The invention disclosed herein provides a novel thermally reversible copolymer having up to about 11 percent by weight of an N-(alkoxymethyl)acrylamide, a diene-containing synthetic rubber and a metal. The thermally reversible copolymer can be blended with conventional styrene-butadiene rubbers to improve the green strength thereof. Also disclosed herein is a process for improving the green strength of styrene-butadiene rubber which includes the steps of charging a reaction vessel with water, an emulsifier, an initiator, an N-(alkoxymethyl)acrylamide monomer and a vinyl monomer where one is desired, purging the vessel with an inert gas, charging a diene monomer and an activator to the reaction vessel, polymerizing the monomers to form a colymer latex, coagulating the latex with an aqueous metal salt to form a thermally reversible polymer and compounding the thermally reversible polymer with styrene-butadiene rubber.