Synthesis Of Silica Particles Grafted With Well-defined Homopolymers And Block Copolymers

In this study, silica particles grafted with well-defined homopolymers, di-, tri- and tetrablock copolymers were synthesized by combination of RAFT polymerization and coupling reaction, and a series of highly pure block copolymers were obtained by aminolysis. The results were shown as follows.In part 1, a combinational approach based on reversible addition-fragmentation chain transfer (RAFT) polymerization and coupling reaction was used to prepare well-defined silica-polymer hybrids. Chain-end-functionalized homopolymers were synthesized by RAFT polymerization of vinyl monomers such as methyl acrylate, and styrene mediated by S-methoxycarbonylphenylmentyl S'-trimethoxysilpropyl thithiocarbonate (MPTT) in toluene or dioxane at 60 oC, and di-, tri-, and tetrablock copolymers were futher synthesized by successive chain end wee then grafted to the surface of flash silica by coupling reaction between trimethoxysilane and hydroxyl groups. IR and thermogravimetric analyses results indicated the grafting ratios of polymeric chains on the surface of silica were relatively high. The grafted polymeric chains were cleaved from the surface of silica by aminolysis, and gel permeation chromatography results revealed all the grafted polymers possessed low polydispersity (typically less than 1.2), and molecular weight similar to that of the"as-prepared"polymers. Furthermore, the solid-suppourted polymeric chains were almost 100% living, evident from highly efficient chain extension polymerization to prepare well-defined block copolymers grafted onto silica particles.In part 2, two approaches based on RAFT polymerization and click reaction were used to synthesize silica-polymer hybrids. Some functional RAFT agents comprising azide, propynyl and alkyl bromide in Z group were designed and synthesized, and S-azidepropoxycarbonylethyl S'-methoxycarbonylphenylmethyltrithiocarbonate (AMP) was chosen as a typcal chain transfer agent to prepare silica-polymer hybrids. The stepwise method comprised (a) RAFT polymerization to synthesize polymers with an azide functionality in Z group and (b) subsequent click reaction between azide-functionalized polymers and alkyne-functionalized silica to prepare silica-polymer hybrids, which could afford well-defined grafted polymers with polydispersity indices slightly lower than their"as-prepared"precursors due to lack of dead chains. The one-pot method using clickable RAFT agents was to perform RAFT polymerization and click reaction simultaneously, which could afford grafted polymers with polydispersity typically less than 1.25, whilst the chain length of grafted polymers was usually shorter than that of free polymers formed in solution due to shielding effect and heterogeneous reaction condition. A series of well-defined homopolymers, di-, tri- and tetrablock copolymers could be efficiently grafted onto silica particles, evident from GPC, IR, elemental and thermogravimetric analyses. The grafted chains comprised poly(methyl acrylate), poly(butyl acrylate), poly(tert-butyl acrylate), polystyrene, poly(N,N-dimethylacrylamide), poly(N-isopropylacrylamide), poly(solketal acrylate), poly(N-acrylomorpholine) and poly(methyl methacrylate) and the grafting density depending on the chain length of grafted polymers was usually ranged between 0.017 and 0.085 chains/nm2. As compared with the stepwise approach, the one-pot method may be more promising due to lack of one step reaction and maintained controllability over surface modification.