The mechanism of the recently reported photocontrolled cationic polymerization of vinyl ethers was investigated using a variety of catalysts and chain-transfer agents (CTAs) as well as diverse spectroscopic and electrochemical analytical techniques. Our study revealed a complex activation step characterized by one-electron oxidation of the CTA. This oxidation is followed by mesolytic cleavage of the resulting radical cation species, which leads to the generation of a reactive cation–this species initiates the polymerization of the vinyl ether monomer–and a dithiocarbamate radical that is likely in equilibrium with the corresponding thiuram disulfide dimer. Reversible addition–fragmentation type degenerative chain transfer contributes to the narrow dispersities and control over chain growth observed under these conditions. Finally, the deactivation step is contingent upon the oxidation of the reduced photocatalyst by the dithiocarbamate radical concomitant with the production of a dithiocarbamate anion that caps the polymer chain end. The fine-tuning of the electronic properties and redox potentials of the photocatalyst in both the excited and the ground states is necessary to obtain a photocontrolled system rather than simply a photoinitiated system. The elucidation of the elementary steps of this process will aid the design of new catalytic systems and their real-world applications.

Herein, we report the transformation of bmonomethyl itaconate, an inexpensive and biorenewable alternative to petroleum feedstocks, to the high-value monomer amethylene-c,c-dimethyl-c-butyrolactone (Me 2 MBL) through a selective addition strategy. This strategy is also applied to the synthesis of a-methylene-c-butyrolactone (MBL, tulipalin A), a monomer that can be polymerized to give materials with desirable properties (high decomposition temperature, glass transition temperature, and refractive index). Subsequent polymerization of both Me 2 MBL and MBL through reversible addition-fragmentation chain-transfer polymerization generates well-defined poly(Me 2 MBL) and poly(MBL) (PMBL). Physical characterization of poly(Me 2 MBL) shows good physical properties comparable with known PMBL materials.

The synthesis and processing of most thermoplastics and thermoset polymeric materials rely on energy-inefficient and environmentally burdensome manufacturing methods. Frontal polymerization is an attractive, scalable alternative due to its exploitation of polymerization heat that is generally wasted and unutilized. The only external energy needed for frontal polymerization is an initial thermal (or photo) stimulus that locally ignites the reaction. The subsequent reaction exothermicity provides local heating; the transport of this thermal energy to neighboring monomers in either a liquid or gel-like state results in a self-perpetuating reaction zone that provides fully cured thermosets and thermoplastics. Propagation of this polymerization front continues through the unreacted monomer media until either all reactants are consumed or sufficient heat loss stalls further reaction. Several different polymerization mechanisms support frontal processes, including free-radical, cat-or anionic, amine-cure epoxides, and ring-opening metathesis polymerization. The choice of monomer, initiator/catalyst, and additives dictates how fast the polymer front traverses the reactant medium, as well as the maximum temperature achievable. Numerous applications of frontally generated materials exist, ranging from porous substrate reinforcement to fabrication of patterned composites. In this review, we examine in detail the physical and chemical phenomena that govern frontal polymerization, as well as outline the existing applications.

Recent advances in frontal ring-opening metathesis polymerization (FROMP) have enabled the rapid and energy-efficient fabrication of high-performance and thermoset materials. The second-generation Grubbs complex [(SIMes)­RuCl2(PCy3)] is the most exploited FROMP catalyst to date despite the availability of several other commercial variants. Changes in the nature of the catalytic species may provide potential advantages for controlling FROMP conditions, polymer microstructure, and monomer selectivities. Herein, nine catalysts are employed for the FROMP of dicyclopentadiene and ethylidene norbornene mixtures to generate copolymers, and the associated polymerization process parameters (front temperatures and velocities) are measured for each system. Dynamic mechanical analysis, differential scanning calorimetry, and quasistatic tensile testing reveal significant differences in the mechanical and material properties of the resultant polymers.

In this work, a simple method is reported for control over initiation in frontal ring-opening metathesis polymerization (FROMP). This noncontact approach uses 375 nm light to excite Grubbs’ second-generation catalyst in the presence of a phosphite inhibitor. Photoinitiated FROMP of dicylcopentadiene (DCPD) displays a similar cure profile to that of its thermally initiated counterpart, yielding a robust polymer with high glass transition temperature. Furthermore, this system is applied to enhance reaction rates in conventional ring-closing metathesis reactions.

Frontal ring-opening metathesis polymerization (FROMP) catalyzed by Grubbs-type Ru complexes enables new, rapid, and energy-efficient syntheses of high-performance, structural plastics. Ideal catalysts survive the extended time periods associated with resin preparation, storage, and transportation. Current catalysts, however, induce premature polymerization within hours to days under ambient conditions. In this work, a thermally latent bis-N-heterocyclic carbene complex provides exceedingly robust resins, which are viable for 8 weeks. When mixed with CuI coreagents, precatalyst activation primes the system for rapid reactivity after thermal initiation. In this study, more than 40 dual-component formulations successfully catalyzed FROMP of dicyclopentadiene. The polymerization process parameters (front temperatures and velocities), resin storability, and resultant polymer properties (e.g., T g) were determined for each composition. Intriguingly, the Cu to Ru ratio dramatically impacts the observed frontal velocity and temperature, as well as the polymer glass-transition temperature; slower, colder reaction fronts result from formulations with large Cu to Ru ratios. The resultant polymers display lower T g values. Mechanistic analysis of a related model system demonstrated that an excess Cu reagent decreases the activation and polymerization rates.

Herein, we report the development of a photoredox-initiated frontal ring-opening metathesis polymerization (FROMP) chemical system. We found that a ruthenium-based, bis-N-heterocyclic carbene metathesis precatalyst was activated with 9-mesityl-10-phenylacridindium tetrafluoroborate, copper­(II) triflate, and a 455 nm light source. This chemistry was used to initiate the FROMP of dicyclopentadiene; once initiated, the heat released from the polymerization sustained a well-controlled reaction front. Variation in copper or metathesis precatalyst loading yielded front speeds ranging from 0.15 to 0.43 mm s–1 and front temperatures ranging from 140 to 205 °C. While the glass transition temperatures of the resultant polymers are lower than those derived with Grubbs’ second-generation catalyst, this chemical system provides extended pot life.

The synthesis and processing of thermoplastics and thermoset polymeric materials rely on energy-inefficient and environmentally burdensome manufacturing methods. Frontal polymerization has emerged as an attractive, scalable alternative due to its exploitation of the heat of polymerization, which generally is wasted as unutilized heat-loss. The only external energy needed for frontal polymerization is an initial thermal (or photo) stimulus that locally ignites the reaction. The subsequent reaction exothermicity provides local heating; the transport of this thermal energy to neighboring monomers in either a liquid or gel-like state results in a self-perpetuating reaction zone that provides fully-cured polymeric thermosets and thermoplastics. Propagation of this polymerization front continues through the unreacted monomer media until either all reactants are consumed or sufficient heat-loss stalls further reaction. Several different polymerization mechanisms support frontal processes, including free-radical, cat- or anionic, amine-cure epoxides, and ring-opening metathesis polymerization. The choice of monomer, initiator/catalyst, and additives dictates how fast the polymer front traverses the reactant medium, as well as the maximum temperature achievable. Frontal polymerization enables the preparation of moldable thermoplastics derived from linear polymers. When crosslinking is involved, polymeric networks (e.g., rigid thermosets) are obtainable through a related process best described as a frontal curing reaction. Numerous applications of frontally-generated polymers and thermosets exist, ranging from the reinforcement of porous substrates to the design of patterned composite materials. In this review, we examine in detail the physical and chemical phenomena that govern frontal polymerization, as well as outline the existing applications.