PURPOSE: To obtain a specific silicon-functional copolymer with a suitable hydrolyzability by reacting a specific triorganosilyl copolymer with a specific silicon-functional compd.

CONSTITUTION: A triorganosilyl copolymer comprising triorganosilyl (meth) acrylate units of formula I (wherein R1 is H or CH3; and R2 is a monovalent hydrocarbon group having no aliph. unsatd. bond provided at least one of R2' s is a 4C or higher monovalent hydrocarbon group) and(meth)acrylate or vinyl units of formula II (wherein R3 is R1; and Z is phenyl or a monovalent polar group contg. no Si) is reacted with a silicon-functional compd. of the formula: R8aSiX4-a (wherein R8 is a monovalent hydrocarbon group; X is OH or a hydrolyzable group; and a is 0-2) to subject part of side triorganosily1 groups of units of formula I of the above copolymer to transesterification, thus giving a silicon-functional copolymer comprising units of formulas I and II and silicon- functional silyl (meth)acrylate units of formula III.