone) (PCL), with 90/10, 80/20 and 70/30 wt% compositions, as well as ternary PLLA/PCL blends containing 0.5–5 wt% of a triblock PLLA/PCL/PLLA copolymer, were obtained by melt mixing using a twin screw extruder. Optical microscopy investigation of binary blends revealed the immiscibility of the components. The thermal behaviour of the blends was investigated by DSC and DMTA and compared with that of pure PLLA. The PLLA crystallization rate was enhanced in the presence of PCL domains. Morphological analysis of the cryofractured and etched–smoothed surfaces was carried out by SEM on both binary and ternary blends. A dimensional analysis of the PCL domains in binary and ternary blends was also performed in order to evaluate the influence of the presence of the triblock copolymer on the dispersion mode of PCL in the PLLA matrix.
Conditions of the living homopolymerization of ε-caprolactone (CL), lactides (LA), and of the homo-oligomerization of γ-butyrolactone (BL) are briefly described. Then block and random copolymerizations of CL with LA are shortly reviewed. The microstructure of the resulting copolyesters in relation to some peculiarities of these processes is discussed in more detail. It is also shown that the otherwise ‘non-polymerizable’ BL does form high molecular weight copolymers with CL, containing up to 50 mol% repeating units derived from BL. Their molecular weight is controlled by the concentrations of the consumed comonomers and the starting concentration of the initiator. NMR and DSC data indicate the random structure of copolymers. TGA traces of the BL/CL copolymers show that the presence of the γ-oxybutyryl repeating units randomly distributed within the poly(CL) chains improves the thermal stability of the latter.
Investigations are made of the 13C{1H} n.m.r. spectra of a series of poly(lactic acid) stereocopolymers obtained by different means, i.e. ring-opening polymerization of l-, rac- and meso-lactides initiated by powdered Zn, or copolymerization of l- and rac-lactides in different proportions, or condensation polymerization of rac-lactic acid. Resonances arising from the stereosensitivity of the carbonyl carbon atoms have been resolved using the resolution enhancement technique. The fine structures thus obtained are discussed in terms of configurational sequences. Comparison is made of the peak intensities with theoretical stereosequence distributions obtained when assuming single and pair additions of repeating units according to the Bernoullian statistics. Neither of the models for triad, tetrad and pentad stereosequences agrees with the area of the experimental lines. It is shown that redistribution of stereosequences in poly(lactic acid) occurs because of transesterification at the ester bonds in the melt. The final stereosequence distribution in the polymers obtained by ring-opening polymerization of lactides is discussed in relation to the pair-addition mechanism and to the redistribution due to transesterification reactions.
AB block copolymers of ε-caprolactone and (L)-lactide could be prepared by ring-opening polymerization in the melt at 110°C using stannous octoate as a catalyst and ethanol as an initiator provided ε-caprolactone was polymerized first. Ethanol initiated the polymerization of ε-caprolactone producing a polymer with ε-caprolactone derived hydroxyl end groups which after addition of L-lactide in the second step of the polymerization initiated the ring-opening copolymerization of L-lactide. The number-average molecular weights of the poly(ε-caprolactone) blocks varied from 1.5 to 5.2 × 103, while those of the poly(L-lactide) blocks ranged from 17.4 to 49.7 × 103. The polydispersities of the block copolymers varied from 1.16 to 1.27. The number-average molecular weights of the polymers were controlled by the monomer/hydroxyl group ratio, and were independent on the monomer/stannous octoate ratio within the range of experimental conditions studied. When L-lactide was polymerized first, followed by copolymerization of ε-caprolactone, random copolymers were obtained. The formation of random copolymers was attributed to the occurrence of transesterification reactions. These side reactions were caused by the ε-caprolactone derived hydroxyl end groups generated during the copolymerization of ε-caprolactone with pre-polymers of L-lactide. The polymerization proceeds through an ester alcoholysis reaction mechanism, in which the stannous octoate activated ester groups of the monomers react with hydroxyl groups.
This review focuses on the properties of lactic acid based polymers and the correlation to the structure of the polymers. Lactic acid based polymers prepared by polycondensation (PC), ring-opening polymerization (ROP), and other methods (chain extension, grafting) are discussed as well as modifications where structural changes have occurred due to post-polymerization reactions (peroxide melt-modification, radiation processing). The different types of polymers include copolymers prepared by ROP from l,l-lactide and d,d-lactide, glycolide (GA), ε-caprolactone (CL), trimethylene carbonate (TMC), 1,5-dioxepan-2-one (DXO), and other cyclic analogues. The thermophysical properties, the solubility, the miscibility, and the mechanical properties have been reviewed. In addition the hydrolytic stability, the thermal stability, the radiation degradation, and the biodegradation of the polymers have been covered.
A binary poly(L-lactide)/poly(ε-caprolactone) (PLLA/PCL) (70/30 w/w) blend and a ternary PLLA/PCL/PLLA-PCL-PLLA blend of the same composition which contains 4 wt.-% of a triblock PLLA-PCL-PLLA copolyester as compatibilizing agent were prepared by melt mixing at 200°C. Investigation of the thermal and mechanical properties of the blends and scanning electron microscopy of their fracture surfaces showed in the case of the ternary blend a better state of dispersion of PCL in the PLLA matrix and an improved toughness.
Poly (ε-caprolactone) (PCL) and poly (l-lactide) (PLA) were prepared by ring-opening polymerization catalyzed by organic amino calcium catalysts (Ca/PO and Ca/EO) which were prepared by reacting calcium ammoniate Ca(NH3)6 with propylene oxide and ethylene oxide, respectively. The catalysts exhibited high activity and the ring-opening polymerization behaved a quasi-living characteristic. Based on the FT-IR spectra and the calcium contents of the catalysts, and based on the 1H NMR end-group analysis of the low molecular weight PCL prepared using catalysts Ca/PO and Ca/EO, it was proposed that the catalysts have the structure of NH2–Ca–O–CH(CH3)2 and NH2–Ca–O–CH2CH3 for Ca/PO and Ca/EO, respectively. The ring-opening polymerization of CL and LA follows a coordination-insertion mechanism and the active site is the Ca–O bond.
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