Synthesis and characterization of a brush-like copolymer of polylactide grafted onto chitosan

A brush-like poly(DL)-lactide grafted onto chitosan as the backbone was investigated. The graft copolymerization was carried out with triethylaluminum as catalyst in toluene at 70 degrees C. It was found that a greater lactide content in the feeding ratio results in a higher grafting percentage. FTIR spectrometry, (1)H NMR, DSC scanning, and wide-angle X-ray scattering, respectively, are used to characterize these branch copolymers. A copolymer has a definite melting point when the molar feeding ratio of lactide to chitosan is more than 10:1, and the deltaH of the copolymers increases with the feed ratio of lactide to chitosan in feeding.     

Lipase-catalyzed copolymerization of omega-pentadecalactone with p-dioxanone and characterization of copolymer thermal and crystalline properties

Candida antarctica Lipase B (CALB), a metal-free enzyme, was successfully employed as catalyst for ring-opening copolymerization of omega-pentadecalactone (PDL) with p-dioxanone (DO) under mild reaction conditions (<80 degrees C, atmospheric pressure). Poly(PDL-co-DO) with high molecular weight (Mw > 30 000) and a wide range of comonomer contents was synthesized using various PDL/DO feed ratios. During the copolymerization reaction, large ring PDL was found to be more reactive than its smaller counterpart DO, resulting in higher PDL/DO unit ratios in polymer chains than the corresponding PDL/DO monomer ratios in the feed. The copolymers were typically isolated in 50-90 wt % yields as the monomer conversion was limited by the equilibrium between monomers and copolymer. 1H and 13C NMR analysis on poly(PDL-co-DO) formed by CALB showed that the copolymers contain nearly random sequences of PDL and DO units with a slight tendency toward alternating arrangements. Copolymerization with PDL was found to remarkably enhance PDO thermal stability. Differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS) results demonstrate high crystallinity in all copolymers over the whole range of compositions. Depending on copolymer composition, the crystal lattice of either PDO or PPDL hosts units of the other comonomer, a behavior typical of an isodimorphic system. In poly(PDL-co-DO), both melting temperature and melting enthalpy display a minimum at 70 mol % DO, that is, at the pseudoeutectic composition. WAXS diffractograms show one crystal phase (that of either PPDL or PDO) on either side of the pseudoeutectic and coexistence of PPDL and PDO crystals at the pseudoeutectic.     

Lipase-catalyzed copolymerization of dialkyl carbonate with 1,4-butanediol and ω-pentadecalactone: synthesis of poly(ω-pentadecalactone-co-butylene-co-carbonate)

Candida antarctica lipase B (CALB) was successfully used to promote synthesis of aliphatic poly(carbonate-co-ester) copolymers from dialkyl carbonate, diol, and lactone monomers. The polymerization reactions were carried out in two stages: first-stage oligomerization under low vacuum, followed by second-stage polymerization under high vacuum. Therefore, copolymerization of ω-pentadecalactone (PDL), diethyl carbonate (DEC), and 1,4-butanediol (BD) yielded PDL-DEC-BD copolymers with a M(w) of whole product (nonfractionated) up to 33?000 and M(w)/M(n) between 1.2 and 2.3. Desirable reaction temperature for the copolymerization was found to be ～80 °C. The copolymer compositions, in the range from 10 to 80 mol % PDL unit content versus total (PDL + carbonate) units, were effectively controlled by adjusting the monomer feed ratio. Reprecipitation in chloroform/methanol mixture allowed isolation of the purified copolymers in up to 92% yield. (1)H and (13)C NMR analyses, including statistical analysis on repeat unit sequence distribution, were used to determine the polymer microstructures. The synthesized PDL-DEC-BD copolymers possessed near random structures with all possible combinations of PDL, carbonate, and butylene units via either ester or carbonate linkages in the polymer chains and are more appropriately named as poly(PDL-co-butylene-co-carbonate).     

Poly(carbonate-ester)s of dihydroxyacetone and lactic acid as potential biomaterials

The synthesis of new polymeric biomaterials using biocompatible building blocks is important for the advancement of the biomedical field. We report the synthesis of statistically random poly(carbonate-ester)s derived from lactic acid and dihydroxyacetone by ring-opening polymerization. The monomer mole feed ratio and initiator concentration were adjusted to create various copolymer ratios and molecular weights. A dimethoxy acetal protecting group was used to stabilize the dihydroxyacetone and was removed using elemental iodine and acetone at reflux to produce the final poly(lactide-co-dihydroxyacetone) copolymers. The characteristics of the copolymers in their protected and deprotected forms were characterized by (1)H NMR, (13)C NMR, GPC, TGA, and DSC. Hydrolytic degradation of the deprotected copolymers was tracked over an 8-week time frame. The results show that faster degradation occurred with increased carbonate content in the copolymer backbone. The degradation pattern of the copolymers was visualized using SEM and revealed a trend toward surface erosion as the primary mode of degradation.     

Crystallization behavior and hydrophobic properties of biodegradable ethyl cellulose-g-poly(3-hydroxybutyrate-co-3-hydroxyvalerate): The influence of the side-chain length and grafting density

The copolymerization of grafting poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) onto ethyl cellulose (EC) was carried out through the homogeneous acylation reaction between EC as a backbone and telechelic OH-terminated PHBV oligomer as side chains in 1,2-dichloroethane by using 1,6-hexamethylene diisocyanate (HDI) as a coupling agent and dibutyltin dilaurate as catalyst. The resulting copolymers were studied by using NMR, FT-IR, WAXD, DSC, and contact angle measurements. It is found that with the increasing of the HDI/PHBV fraction, a transition exhibition occurred on crystallization behavior and hydrophobic properties, which could be modulated through controlling the lengths and grafting densities of PHBV side chains. Compared with those of neat PHBV, the degree of crystallinity for EC-g-PHBV_1.8 decreased from 58.1% to 39.1%, the maximum decomposition temperature increased from 259.6 to 266.3 °C, and the contact angle increased from 60.1° to 95.7°.     

Synthesis and characterization of biodegradable poly(ethylene glycol)-block-poly(5-benzyloxy-trimethylene carbonate) copolymers for drug delivery

Amphiphilic diblock copolymers with various block compositions were synthesized with monomethoxy-terminated poly(ethylene glycol) (MePEG) as the hydrophilic block and poly(5-benzyloxy-trimethylene carbonate) (PBTMC) as the hydrophobic block. When the copolymerization was conducted using MePEG as a macroinitiator and stannous 2-ethylhexanoate (Sn(Oct)2) as a catalyst, the molecular weight of the second block was uncontrollable, and the method only afforded a mixture of homopolymer and copolymer with a broad molecular weight distribution. By contrast, the use of the triethylaluminum-MePEG initiator yielded block copolymers with controllable molecular weight and a more narrow molecular weight distribution than the copolymers obtained using Sn(Oct)2. GPC and 1H NMR studies confirmed that the macroinitiator was consumed and the copolymer composition was as predicted. Two of the newly synthesized MePEG-b-PBTMC copolymers were evaluated in terms of properties primarily relating to their use in micellar drug delivery. MePEG-b-PBTMC micelles with a narrow monomodal size distribution were prepared using a high-pressure extrusion technique. The MePEG-b-PBTMC copolymers were also confirmed to be biodegradable and noncytotoxic.     

Preparation, characterization, and properties of fullerene-vinylpyrrolidone copolymers

Water-soluble fullerene (C(60))-N-vinylpyrrolidone copolymers were prepared by the radical polymerization method. The structures of the copolymers were characterized by Fourier transform infrared, UV-Vis, (1)H NMR, (13)C NMR, gel permeation chromatography, thermogravimetric analyses, and scanning electron microscopy (SEM). The results presented show that C(60) and vinylpyrrolidone (VP) can be copolymerized under different conditions. With a constant benzoyl peroxide amount, C(60) contents in the copolymers increase with increasing initial C(60):VP reactant ratio. The assembly behavior of water-soluble C(60)-N-vinylpyrrolidone copolymers was investigated by SEM. The results show that the copolymers create morphology that is sphere-like. Fullerene-containing micro-/nano-sized copolymer fibers were prepared, for the first time, by electrospinning. The cytotoxicity to cancer cell lines of the copolymers was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide and confocal laser scanning microscope. The results show that copolymers exhibit better cytotoxicity against HeLa cells and mouse osteogenic sarcoma cells (cytotoxicity of copolymers is better than that of fullerene complex). The mechanism of fullerene-VP copolymerization was investigated for the first time.     

Novel biodegradable aliphatic poly(butylene succinate-co-cyclic carbonate)s with functional carbonate building blocks. 1. Chemical synthesis and their structural and physical characterization

This study presents chemical synthesis, structural, and physical characterization of novel biodegradable aliphatic poly(butylene succinate-co-cyclic carbonate)s P(BS-co-CC) bearing functional carbonate building blocks. First, five kinds of six-membered cyclic carbonate monomers, namely, trimethylene carbonate (TMC), 1-methyl-1,3-trimethylene carbonate (MTMC), 2,2-dimethyl-1,3-trimethylene carbonate (DMTMC), 5-benzyloxytrimethylene carbonate (BTMC), and 5-ethyl-5-benzyloxymethyl trimethylene carbonate (EBTMC), were well prepared from ethyl chloroformate and corresponding diols at 0 degrees C in THF solution with our modified synthetic strategies. Then, a series of new P(BS-co-CC)s were synthesized at 210 degrees C through a simple combination of poly-condensation and ring-opening-polymerization (ROP) of hydroxyl capped PBS macromers and the prepared carbonate monomers, and titanium tetra-isopropoxide Ti(i-OPr)4 was used as a more suitable catalyst of 5 candidate catalysts which could concurrently catalyze poly-condensation and ROP. By means of NMR, GPC, FTIR, and thermal analytical instruments, macromolecular structures and physical properties have been characterized for these aliphatic poly(ester carbonate)s. The experimental results indicated that novel biodegradable P(BS-co-CC)s were successfully synthesized with number average molecular weight Mn ranging from 24.3 to 99.6 KDa and various CC molar contents without any detectable decarboxylation and that the more bulky side group was attached to a cyclic carbonate monomer, the lower reactivity for its copolymerization would be observed. The occurrences of 13C NMR signal splitting of succinyl carbonyl attributed to the BS building blocks could be proposed due to the randomized sequences of BS and CC building blocks. FTIR characterization indicated two distinct absorption bands at 1716 and 1733 approximately 1735 cm(-1), respectively, stemming from carbonyl stretching modes for corresponding BS and CC units. With regard to their thermal properties, it is seen that the synthesized P(BS-co-CC)s exhibited thermal degradation temperatures 10 approximately 20 degrees C higher than that of PBS. On the basis of the synthesized P(BS-co-BTMC)s, new aliphatic poly(butylene succinate-co-5-hydroxy trimethylene carbonate)s were further synthesized, bearing hydrophilic hydroxyl pendant functional groups through an optimized Pd/C catalyzed hydrogenation. These semi-crystalline new biodegradable aliphatic copolymers with tunable physical properties and functional carbonate building blocks might be expected as potential new biomaterials.     

New hydrolysis-dependent thermosensitive polymer for an injectable degradable system

Novel, bioerodible, thermosensitive poly(NIPAAm-co-dimethyl-gamma-butyrolactone acrylate), with a hydrolysis-dependent thermosensitivity, was synthesized by radical polymerization with a varying dimethyl-gamma-butyrolactone acrylate (DBA) content, and the properties of the copolymers were characterized using differential scanning calorimetry, gel permeation chromatography in conjunction with static light scattering, Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), and acid titration. The lower critical solution temperature of the copolymers decreases with increasing DBA content, but then increases after ring-opening hydrolysis of the DBA side group. FTIR and NMR spectra showed the copolymerization of these two monomers and the hydrolysis-dependent ring-opening of the DBA side group. It was also found that there are no low-molecular-weight byproducts but rather dissolution of the polymer chains at 37 degrees C during the time frame of application. Models of the kinetics suggest that the hydrolysis reaction is self-catalytic due to an increase in hydrophilicity and charge, and thus accessible water concentration, caused by ring-opening of the DBA.     

Preparation, characterization, and in vitro drug release behavior of biodegradable chitosan-graft-poly(1, 4-dioxan-2-one) copolymer

A novel copolymer of chitosan-g-poly(p-dioxanone) (CGP) was synthesized in bulk by ring-opening polymerization of p-dioxanone (PDO) initiated by the hydroxyl group or amino group of chitosan using SnOct₂ as catalyst. The chemical structure was determined by ¹H NMR. It was found that the feed ratio of chitosan to PDO had a great effect on the degree of polymerization (DP) and the substitution (DS) of PDO. The thermal stability and crystallization behavior of graft copolymer CGP were closely related to the values of DP and DS. When the resulting copolymer was used as Ibuprofen carrier, the release rate of Ibuprofen decreased compared with that of pure chitosan carrier. The drug release behavior was also influenced by the structure of graft copolymers.     

Synthesis and characterization of novel biodegradable poly(carbonate ester)s with photolabile protecting groups

Novel biodegradable poly(carbonate ester)s with photolabile protecting groups were synthesized by ring-opening copolymerization of L-lactide (LA) with 5-methyl-5-(2-nitro-benzoxycarbonyl)-1,3-dioxan-2-one (MNC) with diethyl zinc (Et2Zn) as catalyst. The poly(L-lactide-co-5-methyl-5-carboxyl-1,3-dioxan-2-one) (P(LA-co-MCC)) was obtained by UV irradiation of poly(L-lactide acid-co-5-methyl-5-(2-nitro-benzoxycarbonyl)-1,3-dioxan-2-one) (P(LA-co-MNC)) to remove the protective 2-nitrobenzyl group. The free carboxyl groups on the copolymers P(LA-co-MCC) were reacted with paclitaxel, a common antitumor drug. Gel permeation chromatography and NMR studies confirmed the copolymer structures and successful attachment of paclitaxel to the copolymer.     

Novel rubbers from cationic copolymerization of soybean oils and dicyclopentadiene. 1. Synthesis and characterization

Novel thermosetting copolymers, ranging from tough and ductile to very soft rubbers, have been prepared by the cationic copolymerization of regular (SOY) and 100% conjugated soybean oils (C(100)SOY) with dicyclopentadiene (DCP) catalyzed by Norway fish oil (NFO)-modified and SOY- and C(100)SOY-diluted boron trifluoride diethyl etherate (BFE). The gelation time of the reactions varies from 4 to 991 min at 110 degrees C. The yields of the bulk copolymers are essentially quantitative, while the yields of the cross-linked copolymers remaining after Soxhlet extraction with methylene chloride range from 69% to 88%, depending on the monomer stoichiometry and the catalyst used. (1)H NMR spectroscopy and Soxhlet extraction data indicate that these copolymers consist of a cross-linked soybean oil-DCP network plasticized by certain amounts of methylene chloride-soluble linear or less cross-linked soybean oil-DCP copolymers, unreacted oil, and some low molecular weight hydrolyzed oil. The molecular weights of these soluble fractions are in the range from 400 to 10,000 g/mol based on polystyrene standards. The bulk copolymers have glass transition temperatures ranging from -22.6 to 56.6 degrees C, while their tan delta peak values range from 0.7 to 1.2. Thermogravimetric analysis (TGA) indicates that these soybean oil-DCP copolymers are thermally stable below 200 degrees C, with 10% and 50% weight loss temperatures ranging from 280 to 372 degrees C and 470-554 degrees C, respectively. These properties suggest that these biobased thermosets may prove useful alternatives to current petroleum-based plastics and find widespread utility.     

Porous scaffolds from high molecular weight polyesters synthesized via enzyme-catalyzed ring-opening polymerization

Several aliphatic polyesters have been synthesized until now using enzyme-catalyzed ring-opening polymerization (ROP) of different lactones, although their molecular weight, hence mechanical strength, was not sufficient enough to fabricate porous scaffolds from them. To achieve this target, 1,5-dioxepan-2-one (DXO) and epsilon-caprolactone (CL) were polymerized in bulk with Lipase CA as catalyst at 60 degrees C, and porous scaffolds were prepared from the polymers obtained thereof using a salt leaching technique. The CL/DXO molar feed ratio was varied from 1.5 to 10, and the reactivity ratios of CL and DXO were determined using the Kelen-Tudos method under such conditions of polymerization. NMR results showed a slightly lower CL/DXO molar ratio in the copolymers than in the feed due to high reactivity of DXO toward Lipase CA catalysis. The crystallinity of the PCL segment of the copolymers was affected by the presence of soft and amorphous DXO domains. The copolymers having high CL content were thermally more stable. The porosity of the scaffolds was in the range 82-88%, and the SEM analysis showed interconnected pores in the scaffolds. Of the two parameters which could affect the mechanical properties, viz., the copolymer composition and the scaffold pore size, the pore size showed a significant effect on the mechanical properties of the scaffolds. The porous scaffolds developed in this way for tissue engineering are free from toxic organometallic catalyst residues, and they are highly suitable for biomedical applications.     

The potential effects of antioxidant feed additives in mitigating the adverse effects of corn naturally contaminated with <Emphasis Type="Italic">Fusarium</Emphasis> mycotoxins on antioxidant systems in the intestinal mucosa,plasma, and liver in weaned pigs

Seventy-two piglets (6.0 kg BW) were randomly distributed within six different dietary treatments to evaluate the effect of deoxynivalenol (DON) and the potential of four antioxidant feed additives in mitigating the adverse effects of DON on growth performances and oxidative status. Dietary treatments were as follows: control diet 0.8 mg/kg DON; contaminated diet (DON-contaminated diet) 3.1 mg/kg DON; and four contaminated diets, each supplemented with a different antioxidant feed additive, DON + vitamins, DON + organic selenium (Se)/glutathione (GSH), DON + quercetin, and DON + COMB (vitamins + Se/GSH + quercetin from the other treatments). Although DON was the main mycotoxin in the contaminated diet, this diet also contained 1.8 mg/kg of zearalenone (ZEN). The “mycotoxin” effects therefore included the combined effect of these two mycotoxins, DON, and ZEN. The DON-ZEN ingestion did not affect growth performances, average daily gain (ADG), average daily feed intake (ADFI), and feed efficiency (G:F ratio), but partially induced oxidative stress in weaned pigs as shown by increased malondialdehyde (MDA) content in the plasma and superoxide dismutase (SOD) activity in liver (P?<?0.05). However, no change in the activity of other antioxidant enzymes or GSH concentrations was observed in plasma and liver of piglets fed the DON-contaminated diet (P?>?0.05). Supplementation with individual antioxidant feed additive had a limited effect in weaned pigs fed DON-ZEN-contaminated diets. Combination of antioxidants (vitamins A, C, and E, quercetin, and organic Se/GSH) reduced plasma and liver MDA content and SOD activity in liver (P?<?0.05) of piglets fed DON-ZEN-contaminated diets. Furthermore, this combination also reduced MDA content in the ileum (P?<?0.05), although activity of glutathione peroxidases (GPx), SOD or catalase (CAT) in the ileum was not affected by DON-ZEN contamination or antioxidant supplements. In conclusion, DON-ZEN contamination induced oxidative stress in weaned pigs and combination of antioxidant feed additives restored partially the oxidative status. Further studies will be necessary to assess whether the effects of antioxidant feed additives on oxidative status are specific when feed is contaminated with DON-ZEN.     

Protease-catalyzed regioselective polymerization and copolymerization of glutamic acid diethyl ester

Protease-catalyzed polymerization and copolymerization of L-glutamic acid diethyl ester hydrochloride (1) have been performed in a buffer of high concentration. Papain and bromelain showed high catalytic activity toward the polymerization. H-H COSY NMR analysis of the product showed the exclusive formation of poly(alpha-peptide), which was further confirmed by comparison with NMR spectra of poly(alpha-methyl gamma-L-glutamate). The papain-catalyzed polymerization of gamma-methyl L-glutamate did not occur under the similar reaction conditions, supporting the regioselective production of the polymer having an alpha-peptide linkage from 1. The effects of the reaction parameters have been systematically investigated. The copolymerization of 1 with various amino acid esters took place by the papain catalyst to give peptide copolymers.     

Liquid,phenylazide-end-capped copolymers of epsilon-caprolactone and trimethylene carbonate: preparation,photocuring characteristics,and surface layering

Photoreactive phenylazide-end-capped liquid copolymers were prepared by ring-opening copolymerization of epsilon-caprolactone (CL) and trimethylene carbonate (TMC) at an equimolar monomer feed ratio in the presence of a polyol, namely, a low-molecular-weight alcohol (di-, tri-, and tetraol) or poly(ethylene glycol) (PEG) as an initiator and tin(II) 2-ethylhexanoate as a catalyst, followed subsequently by phenylazide derivatization at their hydroxyl terminus. These tri- and tetrabranched liquid copolymers (precursors) with a molecular weight from approximately 2500 to 7000 g/mol were cross-linked to yield insoluble solids by ultraviolet (UV) light irradiation. The photocuring rate increased with increasing functionality of phenylazide and UV intensity and decreasing thickness of the liquid film of precursors. The photo-cross-linkability of phenylazide-derivatized liquid copolymers was found to be higher than that of the corresponding coumarin-derivatized liquid copolymers. Poly(lactide) (PLA) films surface-layered with photocured copolymers were prepared by coating surfaces with phenylazide-derivatized copolymers and their subsequent photoirradiation. Endothelial cells adhered well on the nontreated PLA and low-molecular-weight alcohol-based copolymer-layered and photocured films. Little cell adhesion was observed on the hydrolytically surface-eroded PLA film and the PEG-based copolymer-layered film. When a phenylazide-derivatized hexapeptide with the cell-adhesion tripeptidyl sequence, Arg-Gly-Asp (RGD), common to cell adhesive proteins, was photoimmobilized on these surfaces, the surfaces became cell adhesive. Microarchitectured surfaces, which were prepared by sequential procedures of surface coating and photocuring using a photomask with lattice windows, produced regionally differentiated cell adhesiveness.     

Functionalization of cellulosic fibers by graft copolymerization of acrylonitrile and ethyl acrylate from their binary mixtures

Functionalization of Agave fibers was carried out by graft copolymerization of acrylonitrile (AN) and ethyl acrylate (EA) from their binary solutions in presence of Ce (IV) ions at a temperature of 45 ± 0.1 °C. An increase in the graft copolymerization was obtained with the increase in the feed molarity of the comonomers up to certain extent. Contrary to lesser affinity of acrylonitrile to grafting on Agave fibers, a synergistic effect of ethyl acrylate on acrylonitrile was observed when graft copolymers were prepared using different feed compositions (f_AN). The graft copolymers were characterized by various techniques such as FT-IR, TGA/DTA, X-RD and SEM analysis. Further swelling behavior of grafted fibers in different solvents, moisture absorption behavior and resistance to chemicals was investigated as a function of percent grafting to define their end uses in different environments.     

Synthesis and characterization of branched polymers from lipase-catalyzed trimethylolpropane copolymerizations

Lipase-catalyzed terpolymerizations were performed with the monomers trimethylolpropane (B3), 1,8-octanediol (B2), and adipic acid (A2). Polymerizations were performed in bulk, at 70 degrees C, for 42 h, using immobilized lipase B from Candida antartica (Novozyme-435) as a catalyst. To determine the substitution pattern of trimethylolpropane (TMP) in copolymers, model compounds with variable degrees of acetylation were synthesized. Inverse-gated 13C NMR spectra were recorded to first determine the chemical shift positions for mono-, di-, and trisubstituted TMP units and, subsequently, to determine substitution of TMP units along chains. Variation of TMP in the monomer feed gave copolymers with degrees of branching (DB) from 20% to 67%. In one example, a hyperbranched copolyester with 53 mol % TMP adipate units was formed in 80% yield, with Mw 14 100 (relative to polystyrene standards), Mw/Mn 5.3, and DB 36%. Thermal and crystalline properties of the copolyesters were studied by thermogravimetric analysis and differential scanning calorimetry.     

Structural characterization of a lipase-catalyzed copolymerization of epsilon-caprolactone and D,L-lactide

The copolymerization of epsilon-caprolactone (epsilon-CL) and d,l-lactide catalyzed by Candida antarctica lipase B was studied. Copolymerizations with different epsilon-CL-to-lactide ratios were carried out, and the product was monitored and characterized by MALDI-TOF MS, GPC, and (1)H NMR. The polymerization of epsilon-CL, which is normally promoted by C. antarctica lipase B, is initially slowed by the presence of lactide. During this stage, lactide is consumed more rapidly than epsilon-CL, and the incorporation occurs dimer-wise with regard to the lactic acid (LA) units. As the reaction proceeds, the relative amount of CL units in the copolymer increases. The nonrandom copolymer structure disappears with time, probably due to a lipase-catalyzed transesterification reaction. In the copolymerizations with a low content of lactide, macrocycles of poly(epsilon-caprolactone) and copolymers having up to two LA units in the ring were detected.     

Synthesis and characterization of random and block copolypeptides derived from gamma-methylglutamate and leucine N-carboxyanhydrides

The synthesis of random and block copolypolyeptides derived from gamma-methylglutamate and leucine N-carboxyanhydrides using Al-Schiff's base complexes and allylamine as initiators is here reported. The copolymer structures were confirmed by (1)H and (13)C NMR. The calculation of the statistical average block lengths reveals the presence of longer methylglutamate units in the copolymer. The determination of the reactivity ratios indicated a slightly higher reactivity of gamma-methylglutamateNCA as compared to leucineNCA. Block copolypeptides containing glutamate and leucine units were obtained by sequential polymerization of the two NCAs using Al-Schiff's base complexes or allylamine in dioxane as solvent. Based on (13)C NMR spectra of copolymers exhibiting two signals corresponding to peptide linkages, we confirmed the block structure and concluded that the copolymerization proceeds by attack of an amino group present on a glutamate chain end onto a LeuNCA. The copolymerization with allylamine was also shown, from calculation of the average block lengths of sequences, to exhibit living behavior. Viscometry analysis further showed that molar masses of the copolypeptides obtained with Al-Schiff's base were quite close to those derived from allylamine, supporting the proposed mechanism of copolymerization.