13 research outputs found
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
Quantum Chemistry Investigation of Secondary Reaction Kinetics in Acrylate-Based Copolymers
A density functional theory study of poly (vinyl chloride) (PVC) free radical polymerization
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/6-31GÂ(d,p) as well as a higher level of theory, MPWB1K/6-31GÂ(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