Tandem Unzipping and Scrambling Reactions for the Synthesis of Alternating Copolymers by the Cationic Ring-Opening Copolymerization of a Cyclic Acetal and a Cyclic Ester

Abstract

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.

Unzipping and scrambling reaction‐induced sequence control of copolymer chains via temperature changes during cationic ring‐opening copolymerization of cyclic acetals and cyclic esters

“…The MW of the transesterification product obtained at 90 C was lower than that of the product obtained at 30 C, indicating a decrease in HBVE homosequences in the original copolymer. The MWD of the transesterification product of the copolymer obtained at 90 C was slightly broader than that of the alternating copolymer obtained in the previous study, 25 likely due to the remaining HBVE homosequences. Indeed, the ESI-MS spectrum of the transesterification products also demonstrated the presence of HBVE homosequences (Figure S9(B)).…”

“…The copolymerization of HBVE (0.50 M) and CL (0.50 M) with EtSO 3 H, which was studied previously 25 and is considered a standard system in this section, was conducted in CH 2 Cl 2 at 30 C. Both monomers were simultaneously consumed to yield a polymer with an M n value of 3.8 Â 10 3 (Figure 1(A), black), and the average number of units per block was calculated to be 1.0/5.7 for HBVE/CL. However, to achieve alternating copolymer synthesis upon heating, more frequent crossover reactions from cyclic ester to cyclic acetal are needed.…”

“…Previously, we demonstrated that the cationic ringopening copolymerization of 2-methyl-1,3-dioxepane (MDOP) (generated via the isomerization of 4-hydroxybutyl vinyl ether (HBVE) in the very early stages of polymerization (Scheme 1(B))) and ε-caprolactone (CL) with EtSO 3 H produced copolymers with a wide variety of compositions and sequences (Scheme 1(A)). 25 Polymerization was initiated via the reaction of an MDOP monomer with protonated MDOP, forming a compound with an oxonium ion and a hydroxy group at each end. Starting from this compound, the propagation of MDOP and CL proceeded via the active chain end (ACE) 26 and activated monomer (AM) 27 mechanisms, respectively, (Scheme 1(A)(I) and (II)).…”

“…An alternating copolymer was synthesized via the preparation of a copolymer with no CL homosequences (prepolymer) and the subsequent removal of all the MDOP homosequences (Scheme S1). 25 The prepolymer was obtained by copolymerization with an excess of MDOP relative to CL. Subsequently, the pressure of the system was reduced by a vacuum pump, which induced the transformation of the prepolymer to an alternating copolymer.…”

Hybrid Polymerization of Reversible Complexation Mediated Polymerization (RCMP) and Reversible Addition–Fragmentation Chain-Transfer (RAFT) Polymerization

“…In recent years, multiple polymerization systems including tandem polymerization, orthogonal polymerization, and hybrid polymerization have attracted more and more attention. − In such a complex system, the polymerization is controlled by more than one polymerization mechanism, which may combine the merits of different polymerization mechanisms and thus broaden the potential of the traditional polymerization techniques. For example, Matyjaszewski et al successfully realized the orthogonal polymerization of ATRP and RAFT by using dithioester as both the initiator for ATRP and chain-transfer agent (CTA) for RAFT polymerization .…”