One-Pot Synthesis Of Block Copolymers With Metal-Free Lewis Pairs As Catalysts

As alternatives of traditional non-degradable polymers,biodegradable polymers such asaliphatic polyesters and polycarbonates provide promising solutions to"white pollution"and microplastic pollution.Yet,they may suffer from unsatisfactory mechanical/thermal properties,limited functionalities,and high cost,thus exhibiting poor market competitiveness.In addition,their synthetic protocols typically rely on high concentrations of metal-based catalysts,and the metallic residues in the products greatly hinder their application in the fields of biomedicine,tissue materials,and food packaging.Rational design and synthesis of block copolymers can precisely tailor the physical/chemical properties of aliphatic polyesters and polycarbonates at the molecular scale,and endow them with specific functions.However,block copolymers typically require multi-step synthetic procedures,orthogonal catalysis,and laboratory purification processes.One-pot/one-step methods are therefore highly desirable,yet,current achievements rely on metal-based catalysts,involve two polymerization cycles,and only give access to AB and ABA block copolymers.To address this,we have developed three metal-free synthetic strategies capable of synthesizing block copolymers comprising aliphatic polyester and polycarbonate blocks in one-pot processes,based on bifunctional chain transfer”,“switchable polymerization”,and“kinetic control”.The main contents and results are listed as follows:（1）The synthesis of polyvinyl-b-CO₂-based polycarbonate diblock copolymers via a“bifunctional chain transfer”strategy.The terpolymerization of epoxides,CO₂ and vinyl monomers was carried out using a trithiocarbonate compound with carboxylic group（TTC-COOH）as the bifunctional chain transfer reagent for the combination of reversible addition-fragmentation chain transfer（RAFT）polymerization of vinyl monomers and metal-free Lewis pair composed of triethylborane（Et₃B）/1-tert-butyl-2,2,4,4,4-pentakis（dimethylamino）-2λ⁵,4λ⁵-catenadi（phosphazene）（t-Bu P₂）catalyzed ring-opening copolymerization（ROCOP）of epoxides/CO₂.When the CO₂ pressure is above 3 MPa,ROCOP of epoxides/CO₂ has good selectivity wherein the carbonate content is more than99%in the resultant polymer.By adjusting the ratio of Et₃B/t-Bu P₂,the rate of ROCOP can be accurately matched with RAFT polymerization,and the cyclic by-product content in the product is less than 1%.Structurally diverse polyvinyl-b-CO₂-based polycarbonate diblock copolymers have been synthesized successfully by changing different monomer types and feed ratio.（2）The synthesis of polyester-b-polyacrylate diblock copolymers via a“switchable polymerization”strategy.The terpolymerization of epoxides,cyclic anhydrides and acrylates was carried out using metal-free Lewis pair composed of Et₃B/tert-butylimino-tris（dimethylamino）phosphorene（t-Bu P₁）as the catalyst and TTC-COOH as the chain transfer agent.Experimental results and density functional theory（DFT）calculations revealed that the alkoxide active species of epoxides/anhydrides ROCOP exhibits high binding energy with Et₃B,which completely blocks the the autoxidation of Et₃B to produce the radicals.However,the carboxylate active species exhibited relatively lower binding energy with Et₃B,which attenuates the autoxidation but not completely blocks it.Therefore,with the injection of O₂ before the full consumption of cyclic anhydride,the switch from ROCOP of epoxides/cyclic anhydrides to RAFT polymerization of acrylates was achieved for the one-pot synthesis of polyester-b-polyacrylate diblock copolymers.（3）The synthesis of polyvinyl-b-polyester-b-poly（lactic acid）triblock copolymers by combining two block copolymer strategies involving“switchable polymerization”and“bifunctional chain transfer”.The quaterpolymerization of epoxides,cyclic anhydrides,racemic lactide（rac-LA）and vinyl monomers was carried out with metal-free Lewis pair composed of Et₃B/1,8-diazabicyclo[5.4.0]undec-7-ene（DBU）as the catalyst and TTC-COOH as the bifunctional chain transfer agent.As clearly revealed by the evolution of NMR spectroscopy,the whole polymerization process involves two stages distinguished by the full conversion of cyclic anhydrides.During the first stage,the ROCOP of epoxides/cyclic anhydrides and RAFT polymerization of vinyl monomers simultaneously occurred to produce polyvinyl-b-polyester diblock copolymers.During the second stage,the ring-opening polymerization（ROP）of rac-LA was switched on and RAFT polymerization of vinyl monomers continued to proceed,eventually affording polyvinyl-b-polyester-b-poly（lactic acid）triblock copolymers.This method could be applied to a variety of epoxides,cyclic anhydrides and vinyl monomers.（4）The synthesis of polyether-b-poly（lactic acid）-b-polyester-b-poly（lactic acid）-b-polyether pentablock copolymers by combining two block copolymer strategies involving“switchable polymerization”and“kinetic control”.The terpolymerization of propylene oxide（PO）,cyclic anhydrides and rac-LA was carried out using metal-free Lewis pair composed of Et₃B/DBU as the catalyst and H₂O as the initiator.For ROCOP of epoxides/cyclic anhydrides and ROP of epoxides,the polymerization rate increased significantly with the increase of the Et₃B/DBU feed ratio because Et₃B effectively activated epoxides by the complextion.On the contrary,for ROP of rac-LA,the polymerization rate decreased significantly with the increase of Et₃B/DBU feed ratio because Et₃B inhibited the catalytic activity of DBU.Et₃B/DBU pair with feed ratio of 2/1 enables chemoselective control over PO/cyclic anhydrides ROCOP and rac-LA ROP and results in significant differences in the reactivity ratio between rac-LA and PO in the ring-opening polymerization.Therefore,the three polymerization reactions can orderly proceed without interference between each other,successfully affording polyether-b-poly（lactic acid）-b-polyester-b-poly（lactic acid）-b-polyether pentablock copolymers in a one-step process.