Applications Of A Novel Cyclic Phosphazene In The Ring-opening Polymerization Of Cyclic Esters

Biodegradable polyesters have great application value in biomedical,food packaging and other fields.Among them,polycaprolactone(PCL)and polylactic acid(PLA)have attracted wide attention because of their unique advantages.The synthesis of high-performance PCL and PLA has aroused general interests.The catalyst is the most important factor in polymer synthesis.Traditional organometallic catalysts,because of difficulties in synthesis and storage and metal residues in polymers,have been unable to meet the requirements of polymer materials for biomedical applications.Therefore,organic metal-free catalysts have begun to attract the interest of researchers.In particular,phosphazene catalysts have become one of the most popular anionic ring-opening polymerization(ROP)catalyst due to their characteristics of easy synthesis and high activity.However,due to the late start of the phosphazene catalysts,there are still many problems such as the little variety and high price,which limits its large-scale application.In response to these problems,we designed and synthesized a novel cyclic phosphazene catalyst CTPB,and applied it to the ROP of caprolactone(ε-CL)and lactide(LA)to prepare high-performance PCL and PLA.The influence of different polymerization conditions on the polymerization reaction was systematically studied,which provided a new idea for the preparation of high-performance biomedical biodegradable polyester materials.The main results are as follows:1)It was found that CTPB could directly catalyze ROP ofε-CL at room temperature but in a relatively low activity.After the addition of the alcohol as an initiator,CTPB could catalyze the ROP ofε-CL very efficiently,converted hundreds to thousands of equivalents of monomer into PCLs in a short time,and obtained a high molecular weight,relatively narrow distribution PCL.Either mono-or multi-hydroxy-alcohols were efficient as initiators in this system.And thus the PCLs with different topology were easily prepared.2)We also found that CTPB could efficiently catalyze stereoselective ROP of rac-LA at low temperature to obtain isotactic stereoblock PLA(P_i up to 0.93).The polymerizations were highly controlled,as evidenced by linear relationship between molecular weights(MW)and monomer conversions and the narrow dispersity(D=M_w/M_n)of the resulted polymers with high fidelity of end groups.