Exclusive One-Way Cycle Sequence Control in Cationic Terpolymerization of General-Purpose Monomers via Concurrent Vinyl-Addition, Ring-Opening, and Carbonyl-Addition Mechanisms

Cationic terpolymerization of vinyl ether (VE), oxirane, and ketone successfully proceeded via unprecedented concurrent vinyl-addition, ring-opening, and carbonyl-addition mechanisms. In particular, the use of cyclohexene oxide as an oxirane resulted in terpolymerization via an exclusive one-way cycle, i.e., the reactions occurred only in the VE → oxirane, oxirane → ketone, and ketone → VE directions. Terpolymers that have repeating units of (VE_∼2–oxirane_∼2–ketone)_<i>n</i> were obtained under appropriate conditions. In addition, no two-monomer combination achieved efficient copolymerization, which suggests that three specific types of crossover reactions are required for successful terpolymerization. The presence of a ketone, a compound that has rarely been employed as a monomer, is indispensable for a one-way cycle: terpolymerization also proceeded with an aliphatic aldehyde but resulted in two-way crossover reactions at the aldehyde-derived propagating ends

Major Progress in Catalysts for Living Cationic Polymerization of Isobutyl Vinyl Ether: Effectiveness of a Variety of Conventional Metal Halides

Cationic polymerization of isobutyl vinyl ether (IBVE) was examined using a variety of metal halides. In the presence of an appropriate added base, ester or ether, the living polymerization of IBVE proceeded for almost all Lewis acids (MCln; M: Fe, Ga, Sn, In, Zn, Al, Hf, Zr, Bi, Ti, Si, Ge, Sb) used in conjunction with an IBVE−HCl adduct in toluene at 0 °C. The difference in the polymerization activity of these Lewis acids was significant. As examples, polymerization with some acids, such as FeCl3, proceeded in the order of seconds, whereas it took more than a few weeks with others such as SiCl4 and GeCl4. The difference in activity is based on the strength of the interaction between the Lewis acid and the propagating end chloride anion and/or the basic carbonyl (or ether) oxygen atom of the added base, that is, the chlorophilic or oxophilic nature of each metal halide is a decisive factor. In addition, a suitable combination of a Lewis acid and an additive was indispensable for living polymerization. With metal pentachlorides, NbCl5 and TaCl5, addition of a salt (nBu4NCl) resulted in superior control of the reaction over that for addition of a base. Lewis acids for living cationic polymerization of vinyl ether were categorized into groups depending on the preferences for these additives

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

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

Chain Multiplying Controlled Cationic Polymerization of Isobutyl Vinyl Ether Using Pyrrole: Increment of Propagating Chains by Efficient “Initiator-Like” Transfer Agent

Cationic polymerization of isobutyl vinyl ether using pyrrole was examined with a variety of metal chlorides in the presence of a weak Lewis base. In conjunction with oxophilic acids such as ZrCl4, long-lived species were produced to yield polymers with narrow molecular weight distributions (MWDs) and number-average molecular weight (Mn) values based on the used amounts of pyrrole. Acid-trapping experiments using silyl ketene acetal indicated that pyrrole worked not as an initiator but as a transfer agent. The polymerization started from adventitious water, followed by the reactions between the propagating species and the 2- and 5-positions of pyrrole, accompanied by the generation of HCl. In addition to the propagation from the generated HCl, the produced pyrrole-bonded chain-end structures were also activated by the oxophilic chlorides to generate propagating carbocation via the abstraction of the isobutoxy group. As a result, the number of growing chains increased. Such transfer reactions occurred predominantly in the early stage of the polymerization stemming from the highly nucleophilic nature of pyrrole. Thus, the resulting polymers had expected Mn values and narrow MWDs as if pyrrole worked as an initiator

Living and Alternating Cationic Copolymerization of <i>o</i>‑Phthalaldehyde and Various Bulky Enol Ethers: Elucidation of the “Limit” of Polymerizable Monomers

Cationic copolymerization of various bulky enol ethers, which have been difficult to homopolymerize and/or copolymerize, was shown to proceed when o-phthalaldehyde (OPA) was used as a comonomer. A series of enol ethers with various substituents on the β-carbon was synthesized from aliphatic aldehydes and alcohols. The relationships between the structures of the enol ethers and the copolymerization behavior were systematically investigated. As a result, monomers with one or two methyl and/or primary alkyl groups on the β-carbon were found to undergo alternating copolymerization with OPA. Moreover, living cationic copolymerization of enol ethers and OPA yielded alternating copolymers under appropriate polymerization conditions. To elucidate the limit of polymerizable monomers, the copolymerization of very bulky enol ethers such as β-t-butyl- or norbornenylidene-type monomers with OPA was also examined. OPA was found to be copolymerizable even with such very bulky monomers, indicating that the unique reactivity of the OPA-derived propagating carbocation with small steric hindrance is the key factor for successful copolymerization

Alternating-like Cationic Copolymerization of Styrene Derivatives and Benzaldehyde: Precise Synthesis of Selectively Degradable Copoly(styrenes)

Cationic copolymerizations of styrene derivatives and benzaldehyde (BzA) were systematically investigated for the purpose of both achieving living alternating copolymerization and creating “degradable polystyrene derivatives”. As a result of the optimization of reaction conditions, the copolymerization of p-methylstyrene (pMeSt) and BzA was demonstrated to proceed in a living manner with GaCl3 as a catalyst in the presence of tetrahydropyran, yielding a copolymer with a nearly alternating sequence and a controllable molecular weight. sec-Benzylic ether structures were generated in the main chain of the copolymer due to the crossover reactions between the monomers; hence, the copolymer was degraded into low-molecular-weight compounds under acidic conditions. In addition, the copolymer exhibited a glass transition temperature and degradation temperature that were comparable to those of the pMeSt homopolymer

ABC-Type Periodic Terpolymer Synthesis by a One-Pot Approach Consisting of Oxirane- and Carbonyl-Derived Cyclic Acetal Generation and Subsequent Living Cationic Alternating Copolymerization with a Vinyl Monomer

A one-pot synthesis of ABC-type periodic terpolymers with controllable molecular weights was achieved via an elaborately designed method consisting of sequence-programmed cyclic monomer synthesis and living cationic copolymerization of this cyclic monomer with a vinyl monomer. In this method, a cyclic acetal generated by a selective and quantitative Lewis acid-catalyzed cyclodimerization reaction of an oxirane and a carbonyl compound was subjected to subsequent copolymerization without any isolation or purification. Alternating copolymerization of the cyclic acetal and vinyl ether (VE) proceeded, yielding an ABC-type periodic terpolymer composed of oxirane, a carbonyl compound, and VE. Interestingly, the copolymerization proceeded in a living manner, which allowed simultaneous control of the molecular weight, molecular weight distribution, and chain ends in addition to the periodic sequence. Moreover, the terpolymers could be degraded by acid due to the periodically located acetal moieties. The use of various monomers also produced ABC-type sequence terpolymers. ABC-b-ABD-type periodic block quaterpolymers were synthesized by the sequential addition of vinyl monomers during the living copolymerization. These results surely provide a simple and efficient approach for the design of monomer sequences, polymer lengths, and chain ends in synthetic polymers

Cationic Copolymerization of <i>o</i>‑Phthalaldehyde and Vinyl Monomers with Various Substituents on the Vinyl Group or in the Pendant: Effects of the Structure and Reactivity of Vinyl Monomers on Copolymerization Behavior

Cationic copolymerization of o-phthalaldehyde with various styrene derivatives and vinyl ethers was investigated, particularly focusing on the relationship between the structure and reactivity of vinyl monomers and the copolymerization behavior. In the case of styrene derivatives, the existence of α- or β-substituents (or cyclic structures) on the vinyl group was indispensable for efficient crossover reactions. Styrene derivatives with no substituents on the vinyl group did not undergo copolymerization with OPA. In contrast, various vinyl ethers other than α-substituted vinyl ethers underwent copolymerization with OPA. To realize alternating-like copolymerization, the use of vinyl monomers that have both large steric hindrance and highly reactive β-carbon was effective. In addition, copolymers that are degradable into low-molecular-weight compounds were successfully obtained from styrene derivatives

Alternating Degradable Copolymers of an Ionic Liquid-Type Vinyl Ether and a Conjugated Aldehyde: Precise Synthesis by Living Cationic Copolymerization and Dual Rare Thermosensitive Behavior in Solution

Alternating degradable copolymers consisting of a vinyl ether (VE) unit with an ionic liquid-type moiety and a conjugated aldehyde were synthesized by living cationic copolymerization and subsequent modification. The alternating copolymers exhibited upper critical solution temperature (UCST)-type phase separation behavior in water like the corresponding ionic liquid-type VE homopolymers. In addition, the copolymers exhibited lower critical solution temperature (LCST)-type phase separation behavior in acetone. Both the UCST- and LCST-type thermosensitive behaviors were most likely induced by the interpolymer electrostatic interactions among the pendant ionic liquid-type groups. The alternating copolymers were completely degraded by acid hydrolysis because of the cleavage of the acetal linkages, generated through the crossover reaction from VE to aldehyde, in the main chain

Living Cationic Polymerization of Vinyl Ether with Methanol/Metal Chloride Initiating Systems: Relationship between Polymerization Behavior and the Nature of Lewis Acids

Cationic polymerization of vinyl ether (VE) was examined with methanol/metal chloride initiating systems instead of those with the conventionally used VE−protonic acid adduct initiators. With MoCl5, a highly oxophilic metal halide, the polymerization of isobutyl VE (IBVE) using methanol proceeded in a living fashion in the presence of ethyl acetate, although the reaction was not controlled in conjunction with IBVE−HCl adduct. Furthermore, in the methanol-initiated polymerization, the Lewis acids used could be classified mainly into three categories in terms of polymerization behavior: those producing long-lived species (e.g., MoCl5, NbCl5, and ZrCl4), those inducing uncontrolled polymerization (e.g., GaCl3 and FeCl3), or those showing no activity at all (e.g., ZnCl2 and InCl3). The matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI−TOF−MS) spectra of the product polymers and the direct 1H NMR analysis of the reaction mixtures revealed that the controlled polymerization starts with an exchange reaction between the methoxy group of methanol and the chloride anion of a metal chloride to generate HCl, the true initiator. With some Lewis acids such as NbCl5, the polymerization proceeds only via thus produced C−Cl terminals as dormant species, whereas rapid equilibrium between the C−Cl ends and acetal structures is involved in the propagation reaction with other Lewis acids such as ZrCl4. The trends in Lewis acids correlated with the nature of their central metals, i.e., their “oxophilicity” and “chlorophilicity”