In an autoclave equipped with a 1.0-liter glass polymerization vessel and purged with nitrogen, polymerization reaction was carried out by dissolving 19.4 grams (0.11 mol) of p-butoxystyrene, 13.2 grams (0.11 mol) of p-hydroxystyrene and 25.0 grams (0.20 mol) of t-butyl acrylate in 300 grams of acetone, adding 3.5 grams of t-butylperoxy(2-ethylhexanoate) as a polymerization catalyst, and heating the mixture at 90° C. Thereafter, the catalyst was added in incremental amounts, that is, 1.0 grams after 21/2 hours and 1.0 grams after 41/2 hours from the start of polymerization reaction. After 6 hours of polymerization, the reaction solution was cooled down and the acetone was distilled off. A 9/1 methanol/water mixture was added to the reaction solution, causing the copolymer to crystallize. The copolymer had a weight average molecular weight (Mw) of 6,900, a dispersity (Mw/Mn) of 1.72, and a copolymerization ratio (p-butoxystyrene/p-hydroxystyrene/t-butyl acrylate) of 1:1:2. The yield was 70.0%.
