Evolution Of Molecular Weight Distribution Research Articles

Kinetics of thermal degradation of vinyl polyperoxides in solution

The thermal degradation of vinyl polyperoxides, poly(styrene peroxide) (PSP), poly(α-methylstyrene peroxide) (PAMSP) and poly(α-phenylstyrene peroxide) (PAPSP), was carried out at different temperatures in toluene. The time evolution of molecular weight distributions (MWDs) was determined by gel permeation chromatography (GPC). A continuous distribution model was used to evaluate the random chain degradation rate coefficients. The activation energies, determined from the temperature dependence of the rate coefficients, suggest that thermal degradation of polyperoxides is controlled by the dissociation of the OO bonds in the backbone of the polymer chain. Among the three polyperoxides investigated, the thermal stability is the highest for PAPSP and the lowest for PAMSP.

Modeling molecular weight distribution in emulsion polymerization reactions with transfer to polymer

AbstractA mathematical model was developed for the computation of the dynamic evolution of molecular weight distributions (MWDs) during nonlinear emulsion polymerization reactions. To allow the direct computation of the whole MWD, an adaptive orthogonal collocation technique was applied. The model was validated with experimental methyl methacrylate/butylacrylate (BuA) semicontinuous and vinyl acrylate (VA)/Veova10 continuous emulsion polymerization results. Both systems considered introduce significant chain‐transfer reactions to polymer chains as a result of the presence of BuA and VA, respectively. The model developed was able to represent quite properly the kinetics and MWD of polymer samples during emulsion polymerizations. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3513–3528, 2001

Continuous distribution kinetics for ultrasonic degradation of poly(methyl methacrylate)

AbstractUltrasonic degradation of poly(methyl methacrylate) (PMMA) was carried out in several solvents and some mixtures of solvents. The time evolution of molecular weight distribution (MWD), determined by gel permeation chromatography, is analysed by continuous distribution kinetics. The rate coefficients for polymer degradation are determined for each solvent. The variation of rate coefficients is correlated with the vapour pressure of the solvent, kinematic viscosity of the solution and solvent–polymer interaction parameters. The vapour pressure and the kinematic viscosity of the solution are found to be more critical than other parameters (such as the Huggins and Flory–Huggins constants) in determining the degradation rates.© 2001 Society of Chemical Industry

Continuous distribution kinetics for oxidative degradation of PMMA in solution

The oxidative degradation of poly(methyl methacrylate) (PMMA) was investigated in chlorobenzene at 50–100°C for various peroxide and polymer concentrations. Unlike thermal degradation, the chain scission of PMMA occurs randomly across the polymer backbone. The samples were analyzed by gel permeation chromatography (GPC) to obtain the time evolution of molecular weight distributions (MWDs). A continuous distribution model for polymer degradation and simultaneous peroxide deactivation was developed assuming that the rate coefficients depend linearly on MW. This model was used to determine the rate coefficients for the oxidative degradation of PMMA. The activation energy, determined from the temperature dependence of the rate coefficients, was 48.2 kcal/mol.

Degradation kinetics of poly(vinyl acetate) in the presence of aluminum chloride

The thermal degradation of poly(vinyl acetate) (PVAc) was carried out in the presence of various concentrations of aluminum chloride (AlCl 3) at different temperatures. The time evolution of molecular weight distributions (MWDs) was determined by gel permeation chromatography. A continuous distribution kinetic model was used to evaluate the degradation rate coefficient assuming the reaction is first order in polymer concentration and that the decomposition of AlCl 3 is first order. Experimental data indicated that the presence of AlCl 3 greatly enhanced the rate of degradation of PVAc. The activation energy of the degradation rate coefficient for PVAc in presence of aluminium chloride was 14.2 kcal/mol.

Effect of solvent on the ultrasonic degradation of poly(vinyl acetate)

The ultrasonic degradation of poly(vinyl acetate) was carried out in six different solvents and two mixtures of solvents. The evolution of molecular weight distribution (MWD) with time was determined with gel permeation chromatography. The observed MWDs were analyzed by continuous distribution kinetics. A stoichiometric kernel that accounts for preferential mid-point breakage of the polymer chains was used. The degradation rate coefficient of the polymer in each solvent was determined from the model. The variations of rate coefficients were correlated with vapor pressure of the solvent, the Flory–Huggins polymer–solvent interaction parameter and the kinematic viscosity of the solution. A lower saturation vapor pressure resulted in higher degradation rates of the polymer. The degradation rate increased with increasing kinematic viscosity.

Evolution of molecular weight distributions in the catalytic chain transfer polymerization of methyl methacrylate up to high monomer conversions

The evolution of molecular weight distributions (MWDs) with monomer conversion in the catalytic chain transfer (CCT) polymerization of methyl methacrylate at 60 °C is investigated by simulation (via the program package PREDICI®) and experiment. A Co(III)-based complex is used as the precursor for the CCT agent, which is formed in situ by initiator-derived (2,2′-azobisisobutyronitrile) radicals to yield the catalytically active Co(II) species. The small shifts seen in the MWD toward lower molecular weights with increasing monomer conversion are shown to be of the same order of magnitude as the associated changes in the MWD in non-CCT controlled free-radical polymerization, indicating that no significant change in the MWD with monomer conversion is associated with the CCT process. These results are compared to the evolution of MWDs in conventional chain transfer polymerizations with thiols as transfer agents. A clear shift toward higher molecular weights is seen with increasing monomer conversion, indicating disparate rates of thiol and monomer consumption. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3303–3312, 2000

Evolution of molecular weight distributions in the catalytic chain transfer polymerization of methyl methacrylate up to high monomer conversions

The evolution of molecular weight distributions (MWDs) with monomer conversion in the catalytic chain transfer (CCT) polymerization of methyl methacrylate at 60 °C is investigated by simulation (via the program package PREDICI®) and experiment. A Co(III)-based complex is used as the precursor for the CCT agent, which is formed in situ by initiator-derived (2,2′-azobisisobutyronitrile) radicals to yield the catalytically active Co(II) species. The small shifts seen in the MWD toward lower molecular weights with increasing monomer conversion are shown to be of the same order of magnitude as the associated changes in the MWD in non-CCT controlled free-radical polymerization, indicating that no significant change in the MWD with monomer conversion is associated with the CCT process. These results are compared to the evolution of MWDs in conventional chain transfer polymerizations with thiols as transfer agents. A clear shift toward higher molecular weights is seen with increasing monomer conversion, indicating disparate rates of thiol and monomer consumption. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3303–3312, 2000

Kinetic model of star-branched polycondensation

A kinetic model was developed for the whole process of star-branched polycondensation of AB type monomers with a multifunctional core, RAf. The evolution of molecular weight distribution and other molecular parameters during reaction were estimated in terms of the derived expressions. The molecular weight distribution first becomes broader with increasing reaction extent of B groups and the functionality of RAf, but suddenly turns to be markedly narrower than the Schulz-Flory distribution when the polycondensation approaches completion.

Kinetic model of star-branched polycondensation

A kinetic model was developed for the whole process of star-branched polycondensation of AB type monomers with a multifunctional core, RAf. The evolution of molecular weight distribution and other molecular parameters during reaction were estimated in terms of the derived expressions. The molecular weight distribution first becomes broader with increasing reaction extent of B groups and the functionality of RAf, but suddenly turns to be markedly narrower than the Schulz-Flory distribution when the polycondensation approaches completion.