Highly active, bifunctional propagating species that contain both nucleophilic and electrophilic catalyst sites as well as bring about efficient living/controlled (meth)acrylate polymerization at ambient temperature via a bimolecular-activated monomer propagation mechanism have been generated by instantaneous oxidative activation of ketene silyl acetal initiators with a catalytic amount (0.05 mol % relative to monomer) of the trityl activator. Studies of structure-reactivity relationships for this novel polymerization system have included effects on the polymerization activity and the degree of control by structures of initiator (alkyl, silyl, germyl, and stannyl acetals), catalyst (variations in alkyl and silyl groups of the silyl acetals), activator (trityl salts of anions differing in their stability, size, coordination, and chirality), and monomer (methacrylates vs acrylates). Of several valuable findings obtained from this study, a most interesting and significant result is that there exhibits remarkable selectivity of the silyl group structure of the acetal initiator (and thus the derived silyl cation catalyst) for monomer structure: initiators having small silyl groups, such as dimethylketene rnethyl trimethylsilyl acetal, promote highly active and efficient polymerization of methacrylates, but they are poor initiators for polymerization of less sterically hindered, active a-H bearing acrylate monomers. On the other hand, initiators incorporating bulky silyl groups, such as the triisobutylsilyl derivative, exhibit low activity toward methacrylate polymerization but exceptionally; high activity, (completed reaction in 1 min), efficiency (achieved quantitative initiator efficiency), and degree of control (regulated low to high M-n (> 10(5)) with narrow molecular weight distributions) for acrylate polymerization at ambient temperature in polar noncoordinating (CH2Cl2), aromatic (toluene), or aliphatic (cyclohexane) solvents.