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

Organoboron-mediated polymerizations

The scientific community has witnessed extensive developments and applications of organoboron compounds as synthetic elements and metal-free catalysts for the construction of small molecules, macromolecules, and functional materials over the last two decades. This review highlights the achievements of organoboron-mediated polymerizations in the past several decades alongside the mechanisms underlying these transformations from the standpoint of the polymerization mode. Emphasis is placed on free radical polymerization, Lewis pair polymerization, ionic (cationic and anionic) polymerization, and polyhomologation. Herein, alkylborane/O2 initiating systems mediate the radical polymerization under ambient conditions in a controlled/living manner by careful optimization of the alkylborane structure or additives; when combined with Lewis bases, the selected organoboron compounds can mediate the Lewis pair polymerization of polar monomers; the bicomponent organoboron-based Lewis pairs and bifunctional organoboron-onium catalysts catalyze ring opening (co)polymerization of cyclic monomers (with heteroallenes, such as epoxides, CO2, CO, COS, CS2, episulfides, anhydrides, and isocyanates) with well-defined structures and high reactivities; and organoboranes initiate the polyhomologation of sulfur ylides and arsonium ylides providing functional polyethylene with different topologies. The topological structures of the produced polymers via these organoboron-mediated polymerizations are also presented in this review mainly including linear polymers, block copolymers, cyclic polymers, and graft polymers. We hope the summary and understanding of how organoboron compounds mediate polymerizations can inspire chemists to apply these principles in the design of more advanced organoboron compounds, which may be beneficial for the polymer chemistry community and organometallics/organocatalysis community.

Polythioethers bearing side groups for efficient degradation by E1cB reaction: reaction design for polymerization and main-chain scission

We have previously reported the polycondensation by the tandem reactions of dithiols and α-(bromomethyl)acrylates, consisting of conjugate substitution (SN2' reaction) and conjugate addition (Michael addition) reactions. The resulting polythioethers underwent a main-chain scission (MCS) by E1cB reaction, which is the reverse reaction of conjugate addition, although it was not quantitative due to the equilibrium. Herein, the modification of the structures of polythioethers led to irreversible MCS, whereby the β-positions of ester moieties were substituted with a phenyl group. This slight modification in the polymer structure influenced the monomer structures and polymerization mechanisms. The understanding of reaction mechanisms by model reactions was required to obtain high molecular weights of polythioethers. It was clarified that the consequent additions of 1,4-diazabicyclo[2.2.2]octane (DABCO), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), and PBu3 were effective to achieve high molecular weight. The resulting polythioethers decomposed by irreversible MCS via E1cB reaction with DBU.

1,3-Pentadiene-Assistant Living Anionic Terpolymerization: Composition Impact on Kinetics and Microstructure Sequence Primary Analysis

The combination of a living anionic technology and a unique alternating strategy provided an exciting opportunity to prepare novel and well-defined poly(1,3-pentadiene-co-syrene-co-1,1-diphenylethylene) resins consisting of three alternating sequences of modules (A/B/C zwitterions). "A" being Styrene (St)/1,3-pentadiene (PD), "B" being diphenylethylene (DPE)/PD, "A" being DPE/St, respectively, A wide composition range of new polyolefin resins, i.e., poly (A-co-B), poly (A-co-C), and poly (B-co-C), with controlled molecular weight and very narrow molecular weight and composition distributions have been prepared by a one-pot living characteristic method. In the section of kinetic analysis, the terpolymer yields and kinetic parameters were strongly dependent on the feed/comonomer ratio as well as the content of the alternating structure. The competition copolymerization behaviors of A/B, B/C, and A/C were studied in detail in this work. By contrast, the microstructure and the thermal property of the resulting terpolymer were investigated via Nuclear magnetic resonance (NMR) and Differential scanning calorimetry (DSC) analysis. The results of 1H NMR tracking the change of [Aromatic ring]/[C=C] value indicated the distinctive copolymer-ization behavior of the selective "alternating-modules". The glass transition temperature (T_g) was very sensitive to the terpolymer composition. By contrast to poly(A-ran-B) with only one obvious T_g, there were two T_gs in the A/C and B/C copolymerization cases. Moreover, the desirable high T_g ~ 140 °C resin was limited to the terpolymers with up to 50 mol % DPE. Finally, the "ABC-X" mechanism was proposed to interpret the unique terpolymerization behavior, which belongs to the classical "bond-forming initiation" theory.

Recent Advances in Sequence-Controlled Ring-Opening Copolymerizations of Monomer Mixtures

Transforming renewable resources into functional and degradable polymers is driven by the ever-increasing demand to replace unsustainable polyolefins. However, the utility of many degradable homopolymers remains limited due to their inferior properties compared to commodity polyolefins. Therefore, the synthesis of sequence-defined copolymers from one-pot monomer mixtures is not only conceptually appealing in chemistry, but also economically attractive by maximizing materials usage and improving polymers' performances. Among many polymerization strategies, ring-opening (co)polymerization of cyclic monomers enables efficient access to degradable polymers with high control on molecular weights and molecular weight distributions. Herein, we highlight recent advances in achieving one-pot, sequence-controlled polymerizations of cyclic monomer mixtures using a single catalytic system that combines multiple catalytic cycles. The scopes of cyclic monomers, catalysts, and polymerization mechanisms are presented for this type of sequence-controlled ring-opening copolymerization.

Hybrid copolymerization of acrylate and thiirane monomers mediated by trithiocarbonate

The composition and structure of polymers have great influence on their performances.

Dependence of the liquid crystalline properties on the exactly controlled single-site functionalized density of mesogens focused on the alternating copolymer model

Fluorinated liquid crystal polymers (FLCPs) with an alternating sequence of mesogenic moieties within their backbones were precisely constructed.

Magnesium bromide (MgBr2) as a catalyst for living cationic polymerization and ring-expansion cationic polymerization

Magnesium bromide (MgBr₂) was found to be an effective catalyst for the ring-expansion cationic polymerizations of isobutyl vinyl ether (IBVE) initiated by a “cyclic” hemiacetal ester (HAE) bond-based initiator leading to the syntheses of cyclic poly(IBVE)s.

tert-Butyl Esters as Potential Reversible Chain Transfer Agents for Concurrent Cationic Vinyl-Addition and Ring-Opening Copolymerization of Vinyl Ethers and Oxiranes

tert-Butyl esters are demonstrated to function as chain transfer agents (CTAs) in the cationic copolymerization of vinyl ether (VE) and oxirane via concurrent vinyl-addition and ring-opening mechanisms. In the copolymerization of isopropyl VE and isobutylene oxide (IBO), the IBO-derived propagating species reacts with tert-butyl acetate to generate a copolymer chain with an acetoxy group at the ω-end. This reaction liberates a tert-butyl cation; hence, a polymer chain with a tert-butyl group at the α-end is subsequently generated. Other tert-butyl esters also function as CTAs, and the substituent attached to the carbonyl group affects the chain transfer efficiency. In addition, ethyl acetate does not function as a CTA, which suggests the importance of the liberation of a tert-butyl cation for the chain transfer process. Chain transfer reactions by tert-butyl esters potentially occur reversibly through the reaction of the propagating cation with the ester group at the ω-end of another chain.

Cationic Ring-Opening Co- and Terpolymerizations of Lactic Acid-Derived 1,3-Dioxolan-4-ones with Oxiranes and Vinyl Ethers: Nonhomopolymerizable Monomer for Degradable Co- and Terpolymers

Lactic acid-derived 1,3-dioxolan-4-ones (DOLOs), which do not undergo cationic homopolymerization, were demonstrated to yield copolymers with oxiranes through a cationic copolymerization via frequent crossover reactions. Acetal and ester moieties were generated in the main chain of the copolymers via crossover reactions from DOLO to oxirane and from oxirane to DOLO, respectively, which is in contrast to the unsuccessful generation of hemiacetal ester moieties in the homopropagation of DOLO. In addition, the terpolymerization of DOLO, oxirane, and vinyl ether (VE) proceeded via crossover reactions, while copolymers could not be generated from VE and DOLO in the absence of oxirane. The obtained co- and terpolymers could be degraded under acidic conditions due to the acetal moieties in the main chain. The strategy devised in this study shows a promising avenue for employing plant-derived "nonhomopolymerizable" compounds as building blocks for the synthesis of degradable co- and terpolymers with general-purpose monomers.

Polymerization of Polar Monomers Mediated by Main-Group Lewis Acid-Base Pairs

The development of new or more sustainable, active, efficient, controlled, and selective polymerization reactions or processes continues to be crucial for the synthesis of important polymers or materials with specific structures or functions. In this context, the newly emerged polymerization technique enabled by main-group Lewis pairs (LPs), termed as Lewis pair polymerization (LPP), exploits the synergy and cooperativity between the Lewis acid (LA) and Lewis base (LB) sites of LPs, which can be employed as frustrated Lewis pairs (FLPs), interacting LPs (ILPs), or classical Lewis adducts (CLAs), to effect cooperative monomer activation as well as chain initiation, propagation, termination, and transfer events. Through balancing the Lewis acidity, Lewis basicity, and steric effects of LPs, LPP has shown several unique advantages or intriguing opportunities compared to other polymerization techniques and demonstrated its broad polar monomer scope, high activity, control or livingness, and complete chemo- or regioselectivity, as well as its unique application in materials chemistry. These advances made in LPP are comprehensively reviewed, with the scope of monomers focusing on heteroatom-containing polar monomers, while the polymerizations mediated by main-group LAs and LBs separately that are most relevant to the LPP are also highlighted or updated. Examples of applying the principles of the LPP and LP chemistry as a new platform for advancing materials chemistry are highlighted, and currently unmet challenges in the field of the LPP, and thus the suggested corresponding future research directions, are also presented.

Exploring Strategies To Bias Sequence in Natural and Synthetic Oligomers and Polymers

Millions of years of biological evolution have driven the development of highly sophisticated molecular machinery found within living systems. These systems produce polymers such as proteins and nucleic acids with incredible fidelity and function. In nature, the precise molecular sequence is the factor that determines the function of these macromolecules. Given that the ability to precisely define sequence emerges naturally, the fact that biology achieves unprecedented control over the unit sequence of the monomers through evolved enzymatic catalysis is incredible. Indeed, the ability to engineer systems that allow polymer synthesis with precise sequence control is a feat that technology is yet to replicate in artificial synthetic systems. This is the case because, without access to evolutionary control for finely tuned biological catalysts, the inability to correct errors or harness multiple competing processes means that the prospects for digital control of polymerization have been firmly bootstrapped to biological systems or limited to stepwise synthetic protocols. In this Account, we give an overview of strategies that have been used over the last 5 years in efforts to program polymer synthesis with sequence control in the laboratory. We also briefly explore how the use of robotics, algorithms, and stochastic chemical processes might lead to new understanding, mechanisms, and strategies to achieve full digital control. The aim is to see whether it is possible to go beyond bootstrapping to biological polymers or stepwise chemical synthesis. We start by describing nonenzymatic techniques used to obtain sequence-controlled natural polymers, a field that lends itself to direct application of insights gleaned from biology. We discuss major advances, such as the use of rotaxane-based molecular machines and templated approaches, including the utilization of biological polymers as templates for purely synthetic chains. We then discuss synthetic polymer chemistry, whose array of techniques allows the production of polymers with enormous structural and functional diversity, but so far with only limited control over the unit sequence itself. Synthetic polymers can be subdivided into multiple classes depending on the nature of processes used to synthesize them, such as by addition or condensation. Consequently, varied approaches for sequence control have been demonstrated in the area, including but not limited to click reactions, iterative solid-phase chemistry, and exploiting the chemical affinity of the monomers themselves. In addition to those, we highlight the importance of environmental bias in possible control of polymerization at the single-unit level, such as using catalyst switching or external stimuli. Even the most successful experimental sequence control approach needs appropriate tools to verify its scope and validity; therefore, we devote part of the present Account to possible analytical approaches to sequence readout, starting with well-established tandem mass spectrometry techniques and touching on those more applicable to specific classes of processes, such as diffusion-ordered NMR spectroscopy. Finally, we discuss progress in modeling and automation of sequence-controlled polymers. We postulate that developments in analytical chemistry, bioinformatics, and computer modeling will lead to new ways of exploring the development of new strategies for the realization of sequence control by means of sequence bias. This is the case because treating the assembly of polymers as a network of chemical reactions will enable the development of control strategies that can bias the outcome of the polymer assembly. The grand aim would be the synthesis of complex polymers in one step with a precisely defined digital sequence.

Synthesis of sequence-determined bottlebrush polymers based on sequence determination in living anionic copolymerization of styrene and dimethyl(4-(1-phenylvinyl)phenyl)silane

Sequence-determined bottlebrush polymers are precisely, efficiently and conveniently synthesized.

Sequence-regulated vinyl copolymers with acid and base monomer units via atom transfer radical addition and alternating radical copolymerization

Sequence-regulated vinyl copolymers with acid and base monomer units were prepared via atom transfer radical addition and alternating radical copolymerization.