Disclosed are processes for preparing a copolymer, a copolymer prepared by the process, a sulfur-crosslinkable rubber mixture, and the use of the sulfur-crosslinkable rubber mixture for production of motor vehicle tires. In one embodiment, the monomer has the formula: A-S-P, wherein A is a chemical group containing at least one aliphatic double bond, S is a sulfur atom, and P is a protecting group.