Controlled cationic vinyl-addition polymerization of an alkyl vinyl ether (VE) and ring-opening polymerization of ε-caprolactone (CL) simultaneously proceeded using HfCl4/Hf­(OBu)4 as a dual-role catalyst for both mechanisms, yielding a graft copolymer consisting of a poly­(VE) main chain and several poly­(CL) side chains. The copolymer of conventionally incompatible monomers was generated via the unprecedented mechanisms consisting of orthogonal propagating reactions and transient merging. Specifically, the poly­(CL) chains were incorporated into a poly­(VE) chain through an exchange reaction between the VE-derived alkoxy group and the propagating poly­(CL) chain at the acetal moiety of the propagating end of the poly­(VE) chain. An appropriate ratio of HfCl4 and Hf­(OBu)4 was indispensable for both the simultaneous vinyl-addition and ring-opening polymerizations and the alkoxy group exchange reaction.

Versatile graft architectures were synthesized in one shot via simultaneous controlled cationic vinyl-addition polymerization of vinyl ethers (VEs) and coordination ring-opening polymerization of cyclic esters (CEs). Graft copolymers were generated via independent propagation reactions and transient incorporation of a poly(CE) chain into the side chain of the poly(VE) propagating end via the exchange of alkoxy groups. In this mechanism, the grafting density and grafting length of a copolymer were designable by tuning the rates of each propagation reaction and the exchange reaction. As a result of a systematic investigation, the effects of polymerization conditions, such as the kinds and concentrations of monomers and catalysts, on the rate of each reaction were revealed and a design principle of various graft architectures was established. Notably, a copolymer with a remarkably high grafting density was obtained [max. 88% of poly(VE) side chains were substituted with poly(CE) chains] when a VE with an ethylenedioxy side chain was used with a titanium catalyst. The specific interaction of an ethylenedioxy unit and a titanium catalyst was key to the high grafting density.

Cationic copolymerization of different types of monomers, 4-hydroxybutyl vinyl ether (HBVE) and ε-caprolactone (CL), was explored using EtSO3H as an acid catalyst, producing copolymers with a remarkably wide variety of compositions and sequences. In the initial stage of the reaction, HBVE was unexpectedly isomerized to 2-methyl-1,3-dioxepane (MDOP), followed by concurrent copolymerization of MDOP and CL via active chain end and activated monomer mechanisms, respectively. The compositions and sequences of the copolymers were tunable, depending on the initial monomer concentrations. Moreover, a unique method was developed for transforming a copolymer with no CL homosequences into an “alternating” copolymer by removing MDOP from the system using a vacuum pump. This was achieved by the tandem reactions of depolymerization (unzipping) and random transacetalization (scrambling) under thermodynamic control. Specifically, the unzipping of HBVE homosequences proceeded at the oxonium chain end until a nondissociable ester bond emerged next to the chain end, while the scrambling of the main chain via transacetalization transferred midchain HBVE homosequences into the polymer chain end.

A temperature change-dependent sequence transformation of copolymer chains was demonstrated by a method based on tandem depolymerization and transacetalization reactions during the cationic ring-opening copolymerization of cyclic acetals and cyclic esters. In this study, the position of polymerizationdepolymerization equilibrium was controlled by the reaction temperature rather than by the decrease in monomer concentration under vacuum conditions, as in our previous study. First, the conditions for efficient copolymeriza-

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