Critically evaluated termination rate coefficients for free-radical polymerization: Experimental methods

Realizing Cross-linking-free Acrylic Pressure-Sensitive Adhesives with Intensive Chain Entanglement through Visible-Light-Mediated Photoiniferter-Reversible Addition-Fragmentation Chain-Transfer Polymerization

A versatile and simplified synthesis scheme for intensively entangled acrylic pressure-sensitive adhesives (PSAs) was developed in this study by leveraging visible-light-driven controlled radical polymerization (photoiniferter/reversible addition-fragmentation chain-transfer polymerization) of acrylic copolymers under a controlled manner; the approach was differentiated by a single factor; molecular weight (Mw up to 2.8 MDa) with identical compositions. By manipulating Mw up to ultra-high ranges, PSAs with diversified viscoelastic properties were prepared and then assessed with a focus on realizing PSAs with a maximized degree of entanglement per chain through domination of high Mw contents, to help achieve excellent cohesiveness without a reinforcing cross-linking network. Moreover, fully linear solvent-soluble poly(acrylate)s were synthesized to facilitate reprocessing and reuse, highlighting the sustainability of the devised method and, consequently, its potential to be applied for effectively reducing industrial or daily waste.

Controversies on the mechanism and kinetics of emulsion polymerization: An updated critical review

Conventional emulsion polymerization (EP) is a process via free radicals whose driving force for its development has been its versatility to generate polymer colloids with ad hoc characteristics for a wide variety of applications, as well as its friendly character to the environment since the continuous medium is water. Although through decades of research, considerable progress has been made in understanding its mechanism and kinetics, some aspects are still not entirely clear. Furthermore, new ideas and experimental results have appeared in the literature that challenge the accepted knowledge about some aspects of EP. This work is a personal vision and an updated critical review on those controversial aspects whose precedent is the review with the same approach published by the author and collaborators almost 20 years ago (J. Macromol. Sci. Part C Polym. Rev., 2004;44:207-229). This review covers advances, aspects that are open to discussion or need improvement regarding what happens in the aqueous phase and in the interface (initiator decomposition, entry and exit of radicals, monomer transport) as well as in the polymer particles (free-radical propagation and termination, swelling, average number of radicals per particle). Special attention is paid to particle formation (nucleation) and its interrelation with colloidal stability and the evolution of the particle size distribution (PSD), which is one of the most fundamental and controversial issues of EP. The Smoluchowski collision rate coefficient to describe diffusion-controlled processes has practically become a paradigm despite the fact that there is evidence that questions its applicability. For this reason, this review also emphasizes this point and the alternatives that have been proposed to mathematically describe the diffusive stages of particle coagulation, the entry of radicals, and the termination reaction. Challenges in improving our understanding of the mechanism and kinetics of emulsion polymerization are pointed out.

Evolution of Molar Mass Distributions Using a Method of Partial Moments: Initiation of RAFT Polymerization

We describe a method of partial moments devised for accurate simulation of the time/conversion evolution of polymer composition and molar mass. Expressions were derived that enable rigorous evaluation of the complete molar mass and composition distribution for shorter chain lengths (e.g., degree of polymerization, Xn = N < 200 units) while longer chains (Xn ≥ 200 units) are not neglected, rather they are explicitly considered in terms of partial moments of the molar mass distribution, μxN(P)=∑n=N+1∞nx[Pn] (where P is a polymeric species and n is its’ chain length). The methodology provides the exact molar mass distribution for chains Xn < N, allows accurate calculation of the overall molar mass averages, the molar mass dispersity and standard deviations of the distributions, provides closure to what would otherwise be an infinite series of differential equations, and reduces the stiffness of the system. The method also allows for the inclusion of the chain length dependence of the rate coefficients associated with the various reaction steps (in particular, termination and propagation) and the various side reactions that may complicate initiation or initialization. The method is particularly suited for the detailed analysis of the low molar mass portion of molar mass distributions of polymers formed by radical polymerization with reversible addition-fragmentation chain transfer (RAFT) and is relevant to designing the RAFT-synthesis of sequence-defined polymers. In this paper, we successfully apply the method to compare the behavior of thermally initiated (with an added dialkyldiazene initiator) and photo-initiated (with a RAFT agent as a direct photo-iniferter) RAFT-single-unit monomer insertion (RAFT-SUMI) and oligomerization of N,N-dimethylacrylamide (DMAm).

100th Anniversary of Macromolecular Science Viewpoint: Achieving Ultrahigh Molecular Weights with Reversible Deactivation Radical Polymerization

Synthetic strategies for achieving ultrahigh molecular weights via reversible deactivation radical polymerization are discussed from the mechanistic, kinetic, and experimental aspects, and their applications as high-performance materials are highlighted. Further development of this field requires continuous effort to improve livingness and polymerization efficiency under greener conditions.

Role of External Field in Polymerization: Mechanism and Kinetics

The past decades have witnessed an increasing interest in developing advanced polymerization techniques subjected to external fields. Various physical modulations, such as temperature, light, electricity, magnetic field, ultrasound, and microwave irradiation, are noninvasive means, having superb but distinct abilities to regulate polymerizations in terms of process intensification and spatial and temporal controls. Gas as an emerging regulator plays a distinctive role in controlling polymerization and resembles a physical regulator in some cases. This review provides a systematic overview of seven types of external-field-regulated polymerizations, ranging from chain-growth to step-growth polymerization. A detailed account of the relevant mechanism and kinetics is provided to better understand the role of each external field in polymerization. In addition, given the crucial role of modeling and simulation in mechanisms and kinetics investigation, an overview of model construction and typical numerical methods used in this field as well as highlights of the interaction between experiment and simulation toward kinetics in the existing systems are given. At the end, limitations and future perspectives for this field are critically discussed. This state-of-the-art research progress not only provides the fundamental principles underlying external-field-regulated polymerizations but also stimulates new development of advanced polymerization methods.

ATRP of Methyl Acrylate by Continuous Feeding of Activators Giving Polymers with Predictable End-Group Fidelity

Atom transfer radical polymerization (ATRP) of methyl acrylate (MA) was carried out by continuous feeding of Cu(I) activators. Typically, the solvent, the monomer, the initiator, and the CuBr2/Me6TREN deactivator are placed in a Schlenk flask (Me6TREN: tris[2-(dimethylamino)ethyl]amine), while the CuBr/Me6TREN activator is placed in a gas-tight syringe and added to the reaction mixture at a constant addition rate by using a syringe pump. As expected, the polymerization started when Cu(I) was added and stopped when the addition was completed, and polymers with a narrow molecular weight distribution were obtained. The polymerization rate could be easily adjusted by changing the activator feeding rate. More importantly, the loss of chain end-groups could be precisely predicted since each loss of Br from the chain end resulted in the irreversible oxidation of one Cu(I) to Cu(II). The Cu(I) added to the reaction system may undergo many oxidation/reduction cycles in ATRP equilibrium, but would finally be oxidized to Cu(II) irreversibly. Thus, the loss of chain end-groups simply equals the total amount of Cu(I) added. This technique provides a neat way to synthesize functional polymers with known end-group fidelity.

Probing into Styrene Polymerization Runaway Hazards: Effects of the Monomer Mass Fraction

Polymerization reactions have caused a number of serious incidents in the past; they are prone to reaction runaways because of their exothermic and autoaccelerating nature. To minimalize the risk, the reaction is commonly performed in a solvent as empirical industrial practice. In this work, the thermal runaway hazards of the ethylbenzene-styrene system with different monomer mass fractions were calorimetrically investigated up to temperatures where decomposition products are unlikely to be produced. Experiments showed that the polymerization runaway "onset" temperature inversely increased with the monomer mass fraction. Experiment and thermodynamic calculations showed that volatile diluent increased system vapor pressure even at a lower adiabatic temperature rise and verified that moderation of the risks could be achieved if the monomer mass fraction is below ca. 85%. A lumped kinetic model developed by Hui and Hamielec was used to predict the runaway profile of this reaction under different dilutions, and the agreement was excellent.

Identification of β scission products from free radical polymerizations of butyl acrylate at high temperature

Unsaturated low molar mass species were identified via ESI-MS after fractionation of poly(butyl acrylate) from high temperature radical polymerization.

RAFT Polymerization of Tert-Butyldimethylsilyl Methacrylate: Kinetic Study and Determination of Rate Coefficients

Well-defined poly(tert-butyldimethylsilyl methacrylate)s (TBDMSMA) were prepared by the reversible addition-fragmentation chain transfer (RAFT) process using cyanoisopropyl dithiobenzoate (CPDB) as chain-transfer agents (CTA). The experimentally obtained molecular weight distributions are narrow and shift linearly with monomer conversion. Propagation rate coefficients (k_p) and termination rate coefficients (k_t) for free radical polymerization of TBDMSMA have been determined for a range of temperature between 50 and 80 °C using the pulsed laser polymerization-size-exclusion chromatography (PLP-SEC) method and the kinetic method via steady-state rate measurement, respectively. The CPDB-mediated RAFT polymerization of TBDMSMA has been subjected to a combined experimental and PREDICI modeling study at 70 °C. The rate coefficient for the addition reaction to RAFT agent (k_β1, k_β2) and to polymeric RAFT agent (k_β) is estimated to be approximately 1.8 × 10⁴ L·mol⁻¹·s⁻¹ and for the fragmentation reaction of intermediate RAFT radicals in the pre-equilibrium (k_-β1, k_-β2) and main equilibrium (k_-β) is close to 2.0 × 10⁻² s⁻¹. The transfer rate coefficient (k_tr) to cyanoisopropyl dithiobenzoate is found to be close to 9.0 × 10³ L·mol⁻¹·s⁻¹ and the chain-transfer constant (C_tr) for CPDB-mediated RAFT polymerization of TBDMSMA is about 9.3.

Hydrogen bonding in radical solution copolymerization kinetics of acrylates and methacrylates: a comparison of hydroxy- and methoxy-functionality

Experimental data obtained via pulsed laser polymerization are used to distinguish the influence of H-bonding on kinetic chain-growth parameters from that of side-chain heteroatoms.

Predictive Model for the Design of Zwitterionic Polymer Brushes: A Statistical Design of Experiments Approach

The performance of polymer interfaces in biology is governed by a wide spectrum of interfacial properties. With the ultimate goal of identifying design parameters for stem cell culture coatings, we developed a statistical model that describes the dependence of brush properties on surface-initiated polymerization (SIP) parameters. Employing a design of experiments (DOE) approach, we identified operating boundaries within which four gel architecture regimes can be realized, including a new regime of associated brushes in thin films. Our statistical model can accurately predict the brush thickness and the degree of intermolecular association of poly[{2-(methacryloyloxy) ethyl} dimethyl-(3-sulfopropyl) ammonium hydroxide] (PMEDSAH), a previously reported synthetic substrate for feeder-free and xeno-free culture of human embryonic stem cells. DOE-based multifunctional predictions offer a powerful quantitative framework for designing polymer interfaces. For example, model predictions can be used to decrease the critical thickness at which the wettability transition occurs by simply increasing the catalyst quantity from 1 to 3 mol %.

Syndiotactic- and heterotactic-specific radical polymerization of N-n-propylmethacrylamide complexed with alkali metal ions

Addition of Li⁺ enhanced the radical polymerization of N-n-propylmethacrylamide, in which the stoichiometry of the monomer–Li⁺ complexes determined the stereospecificity of the radical polymerization.

Absolute potential of the standard hydrogen electrode and the problem of interconversion of potentials in different solvents

The absolute potential of the standard hydrogen electrode, SHE, was calculated on the basis of a thermodynamic cycle involving H(2(g)) atomization, ionization of H((g))* to H((g))(+), and hydration of H(+). The most up-to-date literature values on the free energies of these reactions have been selected and, when necessary, adjusted to the electron convention Fermi-Dirac statistics since both e(-) and H(+) are fermions. As a reference state for the electron, we have chosen the electron at 0 K, which is the one used in computational chemistry. Unlike almost all previous estimations of SHE, DeltaG(aq)(theta)(H(+)) was used instead of the real potential, alpha(aq)(H(+)). This choice was made to obtain a SHE value based on the chemical potential, which is the appropriate reference to be used in theoretical computations of standard reduction potentials. The result of this new estimation is a value of 4.281 V for the absolute potential of SHE. The problem of conversion of standard reduction potentials (SRPs) measured or estimated in water to the corresponding values in nonaqueous solvents has also been addressed. In fact, thermochemical cycles are often used to calculate SRPs in water versus SHE, and it is extremely important to have conversion factors enabling estimation of SRPs in nonaqueous solvents. A general equation relating E(theta) of a generic redox couple in water versus the SHE to the value of E(theta) in an organic solvent versus the aqueous saturated calomel electrode is reported.