A. E. Hamielec

The kinetics of poly(N-isopropylacrylamide) microgel latex formation

Conversion versus time curves were measured for poly(N-isopropylacrylamide) microgel latexes prepared by polymerization in water with sodium dodecyl sulfate, SDS. Polymerization rates increased with temperature with methylenebisacrylamide crosslinking monomer consumed faster thanN-isopropylacrylamide. The particle diameter decreased with increasing concentrations of SDS in the polymerization recipe and there was evidence that the rate of polymerization increased somewhat with SDS concentration. Particle formation occurred by homogeneous nucleation as micelles were absent.Comparison of particle size distributions from dynamic light scattering to those from a centrifugal sizer led to the conclusion that larger particles within a specific latex were less swollen with acetonitrile than were the smaller ones. This was interpreted as evidence for the polymer in larger particles having a higher crosslink density. Particle swelling was estimated from swelling ratios defined as the particle volume at 25 °C divided by the volume at 50 °C. In the absence of crosslinking poly(N-isopropylacrylamide) linear chains would disolve at 25 °C. The swelling results indicated that the average crosslink density in the particles decreased with conversion. This was explained by the observation that the methylenebisacrylamide was consumed more quickly and is typical of crosslinking in emulsion polymerization where polymer particles have high polymer concentrations at their birth.

Polymerization reaction engineering — Metallocene catalysts

Abstract Metallocene catalysts are operative in all existing industrial plants that are presently used for polyolefin manufacture and have the potential to revolutionize the technology for the production of these polymers. A review of metallocene catalysis and its effects on polymer process engineering for the manufacture of polyolefins is provided. This review concentrates on the aspects of polymer reactor engineering, mathematical modelling of polymerization processes, and the characterization of polyolefins made with these novel catalysts.

Temperature rising elution fractionation of linear polyolefins

Abstract Temperature rising elution fractionation (t.r.e.f.) is a technique for the fractionation and characterization of polymers with high crystallizability that has gained some popularity over the past few years, in particular for the copolymers of olefins. This review will cover the general aspects of t.r.e.f. and describe some of the most revealing applications published in the literature. In addition, recent measurements that have been carried out on the t.r.e.f./13C n.m.r. spectroscopic characterization of polypropylenes synthesized using metallocene catalysts, as well as using classical heterogeneous Ziegler—Natta catalysts, are reported on herein.

Deconvolution of chain-length distributions of linear polymers made by multiple-site-type catalysts

Abstract Polyolefins made using most heterogeneous and some homogeneous Ziegler—Natta catalysts show a characteristic broad molecular-weight distribution, generally assumed to be caused by the presence of more than one active site type, each type producing polymer with an instantaneous chain-length distribution that is most probable and having polydispersity of 2. Herein is reported a comparison of two numerical methods for the deconvolution of instantaneous chain-length distribution of linear homo- and copolymers made using multiple-site-type catalysts giving an individual most probable chain-length distribution for each site type, and a methodology is proposed for the efficient deconvolution of measured chain-length distributions.

A kinetic model for olefin polymerization in high-pressure tubular reactors : a review and update

Abstract Free-radical copolymerization in high-pressure tubular reactors is considered. Kinetic mechanisms to describe the polymerization rate and polymer properties, including copolymer composition, molecular weight, branching frequencies, melt flow index and polymer density, have been proposed. Based upon this kinetic scheme, a mathematical model has been derived and implemented as a computer program to simulate commercial tubular reactors. For calculation of molecular-weight averages, the method of moments is used in conjunction with pseudo kinetic rate constants to allow for copolymerization. The model parameters were fitted to industrial data to give useful steady-state simulation software, allowing for multiple feed points, multiple initiators (including oxygen) and non-isothermal polymerization. The effects of the pulse valve and the product cooler are incorporated. Comparisons are made between the model predictions and industrial data. A new approach and considerations for solving the moment closure problem are presented.

Crosslinking kinetics in polyacrylamide networks

Kinetics of network formation in free-radical copolymerization of acrylamide and N,N′-methylenebis-acrylamide in aqueous solution (56.6 g comonomer per litre) at 25°C have been studied. It was found that as high as 80% of pendant double bonds are consumed immediately on polymerization and are wasted in primary cyclization. Primary cyclization is responsible for the delay in the onset of gelation at low mole fractions of divinyl monomer. The effects of decreased reactivity of pendant double bonds and secondary cyclization become significant as the mole fraction of divinyl monomer increases. The decreased reaction rate between huge molecules due to steric hindrance in the pre-gelation period contributes to microgel formation, and consequently to formation of spatially inhomogeneous networks.

Generalized Universal Molecular Weight Calibration Parameter in GPC

Abstract In this report we show by experimental and theoretical investigations that the commonly used GPC universal calibration parameter, the intrinsic viscosity multiplied by the weight average molecular weight ([η] Mw) is incorrect. The error which can arise by using [η] M to calculate the molecular weight across the GPC chromatogram for nonuniformly branched polymers [poly(vinyl acetate) and low density polyethylene] and copolymers with compositional drift, could be very large. We also show conclusively that the number average molecular weight Mn is the correct average to use for the universal calibration parameter. We therefore recommend that our general universal Calibration parameter [η] Mn be used for calculating the molecular weight across the chromatogram for all polymer systems (linear and branched homopolymers, copolymers with or without compositional drift and for polymer blends).

Bulk polymerization of vinyl chloride

The bulk polymerization of vinyl chloride initiated by AIBN at temperature levels of 30°, 50°, and 70°C has been studied. Molecular weight averages and distribution (MWD) were measured by gel permeation chromatography. A model has been proposed which accurately predicts conversion to high levels and MWD. Molecular weight measurements show that transfer to monomer plays the important role in controlling molecular weight averages. Disproportionation is probably the dominant mode of termination.

Experimental investigation of vinyl chloride polymerization at high conversion: mechanism, kinetics and modelling

Abstract The mechanism of vinyl chloride (VCM) polymerization is discussed in detail from both a chemical and a physical point of view. A comprehensive kinetic model for VCM bulk/suspension polymerization is developed based on all of the important elementary reactions for two-phase polymerization. A series of kinetic experiments covering an extensive temperature range with different initiator systems was carried out in an agitated 51 batch reactor. Kinetic parameters were estimated by using these experimental data. The present model is in excellent agreement with experimental rate data measured in different laboratories and can be used to predict polymerization rate, conversion history and other kinetic features over the entire monomer conversion range.

Chemical modification of polyolefins by free radical mechanisms: a modelling and experimental study of simultaneous random scission, branching and crosslinking

Abstract The chemical modification of polyolefins in the melt, using free radical mechanisms suitable for reactive extrusion, was studied. An algorithm to approximate the solution to an equation for simultaneous random scission and crosslinkingl is presented. This solution is compared to the classical two-step solutions of Saito 2–5 , Flory 6 and Charlesby-Pinner 7 , showing significant deviations for the simultaneous random scission and crosslinking case. Suggestions to improve further the scission term of this equation are presented. Experiments were performed using high density polyethylene and peroxides in a minitruder and in small ampoule reactors. Gel fractions, thermal properties and molecular weights were measured. The heats of transition and melting temperatures as measured by differential scanning calorimetry were found to decrease with increasing peroxide level but were independent of reaction temperature. A pure random crosslinking model was found to fit adequately the gel fraction data.