Synthesis And Properties Of Site-specific Functional CO₂-based Polycarbonates

Carbon dioxide-based polycarbonates are regarded as environmentally benign polymer materials prepared by the copolymerization of epoxides and CO2.This approach takes CO2 as abundant,non-toxic and cheap raw materials,which not only realizes the "waste utilization" of CO2,but also helps to reduce the dependence on fossil resources.At the same time,most CO2-based polycarbonates are biodegradable,which are conductive to reduce white pollution.In a manner of speaking,CO2-based polycarbonates embodies the concept of "green chemistry" in the materials’ life cycle of synthesis,application and degradation.Under the background of "carbon neutrality" and "peak CO2 emissions",CO2-based polycarbonates are expected to embrace broader development opportunities.More opportunities also mean more challenges.The continuous development of CO2-based polycarbonates has put forward more requirements for their functions and properties.Functionalization is a powerful method to expand functions and improve properties of materials,which has become an important trend in the research field of CO2-based polycarbonates.In this paper,on the basis of controllable synthesis of CO2-based polycarbonates,three functionalized polycarbonates were studied in terms of synthesis method,functions,properties and structure-function relationship by means of site-specific functionalization and modification.The main research contents are as follows:1.As typical stimuli-responsive materials,thermoresponsive polymers have potential applications in drug delivery,tissue engineering and gene therapy.However,it remains a challenging work to obtain thermoresponsive polymers with both biodegradability and signal "self-reporting" outputs.To solve this problem,we prepared fluorescence signal "self-reporting" biodegradable thermoresponsive polycarbonates based on the mechanism of immortal polymerization,by means of the copolymerization of CO2 and oligoethylene glycol monomethyl ether-functionalized epoxides using hydroxyl-modified tetraphenylethylene(TPE-OH)as chain transfer agent.TPE-OH exhibited good controllability on molecular weight(6.0～17.0 kg mol-1)while endowed the polymer with aggregation-induced emission(AIE)characteristics.Through temperature-dependent fluorescence intensity study,the low critical solution temperature of polycarbonates were determined and the details during thermal-induced phase transition process were monitored.Further study showed that the temperature-controlled aggregation and dissociation of TPE end groups was the main reason for fluorescence intensity variations.This work enables the visualization of thermoresponsive behaviors of CO2-based polycarbonates,which broadens the application in smart materials.2.Aldehyde end-capped polymers are a unique kind of site-specific functional polymers.The high reactivity of aldehyde groups enables the polymers to conjugate with disparate functional entities,leading to various applications in drug delivery,diagnosis and surface modification.However,the present research mainly focused on polyolefins,and their non-biodegradable property may hinder their further applications.To address this issue,we proposed a method to synthesize aldehyde end-capped CO2-based polycarbonates.The copolymerization of propylene epoxide and CO2 were performed in the presence of 4-formylbenzoic acid as the chain transfer agent to obtain well-defined polycarbonates.The resulting polymers own controllable Mn(3.7～19.0 kg mol-1)and narrow molecular weight distribution(D～1.1).The high reactivity of aldehyde end groups permits a variety of post-modification strategies.Several site-specific functional polymers have been derived,including regulating hydrophilicity,changing thermal properties,causing aggregation-induced emission and amino acid conjugation,which indicating that aldehyde end-capped CO2-based polycarbonates have the potential to prepare functional polymers as a green"platform".3.Thermosetting polymers possess cross-linked 3D network polymer,which contribute to their good mechanical properties,thermal stability and solvent resistance.However,cross-linked structure also prevents their reprocessing ability and hinders their recycling.Reversible cross-linked CO2-based polycarbonates containing dynamic covalent bonds were prepared by the curing reaction between epoxidized polymers and carboxylated polymers.Cross-linked polycarbonates exhibited good solvent resistance and could be reprocessed by hot pressing.Crosslinking significantly improved the toughness of polycarbonates,resulting in a substantial increase in elongation at break from 6.4%to 51%.TBD can promote the transesterification reaction and improve the processing ability of cross-linked polycarbonates,but excessive TBD will also accelerate the degradation of polymers,thereby reducing their thermal stability.