Previously published material on the α‐methyl styrene/methyl methacrylate (α‐MS/MMA) copolymer system at temperatures above the ceiling temperature of α‐MS has focused on low‐conversion results. Several attempts have been made to estimate copolymer reactivity ratios from experimental data, but in most cases errors are present in the determination of copolymer composition variables. In this article, the results of rigorous parameter estimations, as applied to two sets of equations developed independently by P. Wittmer (Adv Chem 1971, 99, 140–174) and H. Kruger, J. Bauer, and J. Rubner (Makromol Chem 1987, 188, 2163–2175), are discussed. Experimental data for the copolymer system at low conversions, as well as over the full conversion range, are presented, covering a temperature range of 60–140 °C. A comparison of the data trends with traditional copolymer systems indicates that the reversibility of both MMA and α‐MS must be considered when composition, polymerization rate, or molecular weight equations are being developed. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1981–1990, 2000

In free-radical polymerization, solvents are routinely used to reduce the solution viscosity and molecular weight. In the case of the ␣-methyl styrene/methyl methacrylate copolymer system, reducing the monomer concentration also has significant effects on the copolymer composition and maximum conversion obtained. Reactivity-ratio studies as well as full-conversion-range data were provided, and the results were compared to similar data in the bulk phase from our earlier work. A 30 wt % toluene solution was used for all experimental conditions. Copolymer composition modeling results were also presented.

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