Construction methodologies and sequence-oriented properties of sequence-controlled oligomers/polymers generated via radical polymerization

Switchable Radical Polymerization of α-Olefins via Remote Hydrogen Atom or Group Transfer for Enhanced Battery Performance

Introducing polar groups into non-polar polyolefins can significantly enhance the important properties of materials. However, producing polyolefin backbones consisting of polar blocks remains elusive, due to the substantial difference of reactivity ratios between polar and non-polar olefin monomers in radical polymerization or the poisoning of transition-metal catalysts by polar groups in coordination polymerization. Herein we present a practical way for the preparation of polyethylene-based polymers with distinct polar groups by radical polymerization of α-olefins. A strategy of switchable remote hydrogen atom or group transfer is devised, leading to a diverse range of AAB or ABC sequence-defined carbon-chain polyolefins. The utility of these polymers is demonstrated by using poly(ethyl 2-cyanohept-6-enoate) (P2) as an interphase layer material in anode-free Li metal battery, which effectively improves the cycling stability of the battery and indicates its potential in energy storage applications.

Alternating Graft Copolymer Carrying PLA Graft Chains at Every Other Unit: Sequence Impacts on Crystallization Behaviors

Alternating graft copolymers were precisely synthesized via selective cyclopolymerization of pendant-transformable divinyl monomer (1), post-polymerization modification via aminolysis with alkylamine, and ring-opening polymerization of l-lactide (LLA) from the hydroxy pendant group in alternating sequence. The poly(LLA) (PLLA) graft chain on the alternating copolymer gave a higher crystallization degree on the isothermal treatment than that on the random counterpart likely because of the periodic sequence. The comonomer pendant group from alkylamine in the aminolysis reaction in the alternating sequence affected the crystallization behaviors, and the oligoethylene glycol (OEG) group promoted the crystallization thanks to the larger free volume effect. As for the stereocomplex formation of the racemic mixture of enantiomeric PLLA and poly(d-lactide) (PDLA) chains, the alternating graft copolymer gave a higher degree of stereocomplex crystallization in the mixture with the enantiomer homopolymer than the random analogue.

Design of sec-Benzyl Vinyl Ethers toward the Synthesis of Alternating Copolymers Composed of Vinyl Alcohol and Vinyl Ether Units

In this work, we designed benzyl vinyl ethers carrying alkyl substituents at the benzyl position (i.e., sec-BnVEs) as bulky, reactive, and transformable monomers to realize the alternating cationic copolymerization with an alkyl vinyl ether (VE). In particular, the isopropyl substitution caused not only the bulkiness to suppress the successive propagation but also an enhancement of the vinyl group reactivity to promote crossover propagation with a less bulky VE comonomer. The isopropyl-substituted BnVE (iPr-BnVE) underwent living cationic alternating copolymerization with n-butyl VE (nBVE), and the alternating propagation was strongly suggested by the reactivity ratios. The subsequent deprotection of the sec-benzyl pendant afforded the vinyl alcohol (VA)-nBVE alternating copolymer, and the corresponding statistical copolymer was also synthesized by using the nonsubstituted monomer (BnVE) instead of iPr-BnVE. The alternating copolymer exhibited a higher glass transition temperature, which likely stems from the uniform and efficient hydrogen-bonding formation due to the periodic sequence.

Achieving high molecular weight alternating copolymers of 1-octene with methyl acrylate via Lewis acid catalyzed copolymerization

The radical (co)polymerization of long-chain α-olefins (C4+) to produce high molecular weight (Mw) polymers is of great importance. However, this process is currently faced with significant challenges due to the presence of less reactive allylic radicals during radical (co)polymerization, leading to oligomers or polymers with extremely low Mw (less than 1 × 104 g mol-1). Using copolymerization of 1-octene with methyl acrylate (MA) as a proof-of-concept for addressing this challenge, we present a feasible method for synthesizing high Mw α-olefin copolymers via scandium trifluoromethanesulfonate (Sc(OTf)3)-mediated radical copolymerization. In this case, copolymers of 1-octene and MA (poly(1-octene-alt-MA)) with a Mw exceeding 3 × 104 g mol-1 were successfully synthesized in the presence of Sc(OTf)3. Meanwhile, the presence of alternating 1-octene-MA sequential structures was observed. To further enhance the Mw of poly(1-octene-alt-MA), a difunctional comonomer, 1,7-octadiene, was introduced to copolymerize with 1-octene and MA. The results indicate that the incorporation of difunctional comonomer leads to a significant increase in the Mw of the copolymers synthesized. The addition of 1 mol% of 1,7-octadiene resulted in a copolymer with a remarkably high Mw of up to 13.45 × 104 g mol-1 while still maintaining a high degree of the alternating 1-octene-MA sequence (41%). The influence of polymerization parameters on the molecular weight were also investigated. Increasing the monomer concentration, reducing the dosage of initiator, and lowering the polymerization temperature have been found to be advantageous in enhancing the molecular weight. This approach has also been successfully applied to the synthesis of high molecular weight alternating copolymers of other long-chain α-olefins, including 1-hexene, 1-decene and 1-tetradecane, with methyl acrylate. In summary, this study provides a feasible method for converting "less activated" α-olefins into high Mw olefin copolymers. This approach holds significant potential for the production of value-added polyolefins, thus offering promising prospects for future applications.

Umpolung Isomerization in Radical Copolymerization of Benzyl Vinyl Ether with Pentafluorophenylacrylate Leading to Degradable AAB Periodic Copolymers

This study revealed that benzyl vinyl ether (BnVE) shows a peculiar isomerization propagation in its radical copolymerization with an electron-deficient acrylate carrying a pentafluorophenyl group (PFA). The co-monomer pair inherently exhibits the cross-over propagation feature due to the large difference in the electron density. However, the radical species of PFA was found to undergo a backward isomerization to the penultimate BnVE pendant giving a benzyl radical species prior to propagation with BnVE. The isomerization brings a drastic change in the character of the growing radical species from electrophilic to nucleophilic, and thus the isomerized benzyl radial species propagates with PFA. Consequently, the two monomers were consumed in the order AAB (A: PFA; B: BnVE) and the unique periodic consumption was confirmed by the pseudo-reactivity ratios calculated by the penultimate model: r11 =0.174 and r21 =6600 for PFA (M1 ) with BnVE (M2 ). The pentafluorophenyl ester groups of the resulting copolymers are transformed into ester and amide groups by post-polymerization alcoholysis and aminolysis modifications. The unique isomerization in the AAB sequence allowed the periodic introduction of a benzyl ether structure in the backbone leading to efficient degradation under acid conditions.

A Transformable and Bulky Methacrylate Monomer That Enables the Synthesis of an MMA-nBA Alternating Copolymer: Sequence-Dependent Self-Healing Properties

In this study, we designed a methacrylate molecule with an alkyl-substituted trichloro salicylic acid pendant as a transformable bulky monomer to enable the synthesis of an alternating copolymer of methyl methacrylate (MMA) and n-butyl acrylate (nBA). The adamantyl-substituted methacrylate monomer (1-Ad) showed very low homopolymerization propensity in radical polymerizations, but afforded the alternating copolymer with nBA via copolymerization. The 1-Ad units in the resultant copolymer were quantitatively and selectively transformed into MMA via transesterification with methanol to yield the alternating copolymer of MMA and nBA. Its alternating sequence was clearly demonstrated by a structural analysis via ¹³ C NMR spectroscopy as well as the low reactivity ratios for the 1-Ad and nBA pair. Finally, we verified the superior self-healing ability of the alternating copolymer compared to that of the corresponding 1 : 1 statistical copolymer.

Alternating Terpolymers through Cyclopolymerization and Subsequent Orthogonal Functionalization

A method for the synthesis of functionalized alternating copolymers by reversible deactivation radical polymerization was developed. Copolymerization by reversible addition-fragmentation chain transfer of hexenyl vinyl ether with a novel fluorinated divinyl monomer yields alternating cyclopolymers that can be chemoselectively modified by three distinct orthogonal functionalization reactions. Along the thiol-ene click reaction and amidation, a third functionalization was achieved via NHC-catalyzed transesterification or acylation resulting in a small library of ABC-type alternating terpolymers.

Homopolymer-block-Alternating Copolymers Composed of Acrylamide Units: Design of Transformable Divinyl Monomers and Sequence-Specific Thermoresponsive Properties

In this work, we synthesized an acrylamide-based terpolymer that is a block copolymer composed of an AB alternating copolymer and a C homopolymer. The key to the unprecedented achievement is rational design of an acrylate-acrylamide divinyl monomer carrying CF₃-substituted salicylic acid ester bonds (AAm-CF₃) to realize the efficient and selective cyclopolymerization as well as the quantitative transformation of the resultant cyclorepeating units. The selectivity in the cyclopolymerization and the pendant transformation ability were evaluated through reactivity ratios of the corresponding model monomers and quantitative aminolysis reactions of the model compound. The cyclopolymerization via the photoinduced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) process with a macrochain-transfer agent and subsequent aminolysis reaction afforded the homopolymer-block-alternating copolymer. The sequence-controlled terpolymer exhibited a very unique thermal response behavior in water that was strikingly different from the corresponding sequence-uncontrolled terpolymers, such as homopolymer-block-statistical copolymers and all statistical terpolymers, despite the fact that the structure cannot be distinguished by ¹H NMR.

Backbone-Degradable Polymers via Radical Copolymerizations of Pentafluorophenyl Methacrylate with Cyclic Ketene Acetal: Pendant Modification and Efficient Degradation by Alternating-Rich Sequence

This work deals with syntheses of backbone-degradable polymers via the radical copolymerization of pentafluorophenyl methacrylate (PFMA) with 5,6-benzo-2-methylene-1,3-dioxepane (BMDO), which undergoes ring-opening propagation to afford an ester-bonded backbone. The combination of the electron-deficient methacrylate with the electron-rich cyclic monomer allowed high crossover copolymerization, and the electronic effect was clarified by the comparison with the copolymerization of methyl methacrylate (MMA) and BMDO. The PFMA units of the resultant copolymer underwent quantitative alcoholysis or aminolysis transformation into methacrylate or methacrylamide units along with the pendant functionalization. The alternating-rich sequence was achieved by feeding an excess ratio of BMDO, which was supported by MALDI-TOF-MS of the copolymer obtained by the RAFT copolymerization. The methanolysis-transformed copolymer carrying MMA units was decomposed under basic condition, and the degradation efficiency was superior to that of the copolymer obtained via radical copolymerization of MMA with BMDO because of the alternating-rich sequence.

One-Pot Preparation of Methacrylate/Styrene Alternating Copolymers via Radical Copolymerization and Alcoholysis Modification: Sequence Impacts on Glass Transition Temperature

A series of methacrylate/styrene alternating copolymers were efficiently and systematically synthesized via alternating copolymerization of saccharin methacrylamide (1) with styrene and subsequent one-pot alcoholysis transformation with alcohols. The saccharin amide bond in 1 was stable enough that 1 was used as a bench-stable monomer, but the bond became reactive toward alcohols after the copolymerization. Thanks to the specific feature, the postpolymerization modification could be performed under mild conditions despite easy handling of the monomer. The quantitative transformation as well as the alternating sequence were certainly supported by ¹H NMR and MALDI-TOF-MS analyses. The alternating copolymers carrying relatively short alkyl pendants expressed lower glass transition temperatures than those of the statistical counterparts. Moreover, the alternating copolymerization was controlled via a RAFT polymerization system, affording a unique block copolymer composed of alternating copolymer segments.