Post-polymerization Modification Of CO₂-based Aliphatic Polycarbonate

Carbon dioxide is considered to be one of the main environmental indicators that cause the greenhouse effect.With the development of industry,there is no doubt that carbon dioxide emissions has been increased.Therefore,the use of carbon dioxide for chemical synthesis has attracted widespread attention in the past few decades.One of the cost-effective methods is to use carbon dioxide to produce polymers with biodegradable properties or certain properties.Aliphatic polycarbonates derived from CO₂and epoxides have become the subject of extensive research at present.Although the research in this field has made great progress,the lack of functionality and inert chemical structure of the obtained aliphatic polycarbonates have greatly restricted their use as high value-added and functional materials.In this context,there is an urgent need to design and synthesize more types of carbon dioxide-based polymer materials suitable for existing applications.At present,a variety of synthetic modification strategies have been developed to improve the properties of carbon dioxide-based polycarbonate materials,such as post-polymerization functionalization,ternary copolymerization and crosslinking copolymerization,etc.In particular,the graft and block copolymerization of aliphatic polycarbonate are commonly used methods for post-polymerization modification,which can effectively realize the modification of polymers and synthesize polymer materials with multiple functions,so as to achieve the purpose of expanding the application range of CO₂-based polycarbonate.This paper focus on using graft copolymerization and block copolymerization methods to modify polycarbonate by post-polymerization.First,aliphatic polycarbonate P（CO₂/ECH）synthesized by CO₂and epichlorohydrin（ECH）was selected for graft polymerization modified.For graft polymerization,based on the side groups of the polycarbonate chain in P（CO₂/ECH）with active Cl atoms on the side groups,the side group C-Cl bonds are easily broken to form active sites under external stimulation,thereby initiating the graft polymerization of vinyl monomers.Secondly,the use of Salen metal catalyst to catalyze the copolymerization of CO₂and propylene oxide（PO）to prepare polycarbonate P（CO₂/PO）with Cl end caps.The existence of the C-Cl bond at the end of the polymer chain facilitates block copolymerization with vinyl monomers to obtain corresponding block copolymers.1.Under thermal initiation conditions,the graft copolymerization of aliphatic polycarbonate P（CO₂/ECH）with diacetone acrylamide（DAAM）,N-vinylpyrrolidone（NVP）,methyl methacrylate（MMA）,and styrene（St）have been realized,which prepared various types of graft copolymers.The graft copolymerization of polycarbonate and vinyl monomers has been systematically studied,the research results showed that it was possible to effectively improve the conversion of vinyl monomers and graft yield by optimizing the graft copolymerization reaction conditions,including polymerization temperature,monomer dosage and polymerization time.Taking the graft copolymerization of P（CO₂/ECH）and DAAM as an example,the monomer conversion rate can reach up to 93.63%,the grafting yield is 92.58%by optimizing the reaction conditions,and the molecular weight M_nof the prepared graft copolymer can be up to14.7kg/mol.The performance of the graft copolymer obtained by the research showed that the water contact angle of the graft copolymer obtained by the graft copolymerization reaction with DAAM is compared with the water contact angle of polymer P（CO₂/ECH）of 106.65°,Both are less than 90°,and the minimum is 48.29°,which proved that the graft copolymerization modification can effectively improve the hydrophilic properties of polycarbonate.2.In the presence of visible light,the graft copolymerization of P（CO₂/ECH）with diacetone acrylamide（DAAM）,N-isopropylacrylamide（NIPAM）,methyl methacrylate（MMA）and styrene（St）monomers have been achieved.First,the influence of the amount of photocatalyst perylene on the graft polymerization was investigated.The experimental results showed that the graft polymerization could be initiated by the use of a certain concentration of perylene as the photocatalyst.When the feed ratio of P（CO₂/ECH）,DAAM and perylene is 1:1:0.02,after 24 hours of polymerization,the DAAM conversion rate can reach up to 77.45%,and the corresponding grafting yield is74.87%.The proposed photograft mechanism was verified by corresponding control experiments and fluorescence characterization,The results proved that the graft copolymerization reaction was initiated by the interaction of P（CO₂/ECH）with the excited perylene,and the whole graft copolymerization reaction was highly sensitive to light.The results of further light"on"/"off"experiments showed that with the increase of light irradiation time,the monomer conversion rate in the graft copolymerization reaction increased linearly with the M_nof the graft copolymer prepared,which proved that the photografting process was light-controlled living polymerization.The thermal stability of the four graft copolymers prepared was tested.The results of the study showed that the thermal stability of the polycarbonate can be effectively improved by photografting strategy.Among them,P（CO₂/ECH）-g-PNIPAM copolymer has temperature-sensitive properties.3.The polycarbonate P（CO₂/PO）obtained by the copolymerization of CO₂with propylene oxide（PO）catalyzed by Salen metal catalyst.It was used as the macromolecular initiator for the photocatalytic block copolymerization,and perylene was selected as the organic photocatalyst to achieve the photocatalytic block copolymerization with polycarbonate and vinyl monomers.The effect of catalyst concentration and polymerization time on the block copolymerization reaction was studied,the experiments showed that block copolymer reaction required a quantitative photocatalyst,and with the prolonged light irradiation time,the conversion rate of vinyl monomers increased.When the feed ratio of P（CO₂/PO）,DAAM and perylene is1:1:0.02,the DAAM conversion rate can reach 55.17%after 24 hours of polymerization.Through investigating the mechanism of the photocatalytic block copolymerization reaction,the results showed that the copolymerization reaction was initiated by the free radicals generated by the cleavage of the C-Cl bond at the end of the polycarbonate,and the polymerization was highly sensitive to light.The subsequent chain extension polymerization reaction proved the existence of Cl atoms at the end of the block copolymer growing chains.Moreover,good control of the photocatalytic block copolymerization reaction can be achieved through the light"on"/"off"experiment.Further analysis of thermal properties showed that the thermal decomposition temperature(5%weight loss temperature:T_-5%;maximum weight loss temperature:T_max)of polycarbonate P（CO₂/PO）was 148°C and 217°C,respectively.The T_-5%of the block copolymer products increased by 80°C,and the T_maxincreased to 362-387°C,which proved that the photocatalytic block copolymerization strategy can significantly improve the thermal stability of polycarbonate-based materials.