HINDERED LITHIUM DIALKYLAMIDE INITIATORS FOR THE LIVING ANIONIC-POLYMERIZATION OF METHACRYLIC ESTERS

Lithium diisopropylamide is an efficient initiator for the anionic pol ymerization of methyl methacrylate in polar solvents at -78 degrees C. Predictable molecular weights based on the initiator concentration we re obtained, yields were quantitative, and molecular weight distributi ons were relatively narrow (<1.30). The presence of an amino end group and confirmation of the initiation mechanism were confirmed by potent iometric titration using HClO4, Number-average molecular weights deter mined by titration agreed well with values determined by SEC. Other li thium dialkylamides that contain larger alkyl groups such as lithium d iisobutylamide were less efficient initiators. This was attributed to the more nucleophilic anion due to less steric hindrance near the nitr ogen atom. Molecular weight distributions were significantly broadened (>2.0), and molecular weight control was not achieved. However, polym er yields were quantitative. In all cases, PMMA stereochemistry was in dicative of a solvent-separated lithium counterion, and triad composit ions were identical to organolithium initiated homopolymers, i.e., 78% syndiotactic, 20% heterotactic, and 2% isotactic. (C) 1994 John Wiley & Sons, Inc.