The Research On Enzyme-catalyzed Preparation Of Biodegradable Polyesters By Reactive Extrusion

The aliphatic polyester obtained by the ring-opening polymerization of cyclic esters has good biodegradability and biocompatibility.It not only can be used as environmentally friendly degradable plastics to reduce "white pollution",but also can be used as medical materials in vivo,such as absorbable surgical sutures,drug delivery carriers and tissue engineering scaffolds,etc.Usually,the catalysts used for the ring-opening polymerization of cyclic esters are metal catalysts,but this will leave metal in the product polyesters,which may cause gene mutation and loss of partial cell functions.This also limits wide application of such polyesters in vivo medical field.Enzymes can be used as effective and green catalysts for ring-opening polymerization of cyclic esters.The main advantages of enzyme catalysts are:biological enzymes derived from nature and are degradable,generally non-toxic,have enantioselectivity and regioselectivity,and are still effective at lower reaction temperatures.Novozyme 435(N435)is an immobilized Candida antarctica lipase B(CALB),which has high catalytic activity for the ring-opening polymerization of macrolides.Domestic and foreign researches on enzyme-catalyzed ring-opening polymerization of cyclic esters,mainly based on bulk polymerization and solution polymerization,and there are few reports on the research of reactive extrusion technology.Reactive extrusion is a combination of polymerization and processing,which can effectively improve the efficiency of mass and heat transfer,also can avoid using solvents.The biodegradable polyester obtained by enzyme-catalyzed reaction extrusion has no metal residues,no solvent is used in the process,so the safety of the polyester is improved,and it can be potentially used as vivo medical material.In this paper,biodegradable polyesters ?-pentadecalactone/?-valerolactone(PDL/VL)copolymers,?-caprolactone/?-pentadecalactone(CL/PDL)copolymers,poly(?-caprolactone)(PCL),and methoxy poly(ethylene glycol)-b-poly(?-caprolactone)(mPEG-b-PCL)and methoxy poly(ethylene glycol)-b-poly(?-pentadecalactone)(mPEG-b-PPDL)were synthesized by lipase N435 catalyzed reactive extrusion process.The main research content includes the following four aspects:(1)The research on lipase N435 catalyzed preparation of PDL/VL copolymer by reactive extrusion shows:The total monomer conversion of PDL/VL and the molecular weight of PDL/VL copolymer increase with the extension of the reaction time,while the molecular weight polydispersity(D)of the product decreases with the increase of the reaction time.The total monomer conversion of PDL/VL increased from 68.7%(1 h)to 90.3%(6 h),the molecular weight(Mn)of the synthesized PDL/VL copolymer increased from 8.3 kDa(1 h)to 71.3 kDa(6 h),the dispersity(D)decreased from 2.39(1 h)to 1.67(6 h).PDL/VL copolymers with higher molecular weight(Mn=71.3 kDa)and lower dispersion(D=1.67)can be prepared by N435 catalyzed reactive extrusion,which is larger than the highest molecular weight of PDL/VL copolymers obtained by solution polymerization(SP)and bulk polymerization(BP).Lipase N435 catalyzed reactive extrusion can effectively promote the transesterification reaction.In the initial stage of the copolymerization reaction,PDL/VL copolymer has a block structure,the microstructure of the copolymer gradually becomes random with the reaction time increases,this is mainly due to the increase in the frequency of the transesterification reaction with the extension of the reaction time,which makes the arrangement of the comonomer units on the polymer chain more random.The random index of the copolymer is 0.42 at 1 h,however,its random index is 0.91 at 6 h.The microstructure of the PDL/VL copolymer can be adjusted by controlling the reaction extrusion time,which makes PDL/VL copolymers with different thermal properties,mechanical properties and degradation properties can be prepared.The reaction time was extended from 1 h to 6 h,the microstructures of the products obtained by SP are close to random structure,the microstructures of the products obtained by BP are all blocky structures,it is difficult to adjust the microstructures of the products by controlling the reaction time under SP and BP conditions.The thermal performance studies of PDL/VL copolymers obtained by REX shows:The melting temperature and crystallization temperature of PDL/VL copolymer are gradually reduced as the microstructure of the PDL/VL copolymer changed from block to randomness.In addition,the PDL unit and VL unit in the random PDL/VL copolymer chain can be co-crystallized due to the isomorphism of comonomer units.The mechanism of lipase N435 catalyzing the ring-opening polymerization of PDL/VL was analyzed.The enzyme intermediate containing hydroxyl formed by the water and enzyme in the system is the initial active species of the reaction and initiates the ring-opening polymerization according to the insertion mechanism.(2)The research on lipase N435 catalyzed preparation of CL/PDL copolymer by reactive extrusion found:Reacting for 3 hours at 90?,the total monomer conversion of CL/PDL can reach 98.8%,the Mn(D)of CL/PDL copolymer can reach 51.3 kDa(1.96),and the random index of its microstructure is 1.02.The total monomer conversion and product molecular weight under reactive extrusion(REX)condition are higher than solution polymerization(SP)and bulk polymerization(BP).The random index of CL/PDL copolymer obtained by reactive extrusion(REX)indicates that the structure of copolymer is close to be random at 1 h.However,the microstructure of the product obtained by conventional bulk polymerization(BP)for 3 h still belongs to the block structure.(3)The ring-opening polymerization of CL was studied by the lipase N435 catalyzed reactive extrusion process,the monomer conversion of CL,molecular weight and dispersity of PCL were investigated at the reaction temperature 70?,90? and 110?.The research result shows:The highest conversion of CL obtained at 70? is 99.23%(14 h),and the highest molecular weight Mn of product PCL is 15.1 kDa(10 h),these are all higher than the corresponding maximum values at 90? and 110??The molecular weight of the product gradually decreased after 10 h at 70?,the water in the system caused the PCL hydrolysis and chain scission,which made the molecular weight of the product reduced.However,the lipase was partially inactivated at 110?,which made the catalytic activity reduced due to the high reaction temperature.(4)The mPEG-b-PCL and mPEG-b-PPDL amphiphilic block copolymers were prepared by N435 catalyzed reactive extrusion at 90?.The structure of the product was analyzed by proton nuclear magnetic resonance spectroscopy,and it was found that the product also contained a poly(cyclic ester)(PCL or PPDL)with terminal carboxyl groups in addition to containing the amphiphilic block copolymer.It was also found that the product contained mPEG that did not participate in the reaction.The research shows:CL conversion increases with the extension of the reaction time,and 92.61%CL conversion can be reached at 6 h;in addition,the efficiency of CL ring-opening polymerization initiated by mPEG increased first and then decreased,the maximum value is 40.13%.The presence of water in the system was the main reason for the decrease in the initiation efficiency of mPEG.The study found that the efficiency of mPEG initiating the ring-opening polymerization of PDL is greater than that of water.Water and unreacted mPEG in the system made the molecular weight of the PPDL segment in the amphiphilic block copolymer mPEG-b-PPDL deviate from the designed molecular weight.