Applications Of Bifunctional Catalysts And Co-catalysts In Polymer Synthesis

Catalysts/co-catalysts significantly influences not only catalytic activity but also microstructure and property of the polymer product.As a result,catalysts/co-catalysts open up an infinity of possibilities in polymer chemistry and polymer industries.Bifunctional catalysts/co-catalysts combine two types of catalysts/co-catalysts and potentially enhance catalytic activity,selectivity,control of polymerization.In addition,bifunctional catalysts/co-catalysts might result in novel polymer structures.However,bifunctional catalysts/co-catalysts generally exhibit relatively complex structure and result in the difficulty in synthesis.Therefore,using commercially available compound as a bifunctional catalyst/cocatalyst is a realistic and challenging issue in polymer synthesis.Our work is focused on the issue presented above.1.Firstly,we demonstrate that the combination of alkali alkoxides with simple and commercially available cyclic amides can catalyze fast and controlled ring-opening polymerization of L-lactide.The constrained cis C=N bond in the catalytic species is essential for fast polymerization.By optimizing the basicity of the catalyst,a good balance between activity and controll(PDI<1.1)is realized.A high cyclic amide/initiator ratio is critical for producing narrow molecular-weight distributions and inhibiting transesterification in alkali alkoxides and cyclic amides catalyzed ring-opening polymerization.2.Secondary,a stereoselective zwitterionic ring-opening polymerization(ZROP)of rac-lactide(rac-LA)was explored for synthesis of isotactic cyclic polylactides by using commercially available N-heterocyclic carbene(NHC)catalysts at low temperature in tetrahydrofuran.The epimerization of lactide during ZROP can be inhibited at low temperature.The best isoselectivity can reach to a high value of Pi=0.91 associated with a high melting point(Tm)of 193 ℃.The resulting polylactide is predominantly cyclic as evidenced by IH NMR and MALDI-TOF MS analyses.The steric-hinderance effect of the imidazolium moiety can influence the reactive site and the stereoselectivity of ZROP.3.Finally,a phenol-containing dibenzhydryl-based a-diimine ligand bearing hydroxy group on para-position of aniline moiety was designed,synthesized and investigated in Ni-and Pd-catalyzed ethylene polymerization.The Ni complex bearing hydroxy groups resulted in significantly increased Tm(up to 123 ℃)and greatly decreased branching density(33/1000C)of polyethylene versus the similar Ni catalyst bearing OMe group.This is consistent with the hypothesis that the deprotonation of the phenol moiety generated a phenoxide bearing strong electron-donating O-substituent by methylaluminoxane(MAO)cocatalyst.