RAFT polymerization from nanoparticles and development of novel chain transfer agents.

摘要

Among the current Controlled Radical Polymerization (CRP) methods, (RAFT) may have the best prospects for large scale commercial applications. However, only a limited number of (CTA's) can be used to control methacrylate monomers and both of these CTA's contain tertiary R groups. In the first part of this work, dithioester Chain Transfer Agents (CTA's) [Z-(C=S)-R] containing a secondary fragmenting R group were designed and prepared with a variety of Z groups. These CTA's were prepared in a straightforward two step procedure and, in most cases, were isolated as low melting solids with very low odor. The new Reversible Addition-Fragmentation chain Transfer polymerization (RAFT) agents were very effective for the controlled polymerization of styrenics, acrylates and methyl methacrylate.;In the second part of this work, well-defined polymer grafted silica nanoparticles were prepared by using RAFT agents anchored onto silica nanoparticles. Two approaches were developed to attach RAFT agents onto silica nanoparticles. The kinetics of St, nBuA and MMA graft polymerizations mediated by RAFT agents anchored onto silica nanoparticles of different surface density were examined and compared with the polymerizations mediated by free RAFT agents. Two unique characteristics of RAFT polymerization on particle surface were identified: a localized high RAFT agent concentration effect and an intermediate macro-RAFT agent radical destabilization effect. HPLC and GPC techniques were used to separate ungrafted polymer from the as-prepared polymer grafted nanoparticles. The grafting efficiency was calculated to be as high as 90%. Well-defined homopolymer and block copolymer grafted silica nanoparticles were prepared and characterized by GPC, TGA, TEM, and UV-vis.;Silica nanoparticles grafted with PSt of various molecular weights were used as fillers to prepare polymer nanocomposites. The effects of grafted molecular weight on the interfacial interactions and dispersion in matrix polymer were studied. The morphology of these polymer nanocomposites was examined by TEM and optical microscopy. The viscoelastic behavior of the nanocomposites was also studied. It was found the molecular weights of grafted polymer and matrix polymer played a very important role in the dispersion of the nanoparticles and the interfacial interaction between nanoparticles and matrix polymer.