Disclosed are such co-polymers of vinylidene substituted aromatic monomers and unsaturated compounds containing nucleophilic groups chain extended by a copolymer of one or more vinylidene aromatic monomers and one or more unsaturated compounds having pendant electrophilic groups which copolymerize with the one or more vinylidene aromatic monomers. Disclosed are compositions comprising vinylidene substituted aromatic monomers and unsaturated compounds containing nucleophilic groups and a copolymer of one or more vinylidene aromatic monomers and one or more unsaturated compounds having pendant electrophilic groups, which may optionally, contain salts of alkaline earth metals, alkali metals, transition metals, post transition metals or metalloids. Disclosed are methods of preparing such chain-extended and/or branched copolymers.