K. F. O'Driscoll

Absolute Rate Constants in Free-Radical Polymerization. III. Determination of Propagation and Termination Rate Constants for Styrene and Methyl Methacrylate

Abstract A spatially intermittent polymerization (SIP) reactor has been used for determination of absolute rate constants in photo-initiated, free-radical polymerization of styrene (STY) and methyl methacrylate (MMA). Experimental data are reported in the temperature range 15-30°C and in the high molecular weight region for MMA and STY. Additional experimental data are reported at 30° C and various lower molecular weights for STY which indicate that the propagation rate constant K is independent of polymer molecular weight, and K is dependent on molecular weight, especially at low molecular weight, approaching an approximately constant value at high molecular weight.

Interpreting the copolymerization of styrene with maleic anhydride and with methyl methacrylate in terms of the bootstrap model

Abstract The bootstrap model was proposed by Harwood to explain copolymer sequence distribution which is dependent only on copolymer composition in solvents which cause changes in the apparent reactivity ratios for a number of pairs of monomers. This work presents an explicit formulation of the equations describing copolymer composition and sequence distribution in terms of the bootstrap model for both the simple Mayo-Lewis model and the penultimate unit model. Inspection of these equations reveals the necessary and sufficient conditions for sequence distribution to be a function only of copolymer composition. The results of copolymerizations of styrene with methyl methacrylate and with maleic anhydride under various conditions are interpreted in terms of the bootstrap model.

The rate of copolymerization of styrene and methylmethacrylate—I. Low conversion kinetics

Abstract Styrene (STY) and methylmethacrylate (MMA) have been copolymerized using the initiator 2,2′-azodiisobutyrate (AIBME) over a wide composition and conversion range. The rate and degree of polymerization have been measured, as well as the composition of the copolymer formed, as a function of conversion. From these data, at low conversion, the first order rate constants for AIBME decomposition in STY and MMA were determined. In addition, the apparent bimolecular rate constants for propagation and termination in this copolymerization were calculated as a function of monomer feed and polymer composition. It is shown that the data conforms to a penultimate unit effect model with the assumptions proposed by Fukuda.

Catalytic Chain-Transfer in Polymerization of Methyl Methacrylate. I. Chain-Length Dependence of Chain-Transfer Coefficient

Abstract Free-radical polymerization of methyl methacrylate has been conducted in the presence of a new catalytic chain-transfer agent at different temperatures. The new catalyst exhibits a strong chain transfer characteristic, and the chain-transfer coefficient varies from 2.8 × 104 to 6.6 × 104 and depends on chain length and temperature. The chain-transfer coefficients decrease with increasing chain length but reach a limiting value for chains more than 8 units in length. The activation energy for the chain transfer coefficient is -10.1 kJ/mol for this catalyst.

Long Chain Branching in Polyethylene

Abstract Long chain branching frequency in polyethylene has been measured. Molecular weights determined directly by low angle laser light scattering of eluting species in gel permeation chromatography were compared with those estimated by universal calibration. Erroneous values for long chain branching frequency are produced if care is not taken to disrupt polyethylene aggregates in the GPC solvent. Ethyl and hexyl aide chains do not register as long branches in this analysis but sixteen carbon sidechains are counted. In low density polyethylene the long chain branching frequency is generally highest at low molecular weights. This is because these polymers are produced in a non-isothermal free radical polymerization. Chain transfer to dead polymer, which produces long branches, occurs most frequently under the reaction conditions that also yield low molecular weight polyethylene.

The effect of benzyl alcohol on pulsed laser polymerization of styrene and methylmethacrylate

The homo- and copolymerizations of styrene (STY) and methylmethacrylate (MMA) have been studied in the presence of several levels of benzyl alcohol (BA). From pulsed laser polymerizations it has been found that the apparent propagation rate constant increased with increasing BA for all systems. In copolymerization it has been found that the reactivity ratios for STY decrease somewhat, but those for MMA change little with increasing BA. The tacticity of poly(MMA) formed in the presence of BA is affected, the amount of mm diads increasing with increasing BA. The results are interpreted as supporting the hypothesis that BA forms a strong complex with radical chain ends terminating in MMA and a weak complex with those terminating in STY.

Polymer reaction engineering: Modelling considerations for control studies

Abstract In polymer reaction engineering, mathematical models of varying complexity are employed to describe reactor behaviour and predict process performance under a variety of operating conditions. When process control is the final aim, it is very important to develop and use models that are simple in mathematical structure, yet capable of adequately describing the essential process characteristics. In this paper we first derive a detailed mechanistic model for a batch solution polymerization of methyl methacrylate. Subsequently, we show how this complex model can be simplified using various assumptions and approximations. Five models of different levels of complexity are presented and examined for their suitability for process optimization and control applications. Model responses are discussed and compared in order to check whether the simplifications degrade the predictive capabilities of the original model.

Use of n.m.r. data to calculate copolymer reactivity ratios

Abstract Reactivity ratios can be determined from n.m.r. analyses of binary copolymers. Measurement of the diad or triad sequence distribution provides an estimate of the number average sequence length of each copolymer. Each average sequence length is directly related to the corresponding reactivity ratio in the terminal copolymerization model. Extension to more complicated reaction models is straightforward. A single copolymerization experiment yields values for both reactivity ratios with this procedure. When pentads or triads, centred on one monomer only, are measured the two reactivity ratios cannot normally be estimated unless the copolymer is highly alternating. It is possible, however, to combine such data with an n.m.r. or other analysis of the chemical composition of the copolymer to obtain r 1 and r 2 . When the sequence distribution of runs, centred on only one of the monomers, can be analysed and the chemical composition of the copolymer is not available the procedure can yield a good value for the reactivity ratio of that monomer and a poor estimate for that of the comonomer.

Mathematical modelling of styrene/butyl acrylate copolymerization

Abstract An experimental investigation of the kinetics of the bulk free radical copolymerization of styrene/butyl acrylate initiated with 2, 2′-azobisisobutyronitrile (AIBN) was conducted at 50°C using various initiator concentrations and initial monomer ratios including the azeotropic composition. The experiments were run through the full conversion range in order to collect composition, rate and molecular weight data. A mechanistic model for the bulk copolymerization of styrene/butyl acrylate was subsequently developed. The dynamic model can predict conversion, instantaneous and accumulated copolymer composition and number-average and weight-average molecular weights. Model predictions were compared with experimental data and were found to be in agreement. From this interplay of experimental and modelling efforts, more light was shed on the kinetic mechanisms of this important copolymer system. Postulations were made concerning branching characteristics and the significance of the penultimate versus terminal unit effects.

The rate of copolymerization of styrene and methyl methacrylate—II. The gel effect in copolymerization

Abstract An analysis of the rate data for the copolymerization of styrene with methyl methacrylate at 60° is given over a conversion range from 0 to 95%. A measure of the increase in rate observed over that to be expected from classical kinetics is calculated. It is termed the gel effect index, γ, and is used to systematize the observations. The results are interpreted in terms of three major developments which can be seen in γ, viz. the onset of translation diffusion control of termination, the onset of a strong decrease in initiator efficiency, and the practical cessation of polymerization.