Copolymerisation kinetics

Of the reports that have appeared [37,72,80-90], only a few deal with more quantitative studies. In [86,89] the copolymerisation kinetics have been studied using the precursor [Pd(Ts0)(H20)(dppp)](Ts0) in MeOH over a temperature range of 70-90 °C and total pressure up to 70 bar. The rate increases linearly by increasing catalyst loading, the orders with respect to dissolved CO and ethene are 0.63 and 0.72, respectively the apparent activation energy is 11.7 kcal/mol. 

Statistical copolymerisation kinetics with the crosslinker lessens the heterogeneity of receptor sites. 

Raman fibre optics has been used to study the emulsion homopolymerisations of styrene and n-butyl acrylate (35). An IR spectroscopic technique for the examination of radical copolymerisations of acryl and vinyl monomers was developed. A comparative study of the copolymerisation of model monomer pairs was made using monofunctional and polyfunctional compounds. The data established the role of structural-physical transformations, involved in the formation of crosslinked polymers, on the copolymerisation kinetics and on the nonuniformity of distribution of crosslinks in the copolymers formed (151). Raman fibre optics of polymerisation of acrylic terpolymers was also used to monitor as well as an on-line measurement of morphology/composition (66). The high temperature (330 °C) cure reaction of 4-phenoxy-4 -phenyl-ethynylbenzophenone was monitored using a modulated fibre optic FT-Raman spectrometer (80). 

This chapter focuses on key features to understand the emulsion copolymerisation kinetics and on the influence of operation on the copolymer composition of the final latex products. Focus is on batch and semi-batch or semi-continuous operation, see Figure 4.1. Only the free-radical emulsion copolymerisation of two monomers is considered but the concepts can be directly applied for formulations containing more than two monomers. The reacting monomers usually having different reactivities, polymerise simultaneously. The reactivities and the individual concentrations of the monomers at the locus of polymerisation, that is, the particle phase, govern the built-in ratio into the polymer chains at a certain time. 

The monomer concentrations of the polymerising monomers in the particle phase together with copolymerisation kinetics, govern the rates by which the monomers are... 

The kinetics of copolymerisation are rather complex since four propagation reactions can take place if two monomers are present... 

On the basis of the kinetic characteristics of chain polymerisation reactions, it is possible to predict the final microstructures available by a so-called random process from a simple mixture of two comonomers. Indeed, the global mechanism of copolymerisation can be illustrated as presented in Figure 30. 

Attempts to polymerise isobutene by free radical catalysis have all failed [16,17] and copolymerisation experiments show that the t-butyl radical has no tendency to add to isobutene. The reasons for these facts are not at all obvious. Evidently, they cannot be thermodynamic and therefore they must be kinetic. One factor is probably that the steric resistance to the formation of polymer brings with it a high activation energy [17], and that the abstraction by a radical of a hydrogen atom from isobutene, to give the methallyl radical, has a much smaller activation energy. This reaction will also be accelerated statistically by the presence of six equivalent hydrogen atoms. 

The details of which type of complex (it- or n-) has what kinetic and other effects, e.g., on termination and transfer constants, copolymerisation ratios, tacticity, etc., need to be worked out on the basis of published results and well-aimed new experiments. 

There are many studies of the mechanism and kinetics of the polymerisation of cyclic oxygen compounds, but only relatively few of these are concerned with cyclic formals. In the present paper I will review this field, and I hope to show that formals have some special characteristics which distinguish their polymerisations from those of other cyclic oxygen compounds. Since it is not possible to deal with all aspects of this group of reactions in one lecture, I will concentrate attention on questions of chemistry and mechanism, and I will not deal with other aspects, such as the thermodynamics and kinetics of these polymerisations. Most of the published work has been done with 1,3-dioxolan (I) and there are only very few papers on any other cyclic formals, although the patent literature on the homo- and copolymerisation of (I) and other cyclic formals is quite extensive. 

In the work of Belov et al. the kinetic model that has been developed quantitatively describes the initial rate of copolymerisation, the kinetic curves of the consumption of the two monomers and the molecular weight characteristics of the resulting copolymers and their composition as mixture of ketoesters, diesters, diketones as function of the total pressure up to 40 bar,... 

Staffer et al. [81] have investigated the sonochemical polymerisation of both methyl methacrylate and acrylamide. No polymerisation was observed in the absence of an initiator. However in the presence of initiator and ultrasound, polymerisation conformed to the usual radical kinetics. Orszulik [82] has also been able to show that whilst polymerisation and copolymerisation of acrylic monomers did not occur in the absence of the initiator, in the presence of AZBN as initiator moderately high yields were produced after prolonged sonication (17 h). 

A number of ex situ spectroscopic techniques, multinuclear NMR, IR, EXAFS, UV-vis, have contributed to rationalise the overall mechanism of the copolymerisation as well as specific aspects related to the nature of the unsaturated monomer (ethene, 1-alkenes, vinyl aromatics, cyclic alkenes, allenes). Valuable information on the initiation, propagation and termination steps has been provided by end-group analysis of the polyketone products, by labelling experiments of the catalyst precursors and solvents either with deuterated compounds or with easily identifiable functional groups, by X-ray diffraction analysis of precursors, model compounds and products, and by kinetic and thermodynamic studies of model reactions. The structure of some catalysis resting states and several catalyst deactivation paths have been traced. There is little doubt, however, that the most spectacular mechanistic breakthroughs have been obtained from in situ spectroscopic studies. 

The activation of (P-P)Pd" promoters in MeOH proceeds via formation of Pd"-OMe (Eq. (1)) that can straightforwardly initiate the catalysis cycle or generate Pd"-H via P-H elimination, yielding formaldehyde (Eq. (2)) [16]. The fast kinetics under real copolymerisation conditions do not allow for the spectroscopic detection of Pd-H initiators. However, their formation has been unambiguously assessed by end-group analysis, isotopic labelling experiments and model reactions [Ij. 

Kinetic and Thermodynamic Studies of Ethene/CO Copolymerisation in Aprotic Solvents... 

Besides the isolation and characterisation of several catalytically relevant intermediates, model reaction studies, generally based on variable-temperature NMR experiments in CD2CI2, with isolated Pd" complexes have provided valuable kinetic and thermodynamic information on the mechanism of the alternating ethene/CO copolymerisation. 

Kinetic studies of migratory insertion reactions of the ligands that are involved as (P-P)Pd" fragments in either the propagation cycle of ethene/CO copolymerisation or ethene dimerisation to butenes have been reported by Brookhart [28] and Bian-chini [5e, fj. 

In situ NMR analysis has also been used to determine the kinetic barriers for the migratory insertions of methyl carhonyl complexes [Pd CO) Me)(PPh2 CH2) PPh2)] (n = 2-4) relevant to propagation in ethene/CO copolymerisation. It was found that the steric bulk of the diphosphine has a significant effect on the insertion barriers with the most bulky ligand having the lowest barrier. 

The rate of copolymerisation of ethylene and odd-membered ring cycloolefins is higher than the rate of copolymerisation involving even-membered cycloolefins [467]. This indicates that both the polymerisation kinetics and the spatial configuration of the copolymer are influenced by steric factors [2]. 

By setting the temperature of the reaction medium at 60 C from the beginning of the IPN formation, the PUR synthesis is accelerated, and that of the methacrylic system begins after the usual inhibition period. The competition between the two processes can still favour the complete formation of PUR before appreciable radical copolymerisation may have taken place, though the kinetic curves may change or even cross. For this reason, a second factor, the content of PUR catalyst, is varied too with less stannous octoate, the formation of the first network is more or less delayed, even at 60 C, and counterbalances to some extent the effect of temperature. In such a case, the conversion of the methacrylic phase may proceed further before higher or even post-gel conversions are reached for polyurethane. Thus, IPNs in which both networks have been formed more or less simultaneously, are obtained by this... 

Experimental System The copolymerisation of styrene with methyl acrylate in toluene using azo-bis-iso- butyronitrile (AIBN) was selected as the model experimental system because the overall rate of reaction is relatively fast, copolymer analysis is relatively simple using a variety of techniques and the appropriate kinetic and physical constants are available in the literature. This monomer combination also has suitable reactivity ratios (i = 0.76 and r4 =0.175 at 80 C),(18) making control action essential for many different values if compositionally homogeneous polymers are to be prepared at higher conversions in a semi-batch reactor. 

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