Precise Synthesis of Poly(dimethylsiloxane) Copolymers through C- H Bond-Activated Macroinitiators via Yttrium-Mediated Group Transfer Polymerization and Ring-Opening Polymerization

This article reports on the generation of copolymers from a hydrophobic poly(dimethylsiloxane) (PDMS) block, which was used as a macroinitiator in the formation of poly(diethyl vinylphosphonate) (PDEVP) and poly(2-vinylpyridine) (P2VP) by means of rare earth metal-mediated group transfer polymerization (REM-GTP) and the generation of poly(epsilon-caprolactone) via ring-opening polymerization (ROP). In preliminary studies, a binuclear initiator 3 was investigated as a model compound for ensuring the applicability of such siloxane-containing compounds in REM-GTP. Next, functionalization of the PDMS substrates with 2,6-dimethylpyridiyl units yielded the corresponding macroinitiators and enabled the C-H bond activation with Cp2Y(CH2TMS)-(THF) (4) as well as the subsequent REM-GTP and ROP. Two linear (MI1-2) and one side group (MI3)-functionalized macroinitiators were synthesized for the subsequent polymerization of diethyl vinylphosphonate, in order to elucidate the versatility of this route with different initiating substrates. In addition, 2-vinylpyridine was employed as an alternative Michael-type monomer, while the ROP of e-caprolactone showed that this approach is not only limited to REM-GTP but also enables the utilization of a wide array of catalytic systems and monomers. The initial C-H bond activation process was tracked by nuclear magnetic resonance (NMR) spectroscopy. The resulting homo- and copolymers were characterized by NMR spectroscopy, gel permeation chromatography, and elemental analysis, which confirmed the compositions of the resulting copolymers calculated by H-1-NMR spectroscopy. Differential scanning calorimetry and dynamic light scattering in a variety of solvents provided basic insights into the thermal and solution properties of these materials. Furthermore, turbidity measurements concluded an effect of the PDMS block length on the cloud point of PDEVP.