Styrene polymerization thermodynamics

The presence of a carbon-halogen bond is not absolutely essential for the occurrence of tranfer to monomer. Moore et al. [37] studied styrene polymerization with y-irradiation. They measured thermodynamic quantities, particularly the Gibbs energy, enthalpy, entropy and volume changes by the method of rotating sectors and found that transfer to monomer is negli-... 

However, some very early work on styrene polymerization by Walling, Briggs and Mayo pointed out the necessity of using the thermodynamic activity of the monomer, and not its concentration, which Rosen et al. had done in their work. The presence of polymer in a solvent dramatically reduces the activity of the solvent, as indicated by vapour pressure measurements. Cameron and Cameron and Lissi et have made pertinent observations suggesting this... 

This is one of the reasons we decided to prepare oligomers containing styrene-type functional groups. Styrene s thermal initiation mechanism is fairly well understood, and the same is true for the kinetics and thermodynamics of its radical polymerization. In addition, thermal and radical polymerization of styrene is much faster than any of the other previous classes of reactive groups and at the same time, the microstructure of the crosslinking points is known. 

In conclusion, phase transfer catalyzed Williamson etherification and Wittig vinylation provided convenient methods for the synthesis of polyaromatics with terminal or pendant styrene-type vinyl groups. Both these polyaromatics appear to be a very promising class of thermally reactive oligomers which can be used to tailor the physical properties of the thermally obtained networks. Research is in progress in order to further elucidate the thermal polymerization mechanism and to exploit the thermodynamic reversibility of this curing reaction. 

Lebedev, B.V., Lebedev, N.K., Smirnova, N.N., Kozyreva, N.M., Kirillin, A., and Korshak. The isotope effect in the thermodynamic parameters of polymerization of styrene, Dokl. Akad. Nauk, SSSR 281 379-383, 1985. 

The analysis of the reaction serum (the continuous phase without polymer particles) at the end of polymerization led to the conclusion that the molecular weight of the soluble oligomers of styrene and PEO macromonomer varied from 200 to 1100 g mol-1. This indicates that the critical degree of polymerization for precipitation of oligomers in this medium is more than ten styrene units and only one macromonomer unit per copolymer chain. Several reasons for the low molecular weight of the soluble copolymers were proposed, such as the thermodynamic repulsion (or compatibility) between the PEO chain of the macromonomer and the polystyrene macroradical, the occurrence of enhanced termination caused by high radical concentration, and, to a lower extent, a transfer reaction to ethanol [75]. 

For some particular formulations (e.g., unsaturated polyesters formulated with a high styrene concentration), the primary chains that are first generated are not miscible with the unreacted monomers. In this case, there is a phase separation phenomenon characterized by the appearance of relatively large polymer-rich particles. These microgels are formed by a thermodynamic driving force and their sizes are large enough to be detected in both the course of polymerization and the final materials. 

Polymeric supports can also be used with advantage to form monofunctional moieties from difunctional (Hies. Leznoff has used this principal in the synthesis of sex attractants on polymer supports (67). Starting from a sheap symmetrical diol he blocked one hydroxyl group by the polymer. Functionalization of cross-linked polymers is mostly performed by chloromethylation (65). A very promising method to introduce functional groups into crosslinked styrene-divinylbenzene copolymers is the direct lithiation with butyllithium in presence of N,N,N, N -tetramethyl-ethylenediamine (TMEDA) (69, 70). Metalation of linear polystyrene with butyl-lithium/TMEDA showed no exchange of benzylic hydrogen and a ratio of attack at m/p-position of 2 1 (71). In the model reaction of cumene with amylsodium, a kinetic control of the reaction path is established. After 3h of treatment with amyl-sodiuni, cumene is metalated 42% in a-, 39% m-, and 19% p-position. After 20h the mixture equilibrates to affort 100% of the thermodynamically more stable a-prod-uct (72). 

They also tested (10) Case II for the free radical polymerization of styrene (Mi) and methyl methacrylate at 132°C. by the dilution technique. These data are also shown in Table I, where the good agreement between theory and experiment is apparent. The applicability of the theory to different mechanisms of polymerization is a nice verification of the statement that the composition is governed by end-state thermodynamics rather than by mechanism. 

Studies by many authors, e. q. on copolymerizations of styrene with a-methylstyrene (characterized by low Tc, 334 K), appear to agree with the ideas of Lowry. Some author claim, however, that even copolymerization of this pair can be described by the simple copolymerization equation [221], Johnston and Rudin are of the opionion that the depropagation reaction is not as important in this case because only short sequences of a-methylstyrene are produced. The formation of short blocks is accompanied by relatively high polymerization enthalpy. They are thermodynamically more stable than the homopolymer and have a higher Tc. Only at considerably higher copolymerization temperatures (with the pair styrene—a-methylstyrene > 420 K) does the depropagation effect become important. 

Application As is well-known in the industry, any microporous material which is formed through a nonequilibrium process is subject to variability and nonuniformity, and thus limitations such as block thickness, for example, due to the fact that thermodynamics is working to push the system toward equilibrium. In the present material, the microstructure is determined at thermodynamic equilibrium, thus allowing uniformly microporous materials without size or shape limitations to be produced. As an example, the cubic phase consisting of 44.9 wt% DDAB, 47.6% water, 7.0% styrene, 0.4% divinyl benzene (as cross-linker), and 0.1% AIBN as initiator has been partially polymerized in the authors laboratory by themal initiation the equilibrated phase was raised to 8S°C, and within 90 minutes partial polymerization resulted S AXS proved that the cubic structure was retained (the cubic phase, without initiator, is stable at 65°C). When complete polymerization by thermal initiation is accomplished, then such a process could produce uniform microporous materials of arbitrary size and shape. 

The formulation of two types of ion-pair is an attractive hypothesis which has been used for other systems [130] to explain differences in reactivity. The polymerization of styrene-type monomers in ether solvents, all of which solvate small cations efficiently, seems to be a particularly favourable case for the formation of thermodynamically distinct species. Situations can be visualized, however, in which two distinct species do not exist but only a more gradual change in properties of the ion-pair occurs as the solvent properties are changed. These possibilities, together with the factors influencing solvent-separated ion-pair formation, are discussed elsewhere [131, 132]. In the present case some of the temperature variation of rate coefficient could be explained in terms of better solvation of the transition state by the more basic ethers, a factor which will increase at lower temperatures [111]. This could produce a decrease in activation energy, particularly at low temperatures. It would, however, be difficult to explain the whole of the fep versus 1/T curve in tetrahydrofuran with its double inflection by this hypothesis and the independent spectroscopic and conductimetric evidence lends confidence to the whole scheme. 

The first application of living polymers in thermodynamic studies was reported by Worsfold and Bywater181) and by McCormick182). In both studies anionic polymerization of a-methyl styrene, initiated in tetrahydrofuran by electron-transfer, was investigated over temperatures ranging from 0°C to -40°C. The results are shown graphically in Fig. 3, and lead to heat of polymerization of — 8 kcal/mol, entropy of polymerization in solution of 29 eu, and to about 1 M equilibrium concentration of a-methyl styrene at ambient temperature. 

Anionic polymerization was utilized again in studies of thermodynamics of styrene propagation185. The equilibrium concentration of that monomer is exceedingly low at ambient temperature, and hence the experimentation required elevated temperatures. However, living polystyrene in THF is rapidly destroyed at those temperatures. To avoid these difficulties, living polystyrene formed by BuLi initation in cyclohexane or benzene was used in the studies. The results are presented in Fig. 5 and in Table 1. The effect of the solvent s nature on Me is revealed by these data. 

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