Grafting of cellulose fibers with poly(epsilon-caprolactone) and poly(L-lactic acid) via ring-opening polymerization

In this study, ring-opening polymerization (ROP) of epsilon-caprolactone (epsilon-CL) and L-lactide (L-LA) has been performed from cellulose fibers. The hydroxyl groups on cellulose act as initiators in the polymerization, and the polymers are covalently bonded to the cellulose fiber. As an attempt to introduce more available hydroxyl groups on the surface, and thereby obtain higher grafting efficiency in the ROP of epsilon-CL and L-LA, unmodified paper was modified with xyloglucan-bis(methylol)-2-methylpropanamide (XG-bis-MPA) and 2,2-bis(methylol)propionic acid (bis-MPA), respectively. The grafted substrates were characterized via Fourier transform infrared spectroscopy (FTIR), contact angle measurement, atomic force microscopy, and enzymatic degradation. The results showed a successful grafting of poly(epsilon-caprolactone) (PCL) and poly(L-lactic acid) (PLLA) from the cellulose fiber surfaces. Furthermore, the results showed an improved grafting efficiency after activation of the cellulose surface with bis-MPA, and showed that the amount of grafted polymer could be controlled by the ratio of added free initiator to monomer.     

Lipase-catalyzed ring-opening polymerization of lactones to polyesters and its mechanistic aspects.

Lipase catalysis induced a ring-opening polymerization of lactones with different ring-sizes. Small-size (four-membered) and medium-size lactones (six- and seven-membered) as well as macrolides (12-, 13-, 16-, and 17-membered) were subjected to lipase-catalyzed polymerization. The polymerization behaviors depended primarily on the lipase origin and the monomer structure. The macrolides showing much lower anionic polymerizability were enzymatically polymerized faster than epsilon-caprolactone. The granular immobilized lipase derived from Candida antartica showed extremely efficient catalysis in the polymerization of epsilon-caprolactone. Single-step terminal functionalization of the polyester was achieved by initiator and terminator methods. The enzymatic polymerizability of lactones was quantitatively evaluated by Michaelis-Menten kinetics.     

Enzymatic synthesis of poly(hydroxyalkanoates) in ionic liquids

Ring-opening polymerization of five lactones catalyzed by Candida antarctica lipase B in ionic liquids yielded poly(hydroxyalkanoates) of moderate molecular weights up to Mn=13,000. In the ionic liquid 1-butyl-3-methylimidazolium bis(trifluoromethane)-sulfonimide and with a low weight ratio of enzyme to lactone (1:100) we obtained polymers from beta-propiolactone, delta-valerolactone, and epsilon-caprolactone with degrees of polymerization as high as 170, 25, and 85, respectively; oligomers from beta-butyrolactone and gamma-butyrolactone with degrees of polymerization of 5; and a copolymer of beta-propiolactone and beta-butyrolactone with a degree of polymerization of 180. Water-immiscible ionic liquids were superior to water-miscible ionic liquids. Reducing the water content of the enzyme improved the degree of polymerization by as much as 50% for beta-propiolactone and epsilon-caprolactone.     

Surface modification of natural fibers using bacteria: depositing bacterial cellulose onto natural fibers to create hierarchical fiber reinforced nanocomposites

Triggered biodegradable composites made entirely from renewable resources are urgently sought after to improve material recyclability or be able to divert materials from waste streams. Many biobased polymers and natural fibers usually display poor interfacial adhesion when combined in a composite material. Here we propose a way to modify the surfaces of natural fibers by utilizing bacteria ( Acetobacter xylinum) to deposit nanosized bacterial cellulose around natural fibers, which enhances their adhesion to renewable polymers. This paper describes the process of modifying large quantities of natural fibers with bacterial cellulose through their use as substrates for bacteria during fermentation. The modified fibers were characterized by scanning electron microscopy, single fiber tensile tests, X-ray photoelectron spectroscopy, and inverse gas chromatography to determine their surface and mechanical properties. The practical adhesion between the modified fibers and the renewable polymers cellulose acetate butyrate and poly(L-lactic acid) was quantified using the single fiber pullout test.     

Novel amphiphilic poly(epsilon-caprolactone)-g-poly(L-lysine) degradable copolymers

As part of the search of novel degradable polymers, amphiphilic and cationic poly(epsilon-caprolactone)-g-poly(l-lysine) (PCL-g-PlL) copolymers have been synthesized following a grafting "onto" or a grafting "from" method both applied to a macropolycarbanionic PCL derivative. The first approach led to PCL-g-PZlL containing 36% of epsilon-caprolactone and 64% of N-epsilon-Z-l-lysine units, by reaction of activated poly(N-epsilon-Z-l-lysine) on the macropolycarbanion derived from PCL. The second route was based on the anionic ring opening polymerization of N-carboxyanhydride of N-epsilon-benzyloxycarbonyl-l-lysine initiated by the macropolycarbanion derived from PCL and led to a similar copolymer containing 45% of of epsilon-caprolactone and 55% of N-epsilon-Z-l-lysine units. After deprotection of the lysine units, PCL-g-PlL copolymers were obtained. These copolymers are water-soluble and form nanometric micelle-like objects with mean diameters between 60 and 500 nm in distilled water depending on the synthesis route.     

Intelligent dual-responsive cellulose surfaces via surface-initiated ATRP

Novel thermo-responsive cellulose (filter paper) surfaces of N-isopropylacrylamide (NIPAAm) and pH-responsive cellulose surfaces of 4-vinylpyridine (4VP) have been achieved via surface-initiated ATRP. Dual-responsive (pH and temperature) cellulose surfaces were also obtained through the synthesis of block-copolymer brushes of PNIPAAm and P4VP. With changes in pH and temperature, these "intelligent" surfaces showed a reversible response to both individual triggers, as indicated by the changes in wettability from highly hydrophilic to highly hydrophobic observed by water contact angle measurements. Adjusting the composition of the grafted block-copolymer brushes allowed for further tuning of the wettability of these "intelligent" cellulose surfaces.     

Modeling of lipase catalyzed ring-opening polymerization of epsilon-caprolactone

Enzymatic ring-opening polymerization of epsilon-caprolactone by various lipases was investigated in toluene at various temperatures. The determination of molecular weight and structural identification was carried out with gel permeation chromatography and proton NMR, respectively. Among the various lipases employed, an immobilized lipase from Candida antartica B (Novozym 435) showed the highest catalytic activity. The polymerization of epsilon-caprolactone by Novozym 435 showed an optimal temperature of 65 degrees C and an optimum toluene content of 50/50 v/v of toluene and epsilon-caprolactone. As lipases can degrade polyesters, a maximum in the molecular weight with time was obtained due to the competition of ring opening polymerization and degradation by specific chain end scission. The optimum temperature, toluene content, and the variation of molecular weight with time are consistent with earlier observations. A comprehensive model based on continuous distribution kinetics was developed to model these phenomena. The model accounts for simultaneous polymerization, degradation and enzyme deactivation and provides a technique to determine the rate coefficients for these processes. The dependence of these rate coefficients with temperature and monomer concentration is also discussed.     

Recent developments in lipase-catalyzed synthesis of polymeric materials

In the past three years, enzymatic polymerization has dramatically developed and provided many successful examples in the construction of functional polymeric materials. In this review, the lipase-catalyzed synthesis of polymeric materials is systematically summarized, focusing on the synthesis of complex and well-defined polyesters. Exploration of novel biocatalysts and reaction media is described, with particular emphasis on the enzymes obtained via immobilization or protein engineering strategies, green solvents and reactors. Enzymatic polyester synthesis is then discussed with regard to the different reaction types, including ring-opening polymerization, polycondensation, combination of ring-opening polymerization with polycondensation, and chemoenzymatic polymerization. Using enzymatic polymerization, many polymeric materials with tailor-made structures and properties have been successfully designed and synthesized. Finally, recent developments in catalytic kinetics and mechanistic studies through the use of spectroscopy, mathematics and computer techniques are introduced. Overall, the review demonstrates that lipase-catalyzed synthesis of polymeric materials could be a promising platform for green polymer chemistry, and will be potential to produce biodegradable and biocompatible polymers.     

Syntheses, characterization, and in vitro degradation of ethyl cellulose-graft-poly(epsilon-caprolactone)-block-poly(L-lactide) copolymers by sequential ring-opening polymerization

Well-defined ethyl cellulose-graft-poly(epsilon-caprolactone) (EC-g-PCL) graft copolymers were successfully synthesized via ring-opening polymerization (ROP) of epsilon-caprolactone (CL) with an ethyl cellulose (EC) initiator and a tin 2-ethylhexanoate (Sn(Oct)2) catalyst in xylene at 120 degrees C. Then, novel ethyl cellulose-graft-poly(epsilon-caprolactone)-block-poly(L-lactide) (EC-g-PCL-b-PLLA) graft-block copolymers were prepared by ROP of L-lactide (L-LA) with a hydroxyl-terminated EC-g-PCL macroinitiator and Sn(Oct)2 catalyst in bulk at 120 degrees C. Various graft and block lengths of EC-g-PCL and EC-g-PCL-b-PLLA copolymers were obtained by adjusting the molar ratios of CL monomer to EC and the L-LA monomer to CL. The thermal properties and crystalline morphologies of EC-g-PCL and EC-g-PCL-b-PLLA copolymers were different from those of linear PCL. The in vitro degradation rate of EC-g-PCL-b-PLLA was faster than those of linear PCL and EC-g-PCL due to the presence of PLLA blocks.     

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.     

Enzymatic degradation of block copolymers prepared from epsilon-caprolactone and poly(ethylene glycol)

Block copolymers were prepared by ring-opening polymerization of epsilon-caprolactone in the presence of monohydroxyl or dihydroxyl poly(ethylene glycol) (PEG), using Zn powder as catalyst. The resulting poly(epsilon-caprolactone) (PCL)-PEG diblock and PCL-PEG-PCL triblock copolymers were characterized by various analytical techniques such as NMR, size-exclusion chromatography, differential scanning calorimetry, and X-ray diffraction. Both copolymers were semicrystalline polymers, the crystalline structure being of the PCL type. Films were prepared by casting dichloromethane solutions of the polymers on a glass plate. Square samples with dimensions of 10 x 10 mm were allowed to degrade in a pH = 7.0 phosphate buffer solution containing Pseudomonas lipase. Data showed that the introduction of PEG blocks did not decrease the degradation rate of poly(epsilon-caprolactone).     

酶促聚合：绿色的高分子材料合成技术

以酶促聚合为代表的绿色高分子合成途径，以其反应条件温和、产物多分散性低、无金属催化剂残留、高度立体和区位选择性等优势，成为医用高分子材料合成领域中的研究热点。目前，氧化还原酶、水解酶、转移酶均成功应用于聚合反应，其中脂肪酶催化的缩聚反应及开环聚合反应研究最为广泛，同时，以可逆加成-断裂链转移聚合和原子转移自由基聚合为代表的酶促可逆失活自由基聚合得到了快速发展。针对酶促聚合中单体及合成产物结构与性能单一、应用范围有限等缺陷，基于酶促聚合与原子转移自由基聚合、开环易位聚合等反应的偶联，制备了多种不同结构与性能的聚合物材料，推动了上述材料在药物与基因递送领域中的应用。本文综述了脂肪酶催化聚合、酶促可逆失活自由基聚合、酶促化学偶联催化等方面的研究进展，并探讨了目前研究的局限性和未来研究方向。     

Amphiphilic core-shell nanocarriers based on hyperbranched poly(ester amide)-star-PCL: synthesis, characterization, and potential as efficient phase transfer agent

Amphiphilic biodegradable star-shaped polymer was conveniently prepared by the Sn(Oct)2-catalyzed ring opening polymerization of epsilon-caprolactone (CL) with hyperbranched poly(ester amide) (PEA) as a macroinitiator. Various monomer/initiator ratios were employed to vary the length of the PCL arms. (1)H NMR and FTIR characterizations showed the successful synthesis of star polymer with high initiation efficiency. SEC analysis using triple detectors, RI, light scattering, and viscosity confirmed the controlled manner of polymerization and the star architecture. Because of the hydrophilic PEA core and hydrophobic PCL shell, the obtained star polymers displayed inverted unimolecular micellar structure confirmed by dynamic light scattering. Three water soluble dyes, congo red, methyl orange, and bromophenol blue, were used to investigate the host-guest behavior of the micelles. It proved that the core-shell unimolecular reverse micelles were able to transport polar dyes from water to the organic phase with a high efficiency of up to 22.6 dyes per polymer, indicating a great potential of the micelles as drug carriers. The influence of arm length and core size on the load efficiency of the nanocarrier was also evaluated.     

Synthesis, characterization, and degradation behavior of amphiphilic poly-alpha,beta-[N-(2-hydroxyethyl)-L-aspartamide]-g-poly(epsilon-caprolactone)

A series of biodegradable amphiphilic graft polymers were successfully synthesized by grafting poly(epsilon-caprolactone) (PCL) sequences onto a water-soluble poly-alpha,beta-[N-(2-hydroxyethyl)-L-aspartamide] (PHEA) backbone. The graft copolymers were prepared through the ring-opening polymerization of epsilon-caprolactone (CL) initiated by the macroinitiator PHEA with pendant hydroxyl groups without adding any catalyst. By controlling the feed ratio of the macroinitiator to the monomer, the copolymers with different branch lengths and properties can be obtained. The successful grafting of PCL sequences onto the PHEA backbone was verified by FTIR, 1H NMR, and combined size-exclusion chromatography and multiangle laser light scattering (SEC-MALLS) analysis. The hydrolytic degradation and enzymatic degradation of these graft copolymers were investigated. The results show the hydrolytic degradation rate increases with increasing content of hydrophilic PHEA backbone. While the enzymatic degradation rate is affected by two competitive factors, the catalytic effect of Pseudomonas cepacia lipase on the degradation of PCL branches and the hydrophilicity which depends on the copolymer composition. In situ observation of the degradation under polarizing light microscope (PLM) demonstrates the different degradation rates of different regions in the polymer samples.     

Industrial utilization of tin-initiated resorbable polymers: synthesis on a large scale with a low amount of initiator residue

This article presents the successful large-batch synthesis of a resorbable polymer with a minimal amount of residual tin. Ring-opening polymerization of epsilon-caprolactone was performed in toluene, with a tin (IV) alkoxide as the initiator. A number of parameters were varied in order to study the polymerization with respect to the purity of solvent, batch size, and the residual amount of tin in the polymers. The synthesis of epsilon-caprolactone in undistilled toluene with 1-di-n-butyl-1-stanna-2,5-dioxacyclopentane as the initiator was successfully performed in batches of 5, 20, and 50 g with no differences in the final conversion, molecular weight, or molecular-weight distribution. The residual amount of tin was significantly reduced from over 1000 to 23 ppm. This study examines the industrial utility of the materials regarding the size and purity of the synthesis.     

Evaluation of biocompatibility and mechanical behavior of polyurethane elastomers based on chitin/1,4-butane diol blends

Chitin based polyurethane elastomers with potential as biomedical implants with tunable mechanical properties were synthesized by step growth polymerization techniques using poly(epsilon-caprolactone) (PCL) and 4,4'-diphenylmethane diisocyanate (MDI). The prepolymer was extended with different mass ratios of chitin and 1,4-butane diol (BDO). Molecular characterization was done using FTIR, 1H NMR and 13C NMR techniques. The mechanical properties of these polymers were improved with increase in the chitin contents. Optimum mechanical properties were obtained from elastomers extended with chitin in comparison to elastomers extended with BDO. Cytotoxicity of the synthesized polyurethane samples was affected by varying the chitin contents in the chemical composition of the final polyurethane (PU). It is revealed that the final polymers extended with chitin are preferred candidates for surgical threads with on going investigations into their in vitro biocompatibility and non-toxicity.     

Synthesis of poly(epsilon-caprolactone)-b-poly(gamma-benzyl-L-glutamic acid) block copolymer using amino organic calcium catalyst

A biodegradable two block copolymer, poly(epsilon-caprolactone)-b- poly(gamma-benzyl-L-glutamic acid) (PCL-PBLG) was synthesized successfully by ring-opening polymerization of N-carboxyanhydride of gamma-benzyl-L-glutamate (BLG-NCA) with aminophenyl-terminated PCL as a macroinitiator. The aminophenethoxyl-terminated PCL was prepared via hydrogenation of a 4-nitrophenethoxyl-terminated PCL, which was novelly obtained from the polymerization of epsilon-caprolactone (CL) initiated by amino calcium 4-nitrobenzoxide. The structures of the block copolymer and its precursors from the initial step of PCL were confirmed and investigated by 1H NMR, FT-IR, GPC, and FT-ICRMS analyses and DSC measurements.     

Preparation of a thermosensitive highly regioselective cellulose/N-isopropylacrylamide copolymer through atom transfer radical polymerization

Regioselective copolymerization of N-isopropylacrylamide (NIPAM) onto cellulose was achieved by atom transfer radical polymerization (ATRP) using a regioselectively modified 6- O-bromoisobutyryl-2,3-di- O-methyl cellulose macroinitiator. Varying the ratio of NIPAM to macroinitiator to ligand to transition metal in a Cu(I)Br/ N, N, N', N', N'-pentamethyldiethylenetriamine (PMDETA) catalyst system affected graft yield and degree of polymerization. ATRP proceeded to completion without any trace of the macroinitiator, and a degree of polymerization (DP) of polyNIPAM up to 46.3 was obtained. Increasing the DP of the NIPAM component increased both the thermal decomposition temperature and the glass transition temperature of the copolymer. The grafting of NIPAM also affected the solubility properties of the methylcellulose. The 6- O-polyNIPAM-2,3-di- O-methyl cellulose formed a stable suspension in water at room temperature and underwent a hydrophillic-to-hydrophobic transition and copolymer precipitation when the temperature was raised above 30 degrees C.     

离子液体中固定化脂肪酶催化拆分（±）-薄荷醇

以自制的平均粒径为4．5um磁性高分子微球为载体,采用离子交换法固定化Candida rugosa脂肪酶,催化（±）-薄荷醇的酯化反应,以考察反应时间、pH、反应温度、水活度等因素对酶的固定化以及酯化反应的影响。在固定化反应150min、pH5．0、酯化反应温度30℃、固定化酶的水活度为0．78的条件下,所制备的固定化脂肪酶在离子液体[bmim]PF6中催化拆分（±）-薄荷醇的效果最佳,与游离酶相比固定化脂肪酶的立体选择性有很大的提高,对映体过量率可达93％,对映体选择值为35。