Forced ideal carbocationic copolymerization of α-methylstyrene (αMeSt) with p-tert-butylstyrene (ptBuSt) and (αMeSt) with styrene (St) has been achieved by continuous monomer feed addition to a cumyl chloride/BCl3charge at -50°C by keeping the feeding rate of the monomer mixtures equal to the overall rate of copolymerization, The composition of the copolymers was identical to the composition of the monomer feeds over the entire concentration range. A quantitative expression has been derived to show that under forced ideal copolymerization conditions the composition of the copolymer can be controlled by the composition of the feed. Further, conditions have been found for forced ideal quasiliving copolymerizations, i.e., the number-average molecular weight of the copolymers increased almost linearly with the cumulative weight of consumed monomers by the use of suitably slow, continuous feed addition in the presence of relatively nonpolar solvent mixtures (60/40 v/v n-hexane + methylene chloride). In polar solvent (methylene chloride) the molecular weight increase was less pronounced due to chain transfer to monomer involving indane-skeleton formation; however, with charges containing large amounts of ptBuSt the molecular weight increase was surprisingly strong. Interestingly, ptBuSt does not homopolymerize in 60/40 v/v n-hexane/methylene chloride but it readily copolymerizes with αMeSt. This observation was explained by examining the relative rates of terminations of the cationic species involved. Conditions have been found for the pronounced quasiliving polymerization of St. In forced ideal quasiliving copolymerizations neither the molecular weights of αMeSt/ptBuSt or αMeSt/St copolymers nor the initiating efficiencies of the initiating systems used show a depression. The microstructure of representative αMeSt/ptBuSt copolymers obtained under forced ideal quasiliving conditions has been analyzed by13C-NMR spectroscopy. According to these studies, true copolymers have formed and resonance peaks for various triads have been deduced.