High Molecular Weight Semicrystalline Substituted Polycyclohexene From Alternating Copolymerization of Butadiene and Methacrylate and Its Ambient Depolymerization

Cyclohexene cannot be polymerized via ring-opening polymerization under any conditions due to its lack of ring strain. A hypothetical polycyclohexene would therefore have a strong thermodynamic driving force to depolymerize to monomer if a metathesis catalyst were provided while otherwise having thermal and hydrolytic stability under normal conditions because of its hydrocarbon backbone. We envisioned access to this otherwise unattainable family of polymers via the alternating polymerization of a diene and an alkene. Ethyl aluminum chloride was found to promote highly alternating polymerization of butadiene and methacrylate when radically initiated at room temperature, resulting in formal polycyclohexene structures. Ultrahigh molecular weight (up to 1750 kDa) polymers can be synthesized at the decagram scale in high monomer conversions. The resulting presumably atactic copolymers exhibited semicrystallinity, leading to high toughness. In the presence of a small amount of the Grubbs catalyst, the generated polycyclohexene can be fully depolymerized at ambient temperatures into pure constituent cyclohexene. The strategy of using orthogonal chemistry for the polymerization and depolymerization processes allows access to polymer structures with subambient ceiling temperatures without using ultralow temperature synthesis or relying on the monomer-polymer equilibrium.

Recent Developments on Cationic Polymerization of Vinyl Ethers

In recent times, the evolution of cationic polymerization has taken a multidirectional approach, with the development of cationic reversible addition-fragmentation chain transfer (RAFT) polymerization. In contrast to the conventional cationic polymerization methods, which were typically carried out under inert atmospheres and low temperatures, various novel polymerization techniques have been developed where the reactions are carried out in open air, operate at room temperature, are cost-effective, and are environmentally friendly. Besides, several external stimuli, such as heat, light, chemicals, electrical potential, etc. have been employed to activate and control the polymerization process. It also enables the combination of cationic polymerization with other polymerization methods in a single reaction vessel, eliminating the necessity for isolation and purification during intermediate steps. In addition, significant advancements have been made through various modifications in catalyst systems, resulting in polymers with an exceptionally high level of stereoregularity. This review article comprehensively analyses the recent developments in cationic polymerization, encompassing their applications and offering insights into future perspectives.

Synthesis and Properties of Polyol Copolymer with Alternating Methacrylate-Vinyl Ether Backbone

Radical polymerization of a tailored diphenylsilane-bridged bi-functional monomer consisting of methacrylate and vinyl ether moieties is conducted in diluted monomer concentration, in which both two moieties are consumed at almost the same rate despite their huge difference in monomer reactivity ratio. The vinyl ether content in the backbone is quantified as 45% by ¹ H NMR after removal of the silane bridge. Since vinyl ether alone cannot be polymerized in such radical polymerization, it should be incorporated in an alternating fashion with methacrylate into the copolymer main chain. The cleavage of silane bridge also yields a series of polyol materials composed of ethylene glycol monovinyl ether (EGVE) and hydroxyethyl methacrylate (HEMA), and the EGVE content in the backbone can be regulated from 45% to 18% by increasing the bi-functional monomer concentration. Interestingly, although containing more than 50% HEMA units, the alternating copolymer exhibits new properties totally different from poly(HEMA), but more similar to poly(EGVE), e.g., good water solubility and a markedly low glass transition temperature (T_g ) of -31 °C, which is attributed to the major HEMA-EGVE repeating unit that replaced HEMA-HEMA consecutive segments so that the properties of poly(HEMA) such as 95 °C T_g are completely altered.

Hydrophilic bio-based polymers by radical copolymerization of cyclic vinyl ethers derived from glycerol

In this study, novel functional polymers were obtained by using glycerol as a bio-based precursor, which is abundant and inexpensive renewable feedstock with a polyol skeleton. Cyclic vinyl ethers with acetal linkage were derived from glycerol to yield well-defined copolymers by reversible addition-fragmentation chain transfer (RAFT) radical copolymerization with common vinyl monomers. The resulting acetal-containing copolymers could be hydrolyzed under acidic conditions to afford water-soluble functional polymers with pendent diols.

One-pot synthesis of structure-controlled temperature-responsive polymer gels

The simultaneous use of metal Lewis acids and photo-radical generators for dithioesters, which are the common dormant species for cationic and radical polymerization, made it possible to convert a cationic species into a radical by photoirradiation.

Photoswitching Cationic and Radical Polymerizations: Spatiotemporal Control of Thermoset Properties

The ability to fabricate polymeric materials with spatially controlled physical properties has been a challenge in thermoset manufacturing. To address this challenge, this work takes advantage of a photoswitchable polymerization that selectively incorporates different monomers at a growing chain by converting from cationic to radical polymerizations through modulation of the wavelength of irradiation. By regulating the dosage and wavelength of light applied to the system, the mechanical properties of the crosslinked material can be temporally and spatially tuned. Furthermore, photopatterning can be achieved both on the macroscale and the microscale, enabling precise spatial control of crosslink density that results in high-resolution control over mechanical properties.

Interconvertible Living Radical and Cationic Polymerization using a Dual Photoelectrochemical Catalyst

The necessity of well-tuned reactivity for successful controlled polymer synthesis often comes with the price of limited monomer substrate scope. We demonstrate here the on-demand interconversion between living radical and cationic polymerization using two orthogonal stimuli and a dual responsive single catalyst. The dual photo- and electrochemical reactivity of 10-phenylphenothiazine catalyst provides control of the polymer's molar mass and composition by orthogonally activating the common dormant species toward two distinct chemical routes. This enables the synthesis of copolymer chains that consist of radically and cationically polymerized segments where the length of each block is controlled by the duration of the stimulus exposure. By alternating the application of photochemical and electrochemical stimuli, the on-demand incorporation of acrylates and vinyl ethers is achieved without compromising the end-group fidelity or dispersity of the formed polymer. The results provide a proof-of-concept for the ability to substantially extend substrate scope for block copolymer synthesis under mild, metal-free conditions through the use of a single, dual reactive catalyst.

Controlling Polymer Molecular Weight Distribution through a Latent Mediator Strategy with Temporal Programming

Polymer molecular weight distribution (MWD) is a key parameter of polymers. Here we present a robust method for controlling polymer MWD in controlled cationic polymerizations. A latent mediator strategy was designed and combined with temporal programming to regenerate mediators at different times during polymerization. Both the breadths and shapes of MWD curves were tuned easily by adjusting an external light source. Bimodal, trimodal, and tetramodal distributions were obtained, and the breadths could be varied from 1.06 to 2.09. Polymers with different MWDs prepared by this method had good chain end fidelity, which was demonstrated with successful chain-extension experiments. In addition, the introduction of temporal programming with a computer-controlled single chip for the light source opened an avenue for the use of artificial intelligence in polymer synthesis.

Manganese-Catalyzed Batch and Continuous Flow Cationic RAFT Polymerization Induced by Visible Light

We present a robust manganese-catalyzed cationic reversible addition-fragmentation chain transfer (RAFT) polymerization induced by visible light. Well-defined poly(vinyl ether)s with controlled molecular weight and molecular weight distributions (MWDs) can be conveniently prepared at room temperature without monomer purification. The commercially available manganese carbonyl bromide is used as the photocatalyst for cationic RAFT polymerization. Moreover, this method has been further applied in both batch and continuous flow systems, providing a visible light induced flow cationic polymerization under mild conditions.

Hybridization of Step-/Chain-Growth and Radical/Cationic Polymerizations Using Thioacetals as Key Components for Triblock, Periodic and Random Multiblock Copolymers with Thermoresponsiveness

A novel strategy for synthesizing a series of multiblock copolymers is developed by combining radical/cationic step-growth polymerizations of dithiols and divinyl ethers and chain-growth cationic degenerative chain-transfer (DT) polymerizations of vinyl ethers using thioacetals as key components. The combination of radical step-growth polymerization and a cationic thiol-ene reaction or cationic step-growth polymerization enables the synthesis of a series of macro chain-transfer agents (CTAs) composed of poly(thioether) and thioacetal groups at different positions. The resulting products are 1) bifunctional macro CTAs with thioacetal groups at both chain ends, 2) periodic macro CTAs periodically having thioacetal groups in the main chain, and 3) random macro CTAs randomly having thioacetal groups in the main chain. Subsequently, the obtained macro CTAs are used for chain-growth cationic DT polymerization of methoxyethyl vinyl ether (MOVE) to result in 1) triblock, 2) periodic, and 3) random multiblock copolymers consisting of poly(thioether) and poly(MOVE) segments. All these triblock and multiblock copolymers composed of hydrophobic poly(thioether) and hydrophilic poly(MOVE) segments show an amphiphilic tendency to form characteristic micelles in aqueous solutions. In addition, due to the thermoresponsive poly(MOVE) segments, the obtained copolymers exhibit lower critical solution temperatures that depend on the segment sequences and lengths.

A photo–thermal dual-regulated latent monomer strategy for sequence control of polymers

A photo–thermal dual-regulated latent monomer was used for the synthesis of polymers with advanced sequence structures.

Progress and Perspectives Beyond Traditional RAFT Polymerization

The development of advanced materials based on well-defined polymeric architectures is proving to be a highly prosperous research direction across both industry and academia. Controlled radical polymerization techniques are receiving unprecedented attention, with reversible-deactivation chain growth procedures now routinely leveraged to prepare exquisitely precise polymer products. Reversible addition-fragmentation chain transfer (RAFT) polymerization is a powerful protocol within this domain, where the unique chemistry of thiocarbonylthio (TCT) compounds can be harnessed to control radical chain growth of vinyl polymers. With the intense recent focus on RAFT, new strategies for initiation and external control have emerged that are paving the way for preparing well-defined polymers for demanding applications. In this work, the cutting-edge innovations in RAFT that are opening up this technique to a broader suite of materials researchers are explored. Emerging strategies for activating TCTs are surveyed, which are providing access into traditionally challenging environments for reversible-deactivation radical polymerization. The latest advances and future perspectives in applying RAFT-derived polymers are also shared, with the goal to convey the rich potential of RAFT for an ever-expanding range of high-performance applications.

Mixed Polymer Brushes for "Smart" Surfaces

Mixed polymer brushes (MPBs) are composed of two or more disparate polymers covalently tethered to a substrate. The resulting phase segregated morphologies have been extensively studied as responsive "smart" materials, as they can be reversible tuned and switched by external stimuli. Both computational and experimental work has attempted to establish an understanding of the resulting nanostructures that vary as a function of many factors. This contribution highlights state-of-the-art MPBs studies, covering synthetic approaches, phase behavior, responsiveness to external stimuli as well as novel applications of MPBs. Current limitations are recognized and possible directions for future studies are identified.

Thiol-Ene Cationic and Radical Reactions: Cyclization, Step-Growth, and Concurrent Polymerizations for Thioacetal and Thioether Units

Thiol-ene cationic and radical reactions were conducted for 1:1 addition between a thiol and vinyl ether, and also for cyclization and step-growth polymerization between a dithiol and divinyl ether. p-Toluenesulfonic acid (PTSA) induced a cationic thiol-ene reaction to generate a thioacetal in high yield, whereas 2,2'-azobisisobutyronitrile resulted in a radical thiol-ene reaction to give a thioether, also in high yield. The cationic and radical addition reactions between a dithiol and divinyl ether with oxyethylene units yielded amorphous poly(thioacetal)s and crystalline poly(thioether)s, respectively. Under high-dilution conditions, the cationic and radical reactions resulted in 16- and 18-membered cyclic thioacetal and thioether products, respectively. Furthermore, concurrent cationic and radical step-growth polymerizations were realized using PTSA under UV irradiation to produce polymers having both thioacetal and thioether linkages in the main chain.

Unprecedented Sequence Control and Sequence-Driven Properties in a Series of AB-Alternating Copolymers Consisting Solely of Acrylamide Units

Herein, we report a method to synthesize a series of alternating copolymers that consist exclusively of acrylamide units. Crucial to realizing this polymer synthesis is the design of a divinyl monomer that contains acrylate and acrylamide moieties connected by two activated ester bonds. This design, which is based on the reactivity ratio of the embedded vinyl groups, allows a "selective" cyclopolymerization, wherein the intramolecular and intermolecular propagation are repeated alternately under dilute conditions. The addition of an amine to the resulting cyclopolymers afforded two different acryl amide units, i.e., an amine-substituted acryl amide and a 2-hydroxy-ethyl-substituted acryl amide in alternating sequence. Using this method, we could furnish ten types of alternating copolymers; some of these exhibit unique properties in solution and in the bulk, which are different from those of the corresponding random copolymers, and we attributed the differences to the alternating sequence.

Lewis acid-surfactant complex catalyzed polymerization in aqueous dispersed media: cationic or radical polymerization?

Evidence is presented that shows Lewis acid-surfactant complex catalyzed polymerization proceeds via a radical, not a cationic, mechanism.

Dual Polymerizations: Untapped Potential for Biomaterials

Block copolymers with unique architectures and those that can self-assemble into supramolecular structures are used in medicine as biomaterial scaffolds and delivery vehicles for cells, therapeutics, and imaging agents. To date, much of the work relies on controlling polymer behavior by varying the monomer side chains to add functionality and tune hydrophobicity. Although varying the side chains is an efficient strategy to control polymer behavior, changing the polymer backbone can also be a powerful approach to modulate polymer self-assembly, rigidity, reactivity, and biodegradability for biomedical applications. There are many developments in the syntheses of polymers with segmented backbones, but these developments are not widely adopted as strategies to address the unique constraints and requirements of polymers for biomedical applications. This review highlights dual polymerization strategies for the synthesis of backbone-segmented block copolymers to facilitate their adoption for biomedical applications.

Redox-Switchable Ring-Opening Polymerization with Ferrocene Derivatives

Switchable catalysts incorporate stimuli-responsive features and allow synthetic tasks that are difficult or impossible to accomplish in other ways. They mimic biological processes in that they can provide both spatial and temporal control, unlike most reactions promoted by human-made catalysts that usually occur according to carefully optimized conditions. In the area of switchable catalysis, redox-switchable ring-opening polymerization (ROP) has attracted much attention, emerging as a powerful strategy for the development of environmentally friendly biodegradable copolymers, especially those containing blocks with complementary properties. Controlling the sequence and regularity of each copolymeric building block can affect the material properties significantly since they are directly related to the respective microstructures. Such control can be exerted with a well-designed redox-switchable catalyst by timing the oxidation and reduction events. In highly selective systems, one form of the catalyst reacts with a monomer until the redox state of the catalyst is altered, at which point the altered state of the catalyst reacts with another monomer. The reaction time may be varied from one cycle to another to generate various designer multiblock copolymers. The first instance of redox-mediated ROP was described by N. Long and co-workers in 2006. This example, as well as many early reported redox-switchable catalysts, could only achieve an on/off switch of activity toward a single monomer or substrate. However, our efforts brought on a general strategy for designing redox-switchable metal complexes that can catalyze different reactions in two oxidation states. In recent years, our contributions to this research field led to the synthesis of several multiblock copolymers prepared from biorenewable resources. This Account provides an overview of reported redox-switchable polymerization catalysts that allow for complementary reactivity in different oxidation states and highlights the potential of this strategy in preparing biodegradable materials. First, we define the field of redox-switchable catalysis and illustrate the design and significance of our ferrocene-chelating ligands, in which the oxidation state of iron in ferrocene can control the reactivity of the resulting metal complexes remotely. Next, we illustrate recent advances in the synthesis of new biodegradable copolymers including (1) how to tune the activity of the ROP catalysts by exploring various metal centers and ferrocene-based ligand combinations; (2) how to synthesize new multiblock copolymers of cyclic esters, epoxides, and carbonates by redox-switchable ROP; and (3) how to understand the mechanism of these reactions by discussing both experimental and theoretical investigations. By the application and development of redox-switchable strategies, various novel materials and reactions can be expected in the future.

Hybrid copolymerization via mechanism interconversion between radical vinyl-addition and anion ring-opening polymerization

Here, we report a new hybrid copolymerization via an interconvertible living free radical and anion ring-opening polymerization mechanism, in which the copolymerization of cyclic monomers and vinyl-type monomers can be achieved.

Visible light controlled aqueous RAFT continuous flow polymerization with oxygen tolerance

A fast visible light controlled RAFT polymerization system without the prior removal of oxygen was successfully carried out in a continuous tubular reactor with water as a green solvent.

Investigation of a zirconium compound for redox switchable ring opening polymerization

Redox switchable catalysis with a ferrocene Schiff base zirconium complex allowed the synthesis of diblock and triblock copolymers.

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.

On Demand Switching of Polymerization Mechanism and Monomer Selectivity with Orthogonal Stimuli

The development of next-generation materials is coupled with the ability to predictably and precisely synthesize polymers with well-defined structures and architectures. In this regard, the discovery of synthetic strategies that allow on demand control over monomer connectivity during polymerization would provide access to complex structures in a modular fashion and remains a grand challenge in polymer chemistry. In this Article, we report a method where monomer selectivity is controlled during the polymerization by the application of two orthogonal stimuli. Specifically, we developed a cationic polymerization where polymer chain growth is controlled by a chemical stimulus and paired it with a compatible photocontrolled radical polymerization. By alternating the application of the chemical and photochemical stimuli the incorporation of vinyl ethers and acrylates could be dictated by switching between cationic and radical polymerization mechanisms, respectively. This enables the synthesis of multiblock copolymers where each block length is governed by the amount of time a stimulus is applied, and the quantity of blocks is determined by the number of times the two stimuli are toggled. This new method allows on demand control over polymer structure with external influences and highlights the potential for using stimuli-controlled polymerizations to access novel materials.

Diverse approaches to star polymers via cationic and radical RAFT cross-linking reactions using mechanistic transformation

Core cross-linked star polymers were synthesizedviacationic RAFT polymerization and three different approaches in combination with a radical RAFT mechanism.