Synthesis of ethylene oxide/methyl methacrylate diblock copolymers by group transfer polymerization of methyl methacrylate with poly(ethylene oxide) macroinitiators

Abstract

Ethylene oxide/methyl methacrylate diblock copolymers were prepared by group transfer polymerization (GTP) of methyl methacrylate (MMA) with poly(ethylene oxide) macroinitiators (1). The macroinitiator containing silyl ketene acetal end-groups was synthesized by hydrosilation of poly(ethylene glycol) monomethacrylate which was previously prepared by esterification of poly(ethylene glycol) monomethyl ether (PEO-OH). All synthesized macroinitiators initiated the GTP of MMA in tetrahydrofuran with tetrabutylammonium cyanide or tris(dimethyiamino)sulfonium difluoride as catalysts. The polymerization proceeds rapidly at room temperature to quantitative yield. The block copolymers are contaminated with small fractions of PEO homopolymer as a result of uncomplete macroinitiator synthesis. Unreacted PEO-OH, which is present in very small amounts, reduces the concentration of active centers. Therefore, the degree of polymerization is not in good agreement with the theoretical value calculated for a living polymerization taking the monomer to macroinitiator ratio. However, after extraction of the PEO homopolymer, the block copolymers show a narrow molecular weight distribution. Induction periods in time-conversion curves obtained with 1 as initiator vanished nearly, when the reaction was started with the addition product of 1 and MMA. This is an indication, that in the system under investigation a slow initiation step between 1 and the first MMA unit precedes the propagation reaction. Tacticity measurements yield about 56% syndiotactic, 40% heterotactic and 4% isotactic triads for PMMA in the block copolymers.