Experimental and modeling study of acrylamide copolymerization with cationic monomers in aqueous medium

Aqueous free-radical copolymerization of acrylamide with the cationic monomer acryloyloxyethyltrimethyl ammonium chloride is investigated in this work by both experimental analysis and model development with emphasis on the copolymer composition. Solution polymerization experiments carried out by in-situ 1H-NMR technique at different values of monomer concentration, initial monomer mixture composition, and ionic strength provided accurate data of monomer composition as a function of conversion. The results revealed a remarkable dependence of the composition behavior upon monomer and electrolyte concentration, as it has been observed in similar polyelectrolyte systems. An advanced model of copolymer composition which takes into account the non-conventional features of this system is proposed and applied to estimate the corresponding reactivity ratios. On the one hand, modeling of aqueous free-radical copolymerization systems involving charged monomers is challenging due to the presence of electrostatic effects arising from the interaction between ionic moieties. The repulsive forces between equally-charged monomer units may result in the dependence of reaction rate coefficients and reactivity ratios upon the concentration of charges in the system due to electrostatic screening effects. Namely, the copolymerization behavior is likely to be affected by parameters such as the concentration of the charged monomer and, more generally, the ionic strength of the reaction medium. On the other hand, acrylate polymers are known to be subject to relevant secondary reactions which originate a non-negligible fraction of mid-chain radicals (MCRs) in the system, with consequences on the rate of monomer consumptions as well as copolymer composition. The developed model accounts for both previous aspects. In particular, the electrostatic effect has been simulated through a DLVO-based kinetic approach to express effective rate coefficients as a function of the ionic strength. On the other hand, the role of the secondary reactions has been included by defining reactivity ratios for backbiting and MCR propagation reactions. The proposed model of copolymer composition as a function of conversion explains the experimental dependencies upon monomer and electrolyte concentration for a wide range of experimental conditions. The proposed approach appears to be general enough to be successfully applied to this type of complex copolymerization systems

Quantum Mechanical Investigation on Bimolecular Hydrogen Abstractions in Butyl Acrylate-Based Free Radical Polymerization Processes

The present computational study focuses on the investigation of bimolecular hydrogen abstractions that can occur during free radical polymerization (FRP) processes. In particular, several hydrogen abstractions from four monomers (butyl acrylate, BA; styrene, ST; butyl methacrylate, BMA; vinyl acetate, VA) and three different backbone chains (poly-BA, poly-BA-<i>co</i>-VA, and poly-BA-<i>co</i>-ST) have been studied. The aim is to provide an overview of the kinetics for all possible intermolecular hydrogen abstraction reactions from all chemical species present in a bulk FRP as well as to support the understanding of the influence of chemical environment on hydrogen abstractions. All simulations were performed using density functional theory (DFT) with quantum tunneling factors estimated using the Eckart model. This study provides proof that the presence of an electron donating group in the chemical environment of the abstracted hydrogen atoms can lead to lower activation energies and higher rate coefficients for abstraction whereas the presence of an electron withdrawing group leads to opposite effects

Theoretical Study of Chain Transfer to Agent Kinetics in Butyl Acrylate Polymerization

Reactions of chain transfer to agent (CTA) are conventionally used to regulate the polymer molecular weight during radical polymerization processes, due to the interaction between CTAs and chain-end growing radicals. In acrylate polymerization, the presence of a relatively large amount of midchain radicals (MCRs) opens the way for alternative kinetic pathways involving CTAs, which can result in a modification of the overall kinetics as well as the final polymer properties. In this work, chain transfer reactions from butyl acrylate (BA) radicals of various size and nature to a set of selected CTAs are investigated using quantum chemistry. The different reactivity of chain-end and midchain radicals is emphasized, with particular focus on the kinetic effect of the radical chain length. Eventually, the mechanism of MCR patching and its relevance in decreasing the branching density are critically examined, with reference to the estimated kinetic parameters and experimental evidence about BA polymerization

The RAFT copolymerization of acrylic acid and acrylamide

The reversible addition-fragmentation chain transfer (RAFT) copolymerization of non-ionized acrylic acid with acrylamide in dimethyl sulfoxide is investigated and compared with the conventional free radical process carried out at the same conditions. The effect of the living nature of the RAFT process on overall polymerization rate and on copolymer composition is studied by in situ 1H NMR. The same values of reactivity ratios (rAA = 1.55 ± 0.02 and rAm = 0.75 ± 0.02) are estimated for both the processes, living and non-living. The copolymers prepared by RAFT polymerization exhibit well-controlled composition, with narrow distribution. This approach can be applied to the synthesis of copolymer standards at specific composition for direct calibration of size-exclusion chromatography

Quantum Chemistry Investigation of Fluorinated Polymer Systems of Industrial Interest

In this work, the free-radical polymerization (FRP) of widely used fluorinated monomers was investigated. Computational studies were conducted to assess the FRP kinetics of each binary copolymerization between vinylidene fluoride (VDF), hexafluoropropylene (HFP), and tetrafluoroethylene (TFE). More specifically, all calculations were performed using density functional theory (DFT), and the B3LYP level of theory was used to optimize structures and determine absolute minimum energy geometries, whereas the electronic energies were estimated using B3LYP/6-31G­(d,p) as well as a higher level of theory, MPWB1K/6-31G­(d,p). Transition state theory was employed to determine kinetic parameters according to the terminal model of copolymerization. The homopolymerization of VDF and all of its corresponding copolymerizations were investigated by taking into account every possible propagation reaction (head to head, head to tail, tail to tail, head to monomer, tail to monomer, etc.) to estimate the Arrhenius parameters for each system. This study provides the estimation of a large set of rate coefficients, which gives detailed pictures of the specific copolymerization systems examined and is highly valuable to generate a comprehensive overview of the polymerization kinetics of relevant fluorinated monomers