Organocatalytic ring-opening polymerization of cyclotrisiloxanes using silanols as initiators for the precise synthesis of asymmetric linear polysiloxanes†
The organocatalytic controlled/living ring-opening polymerization (ROP) of cyclotrisiloxanes such as hexamethylcyclotrisiloxane (D3) and 1,3,5-trimethyl-1,3,5-trivinylcyclotrisiloxane (V3) using silanol initiators and guanidine catalysts produced various asymmetric linear poly(dimethylsiloxane) (PDMS) and poly[methyl(vinyl)siloxane] (PMVS) compounds with controlled number-average molar mass (Mn), narrow molar-mass dispersity (ĐM), and well-defined terminal structures. An extensive range of solvents and catalysts were tested in order to optimize the conditions for the ROP of D3 and V3 as well as to minimize undesired side-reactions, particularly the condensation of two Si–OH groups. 1,3-Trimethylene-2-n-propylguanidine (TMnPG) and 1,3-trimethylene-2-ethylguanidine (TMEG) were identified as the most appropriate catalysts for the ROP of D3 and V3. Asymmetric linear polysiloxanes that contain either a functional group on one terminus, i.e., hemitelechelic polysiloxanes, or two different functional groups on each terminus, i.e., heterotelechelic polysiloxanes, were conveniently obtained from choosing a suitable combination of a functionalized silanol initiator and a functionalized chlorosilane end-capping agent. The controlled synthesis of a PDMS/PMVS diblock copolymer was also achieved by consecutive copolymerizations of D3 and V3. These polymerizations are considered to proceed via the initiator/chain-end activation mechanism, and intensive removal of water from the starting materials is necessary to control the polymerizations.