End-Functionalized and Branched Polymers by Anionic Ring-Opening Polymerization

Abstract

The introduction of functional groups and branching to polymers leads to substantial changes in thermal behaviour and rheology. This work focused on two different macromonomer approaches, namely AB2 and ABx, to synthesise branched polysiloxanes by Williamson and hydrosilylation coupling reactions, respectively. For the AB2 approach, anionic ring-opening polymerization (AROP) of hexamethylcyclotrisiloxane (D3) was attempted to be initiated by using (protected) functionalized initiator molecules to introduce dihydroxyl functionality (B2) on to the polydimethylsiloxane chain-end. The hydroxyl functionalities were protected with silyl ethers and successfully deprotected by using tetra-n-butylammonium fluoride or acetic acid after polymerization. Prior to the introduction of an alkyl halide (A-functionality) by end-capping, stability of the PDMS backbone was tested with Williamson coupling reactions. It was understood that the basic conditions of this reaction led to serious damage to the polymer and this approach was abandoned. Synthesis of ABx macromonomers was performed by copolymerizing D3 and 1,3,5-trivinyl-1,3,5-trimethylcyclotrisiloxane (V3) monomers. In this work, A and B represent Si-H and vinyl groups, respectively. The anionic copolymerization was end-capped by using chloro(methyl)phenylsilane. The resulting polymers were coupled by hydrosilylation reactions in the presence of Pt(0) catalyst. These macromonomers were successfully used to form highly branched polysiloxanes. To optimize conditions of the coupling reaction, solution concentration of the reaction was varied from 20 % to 100 % and the vinyl content of the macromonomers was varied from 11 mol% to 52 mol%. It was shown that higher solution concentrations result in higher degree of branching. On the other hand, while an increase in vinyl composition also resulted in higher level of branching, a high vinyl (>30 %) composition resulted in a decrease in branching. This is believed to happen because of poor chain end-capping for high vinyl content copolymers and/or increased likeliness of intramolecular cyclization reactions.