The anionic polymerization of methyl methacrylate was performed ill tetrahydrofuran (THF) at -78 degrees C, using sec-butyllithium/1,1-diphenylethylene (DPE) as the initiation system. The effects of polymerization time and initiator concentration on the branching reaction were studied. High vacuum was used to prevent contamination during the polymerization. Gel permeation chromatography (GPC) was used to characterize the branching effect qualitatively. Experimental results indicated that the monomer conversion reached more than 98%, in a polymerization time of 10 min. The branching reaction occurred after high monomer conversion, resulting in a tail of high molecular weight in the GPC trace. This branching effect, observed by GPC, increased with polymerization time. Rapid termination was thus probably required immediately after all of the monomer was consumed in the preparation of a well-defined PMMA without a high-molecular-weight tail in this diphenylbutylllithium/THF/-78 degrees C system. (C) 2008 Wiley Periodicals, Inc. J Appl Poly Sci 111: 2099-2103, 2009