Termination Mechanism of the Radical Polymerization of Acrylates

Beauty of Explicit Dispersity (Đ) Equations in Controlled Polymerizations

Dispersity (Đ) as a critical parameter indicates the level of uniformity of the polymer molar mass or chain length. In the past several decades, the development of explicit equations for calculating Đ experiences a continual revolution. This viewpoint tracks the historical evolution of the explicit equations from living to reversible-deactivation polymerization systems. Emphasis is laid on displaying the charm of explicit Đ equations in batch reversible-deactivation radical polymerization (RDRP), with highlights of the relevant elegant mathematical manipulations. Some representative emerging applications enabled by the existing explicit equations are shown, involving nitroxide-mediated polymerization (NMP), atom transfer radical polymerization (ATRP), and reversible addition-fragmentation chain transfer (RAFT) polymerization systems. Stemming from the several outlined challenges and outlooks, sustained concerns about the explicit Đ equations are still highly deserved. It is expected that these equations will continue to play an important role not only in traditional polymerization kinetic simulation and design of experiments but also in modern intelligent manufacturing of precision polymers and classroom education.

Additive manufacturing: Frameworks for chemical understanding and advancement in vat photopolymerization

Three-dimensional printing, or additive manufacturing (AM), is a broad term for a wide range of fabrication methods utilizing materials such as small-molecule, polymer, and metal feedstocks. Each method requires different chemical, physical, and engineering needs to be successful. This article will discuss some of the considerations for polymer-based AM methods. Ultimately, we focus on the chemistries of vat photopolymerization, in which light is used to cure a resin from liquid to solid, to provide an example of how chemical advancements have led to increased speed, resolution, and multimaterial printing capabilities not previously possible.

Capturing Free-Radical Polymerization by Synergetic Ab Initio Calculations and Topological Reactive Molecular Dynamics

Photocurable polymers are used ubiquitously in 3D printing, coatings, adhesives, and composite fillers. In the present work, the free radical polymerization of photocurable compounds is studied using reactive classical molecular dynamics combined with a dynamical approach of the nonequilibrium molecular dynamics (D-NEMD). Different concentrations of radicals and reaction velocities are considered. The mechanical properties of the polymer resulting from 1,6-hexanediol dimethacrylate systems are characterized in terms of viscosity, diffusion constant, and activation energy, whereas the topological ones through the number of cycles (polymer loops) and cyclomatic complexity. Effects like volume shrinkage and delaying of the gel point for increasing monomer concentration are also predicted, as well as the stress-strain curve and Young's modulus. Combining ab initio, reactive molecular dynamics, and the D-NEMD method might lead to a novel and powerful tool to describe photopolymerization processes and to original routes to optimize additive manufacturing methods relying on photosensitive macromolecular systems.

Connecting Gas-Phase Computational Chemistry to Condensed Phase Kinetic Modeling: The State-of-the-Art

In recent decades, quantum chemical calculations (QCC) have increased in accuracy, not only providing the ranking of chemical reactivities and energy barriers (e.g., for optimal selectivities) but also delivering more reliable equilibrium and (intrinsic/chemical) rate coefficients. This increased reliability of kinetic parameters is relevant to support the predictive character of kinetic modeling studies that are addressing actual concentration changes during chemical processes, taking into account competitive reactions and mixing heterogeneities. In the present contribution, guidelines are formulated on how to bridge the fields of computational chemistry and chemical kinetics. It is explained how condensed phase systems can be described based on conventional gas phase computational chemistry calculations. Case studies are included on polymerization kinetics, considering free and controlled radical polymerization, ionic polymerization, and polymer degradation. It is also illustrated how QCC can be directly linked to material properties.

Mapping the Pathway to Organocopper(II) Complexes Relevant to Atom Transfer Radical Polymerization

The rare organocopper(II) complex [Cu(Me₆tren)(CH₂CN)]⁺ (Me₆tren = tris(2-(dimethylamino)ethyl)amine) has emerged as an important model of potential byproducts in copper-catalyzed atom transfer radical polymerization. This complex has been generated by controlled potential electrolysis of [Cu(Me₆tren)(NCMe)]²⁺ in the presence of BrCH₂CN. Time-resolved UV-vis and continuous wave and pulse electron paramagnetic resonance (EPR) spectra identified [Cu(Me₆tren)Br]⁺ as an intermediate. Hyperfine sublevel correlation and electron nuclear double resonance spectroscopy of samples at different timepoints reveal signals that are assigned to a C-bound cyanomethylate ligand, with distinct ¹⁴N and ¹H hyperfine coupling constants in comparison with the corresponding N-bound acetonitrile and bromido complexes. The experimental EPR data are supported by density functional theory calculations to understand how the geometries of the species involved produce distinct spectroscopic signatures, and a clear picture of how this unusual organocopper(II) complex is formed has emerged.

Dual Polymerization Pathway for Polyolefin-Polar Block Copolymer Synthesis via MILRad: Mechanism and Scope

This work explores the mechanism whereby a cationic diimine Pd(II) complex combines coordination insertion and radical polymerization to form polyolefin-polar block copolymers. The initial requirement involves the insertion of a single acrylate monomer into the Pd(II)-polyolefin intermediates, which generate a stable polymeric chelate through a chain-walking mechanism. This thermodynamically stable chelate was also found to be photochemically inactive, and a unique mechanism was discovered which allows for radical polymerization. Rate-determining opening of the chelate by an ancillary ligand followed by additional chain walking allows the metal to migrate to the α-carbon of the acrylate moiety. Ultimately, the molecular parameters necessary for blue-light-triggered Pd-C bond homolysis from this α-carbon to form a carbon-centered macroradical species were established. This intermediate is understood to initiate free radical polymerization of acrylic monomers, thereby facilitating block copolymer synthesis from a single Pd(II) complex. Key intermediates were isolated and comprehensively characterized through exhaustive analytical methods which detail the mechanism while confirming the structural integrity of the polyolefin-polar blocks. Chain walking combined with blue-light irradiation functions as the mechanistic switch from coordination insertion to radical polymerization. On the basis of these discoveries, robust di- and triblock copolymer syntheses have been demonstrated with olefins (ethylene and 1-hexene) which produce amorphous or crystalline blocks and acrylics (methyl acrylate, ethyl acrylate, n-butyl acrylate, and methyl methacrylate) in broad molecular weight ranges and compositions, yielding AB diblocks and BAB triblocks.

The Role of Poly(Methyl Methacrylate) in Management of Bone Loss and Infection in Revision Total Knee Arthroplasty: A Review

Poly(methyl methacrylate) (PMMA) is widely used in joint arthroplasty to secure an implant to the host bone. Complications including fracture, bone loss and infection might cause failure of total knee arthroplasty (TKA), resulting in the need for revision total knee arthroplasty (rTKA). The goals of this paper are: (1) to identify the most common complications, outside of sepsis, arising from the application of PMMA following rTKA, (2) to discuss the current applications and drawbacks of employing PMMA in managing bone loss, (3) to review the role of PMMA in addressing bone infection following complications in rTKA. Papers published between 1970 to 2018 have been considered through searching in Springer, Google Scholar, IEEE Xplore, Engineering village, PubMed and weblinks. This review considers the use of PMMA as both a bone void filler and as a spacer material in two-stage revision. To manage bone loss, PMMA is widely used to fill peripheral bone defects whose depth is less than 5 mm and covers less than 50% of the bone surface. Treatment of bone infections with PMMA is mainly for two-stage rTKA where antibiotic-loaded PMMA is inserted as a spacer. This review also shows that using antibiotic-loaded PMMA might cause complications such as toxicity to surrounding tissue, incomplete antibiotic agent release from the PMMA, roughness and bacterial colonization on the surface of PMMA. Although PMMA is the only commercial bone cement used in rTKA, there are concerns associated with using PMMA following rTKA. More research and clinical studies are needed to address these complications.

The Effect of Viscosity on the Diffusion and Termination Reaction of Organic Radical Pairs

The effect of viscosity on the diffusion efficiency (F_dif ) of an organic radical pair in a solvent cage and the termination mechanism, that is, the selectivity of disproportionation (Disp) and combination (Comb) of the geminated caged radical pair and the diffused radicals encountered, were investigated quantitatively by following the photolysis of dimethyl 2,2'-azobis(2-methylpropionate) (V-601) in the absence and presence of PhSD. F_dif and Disp/Comb selectivity outside the cage [Disp_(dif) /Comb_(dif) ] are highly sensitive to the viscosity. In contrast, the Disp/Comb selectivity inside the cage [Disp_(cage) /Comb_(cage) ] is rather insensitive. The difference in viscosity dependence between Disp_(cage) /Comb_(cage) and Disp_(dif) /Comb_(dif) is explained by the spin state of the radical pair inside and outside the cage and the spin state dependent configurational changes of the radical pair upon their collision. Given that the configurational change of the radicals associates the displacement and reorganization of solvents around the radicals, the termination outside the cage, which requires larger change than that inside the cage, is highly viscosity dependent. Furthermore, while the bulk viscosity of each solvent shows good correlation with F_dif and Disp/Comb selectivity, microviscosity is the better parameter predicting F_dif and Disp_(dif) /Comb_(dif) selectivity regardless of the solvents.

Mechanism of Cu(I)/Cu(0)-Mediated Reductive Coupling Reactions of Bromine-Terminated Polyacrylates, Polymethacrylates, and Polystyrene

The mechanism of the Cu(I)/Cu(0)-mediated reductive coupling reactions of bromine-terminated polymer chain-end radicals, so-called atom-transfer radical coupling (ATRC), is studied. Poly(methyl acrylate) (PMA), poly(methyl methacrylate) (PMMA), and polystyrene (PSt), prepared by atom-transfer radical polymerization (ATRP), were activated by an excess amount of Cu(I)Br and Cu(0) in the presence of tris[2-(dimethylamino)ethyl]amine (Me₆TREN), and the structural analyses of the resulting polymer products and deuterium-labeling experiments unambiguously determined the mechanism. While PMMA and PSt reacted by a radical mechanism as expected, PMA-bromide was reduced to an anionic species, which was most likely an organocopper species. Trapping experiments with TEMPO suggested that the polymer chain-end radicals were generated in all cases by the reduction of the bromine-terminated polymers by low-valent Cu species. However, the PMA chain-end radical was further reduced to the anionic species from which coupling products formed in low yield.