A2 and A4 independently are a substituted bridging group comprising either a silicon bridging atom, a germanium bridging atom, a tin bridging atom, a carbon bridging atom, or a bridging chain of 2 to 5 carbon atoms, any substituents on A2 and A4 other than an alkenyl linking group independently are a hydrogen atom, or a substituted or unsubstituted aliphatic, aromatic, or cyclic group, or a combin