Average propagation rate coefficients in the free-radical copolymerization of styrene and alpha-methylstyrene measured by pulsed-laser polymerization

The average propagation rate coefficient, [k(p)], has been measured in the free-radical copolymerization of styrene (STY) and cl-methylstyrene (AMS) using pulsed-laser polymerization. The value of[k(p)] was found to decrease by 2 orders of magnitude as the mole fraction of STY, f(STY), is decreased from 1.00 to 0.12. The effect of temperature on [k(p)] was measured over the range 17.9-47.4 degrees C, and the Arrhenius parameters were determined. The reduction in [k(p)] with decreasing f(STY) is primarily attributed to entropic factors associated with the a-methyl group in AMS, as revealed by a reduction in the Arrhenius preexponential factor, A. Homopolymerizations of AMS did not yield molecular weight distributions suitable for successful evaluation of k(p), which is attributed to transfer to monomer dominating the chain stopping events. Mathematical expressions relating [k(p)] to monomer composition, allowing for different depropagation kinetics, are derived. It was found that depropagation events did not play a significant role in this work and that the terminal model for [k(p)] provided an adequate description of the observed propagation kinetics. Since k(p) for pure AMS could not be measured directly, values were estimated by extrapolation of the copolymerization [k(p)] data and the resulting Arrhenius parameters are E-a = 36.7 kJ.mol(-1) and A = 10(6.17) dm(3).mol(-1).s(-1)

Mechanism of catalytic chain transfer in the free-radical polymerisation of methyl methacrylate and styrene

The mechanism of catalytic chain transfer with bis(boron difluorodimethylglyoximate) colbaltate(II) (COBF) has been studied in the homopolymerisations of methyl methacrylate and styrene. The chain transfer constants were measured using both the Mayo and Chain Length Distribution (CLD) methods over a range of temperatures (40-70 degrees C). The two methods generally agree within 10%. The high values of the chain transfer rate coefficients, k(tr) (similar to 10(7) for MMA), suggest the possibility that the reaction is approaching diffusion control. This is also supported by the high values obtained for the frequency factor (A similar to 10(10)). The chain transfer rate coefficients for styrene are approximately two orders of magnitude lower than those obtained for MMA which can be explained in terms of the formation of cobalt-carbon bonds and the accessibility of beta-H sites for hydrogen abstraction from the two different radical chain ends in the case of styrene. High conversion, solution polymerisation experiments on methyl methacrylate in toluene reveal behaviour inconsistent with a simple catalytic mechanism and may suggest deactivation of the catalyst by solvent. On the assumption that the kinetics of catalytic chain transfer can be explained by a classical free-radical mechanism, it is possible to derive information on the chain length dependence of the average termination rate coefficient, (k(t)). Applying this approach to methyl methacrylate and styrene at different temperatures, we have found that the chain length effect on (k(t)) appears to be independent of both temperature and monomer type

Copolymerization of styrene and alpha-methylstyrene in the presence of a catalytic chain transfer agent

Copolymerizations of styrene (STY) and alpha-methylstyrene (AMS) have been performed at different monomer feed compositions and temperatures (40-70 degrees C) in the presence of the catalytic chain transfer agent bis(boron difluorodimethylglyoximate)cobaltate(II) (COBF). The average chain transfer constant, [C-s], was found to increase approximately 3 orders of magnitude upon going from pure Sm to pure AMS. The addition of only 10% AMS increased the (Cs) by 1 order of magnitude. This behavior can be predicted from the relative fractions of growing radical end groups, which results in the majority of polymer chains being formed with an unsaturated AMS end group. No significant penultimate unit effects in the chain transfer reaction were observed. The addition of 10-20% AMS results in the majority of the growing radicals having AMS end groups, and hence AMS dominates the catalytic chain transfer reaction. The [C-s] values continue to increase as the AMS content is increased beyond 20%; however, this can be mostly attributed to a decrease in [k(p)] rather than an increase in [k(tr)]. The expected end groups in the homopolymerization of AMS in the presence of COBF were confirmed by matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS)

Chain transfer to monomer in the free-radical polymerizations of methyl methacrylate, styrene, and alpha-methylstyrene

The monomer chain transfer constant (C-M) has been evaluated for three monomers in bulk polymerization, viz, methyl methacrylate (MMA), styrene (STY) and alpha-methyl styrene (AMS). The new chain length distribution (CLD) analytical method was utilized to determine values for C-M at the low initiation limit and compared to the traditional Mayo method. The values for C-M obtained at 50 degrees C using the CLD method were (C-M X 10(5)) 5.15, 5.27, and 412 for MMA, STY, and AMS, respectively. The extremely high value for AMS originates in the low propagation rate coefficient for AMS. The values for the chain transfer rate coefficients (k(tr)) were calculated were (k(tr) x 10(2) dm(3).mol(-1).s(-1)), 3.34, 1.25, and 0.711 for MMA, STY, and AMS, respectively. The similarity of the values for k(tr) are discussed in terms of possible mechanisms for the chain transfer to monomer reaction. The strong influence of transfer to monomer as a chain-stopping mechanism in AMS polymerization was confirmed by end group studies using matrix-assisted laser desorption ionization mass spectrometry

COPOLYMERIZATION OF A NOVEL SUBSTITUTED METHACRYLAMIDE WITH METHYL-METHACRYLATE

The copolymerization behaviour of the adduct of N',N'-dimethylaminopropyl methacrylamide and epichlorohydrin (DMAPMA-Epi) and methyl methacrylate (MMA) was investigated. The reactivity ratios for the MMA (1)/DMAPMA-Epi (2) couple in solution, using various regression methods based on the Mayo-Lewis model, are r1 = 1.73 and r2 = 0.03. The Q and e values for DMAPMA-Epi are estimated to be 0.23 and - 1.32 respectively. On the basis of the reactivity ratios, theoretical curves for the instantaneous copolymer composition throughout conversion have been calculated

COBALT-MEDIATED FREE-RADICAL POLYMERIZATION OF ACRYLIC-MONOMERS

The use of square-planar low-spin cobalt complexes to control the free-radical polymerization of vinyl monomers is described. The control exerted depends on both the catalyst and the monomer structure. Monomers with a beta-hydrogen (for example, methacrylates) undergo catalytic chain transfer, which is a powerful synthetic technique for producing macromonomers. Monomers without a beta-hydrogen (for example, acrylates) are capable of pseudo-living polymerization by exploitation of the 'persistent radical' effect. The structure of the cobalt complex is shown to be very important in determining its catalytic activity

Copolymerization of styrene and methyl methacrylate in the presence of a catalytic chain transfer agent

Copolymerizations of styrene and methyl methacrylate have been performed using different monomer feed compositions in the presence of a catalytic chain transfer agent at 40 degrees C. Average chain transfer constants as a function of monomer feed composition were determined with the conventional Mayo procedure using both number (M-n) and weight (M-w) average molecular weights, and with the chain length distribution procedure using both high (Lambda(H)) and peak (Lambda(P)) molecular weight slopes. It is found that the average chain transfer constants determined from M-w and Lambda(P) are generally very similar, with those obtained from M-n and Lambda(H) being larger and smaller, respectively. The average chain transfer constants obtained from M-w and Lambda(P) are compared with model predictions based upon both the terminal and penultimate unit models of free-radical copolymerization and are in satisfactory agreement. These two models are used to predict the fraction of propagating radicals with a terminal styrene unit, and it is found, similar to earlier studies reported in the literature, that this parameter is very sensitive to the penultimate model s values (in contrast to the average propagation rate coefficient), and a satisfactory agreement between model and experiment is obtained for s(M) = 2. This finding possibly suggests a complimentary route to measuring average propagation rate coefficients, for the determination of penultimate model s values

Novel polymers from atom transfer polymerisation mediated by copper(I) Schiff base complexes

The use of copper(I) Schiff base complex catalysed atom transfer polymerisation of methacrylates is described. The use of a range of functional and multi-functional initiators enables the synthesis of a range of functional and star polymers to be prepa