Solvent polarity effects, initiator systems

Research described in this section concerns effects of solvent polarity, temperature, monomer and initiator concentration on the polymerization of a-methylstyrene with Si-H containing initiator/Me3Al system for the synthesis of HSi-PaMeSt and of desirable molecular weight. 

Table 10. The effect of solvent polarity on the polymerization of isobutylene by the HSi(CH3)2CH2CH29 CH2Q/Me3Al initiating system...

Another, more indirect but perhaps more efficient method, would be to determine K for a number of typical systems, perhaps by use of model compounds, and then to select for kinetic experiments initiator systems for which K is so great that [Pn+] is effectively equal to c0, so that then the simple Equation (1) with [Pn+] = c0 is applicable. The trouble is that this method will probably only work for fairly polar solvents, because it is to be expected that Kp will be smaller, the less polar the solvent. This effect is probably one of the factors responsible for the improbably low kp value obtained by Higashimura for styrene in benzene solution [7]. In any case, for solvents of low polarity the participation of paired cations must be taken into account, which makes the relevant equations rather more complicated, but does not alter the relevance and importance of equilibrium (i). 

For example, the polymerization of alkyl vinyl ethers using an HC1/ SnCU (or adduct S/SnCl4) initiating system in methylene chloride is very fast even at - 15° C to give polymers with broad and often bimodal MWDs (Figure 17D) [105], Similar effects of solvent polarity are found in the polymerizations of p-alkoxystyrenes [107], styrene [25], and iV-vinylcar-bazole [108],... 

A considerable amount of work has been done to optimize the preparation of the di-.v-butyllithium adduct of 1,3-diisopropenylbenzene (ra-DIPB). Under the the right conditions, an effective initiator can be prepared with minimal m-DIPB dimerization [13]. The principle drawback of this system is that a polar additive is required, which has implications for the microstructure, as outlined above. The addition of 2mol of butyllithium to l,3-bis(l-phenylethenyl)ben-zene (DDPE) can be carried out in pure hydrocarbon solvents. However, efficient diinitiation requires the addition of a polar modifier or alkoxide salt [14], Another drawback to this approach is the high cost of the diolefin. 

Charge transfer transitions give rise to electronic spectra which suffer medium effects because there is an alteration in charge distribution between the electronic states in the molecular system. The electronic transition involves transfer of an initial state to an excited state where the two ions have partially neutralised each other the interaction of this state with solvent therefore provides a measure of solvent polarity. These systems suffer from the problem that the electronic process of charge transfer is very unlike that of reactions normally under investigation. 

Thus, a dipolar aprotlc solvent such as dlmethylsulfoxide provides the necessary solvation and polarity to render lithium alkoxldes as effective initiators for ethylene oxide polymerization. Work is underway to further explore the scope and kinetics of this Important polymerization system. 

In the classical limit (hcoj k T), the reaction coordinate X t) in each quantum state can be described as a Gaussian stochastic process [203]. It is Gaussian because of the assumed linear response. As follows from the discussion in Section II.A, if the collective solvent polarization follows the linear response, the ET system can be effectively represented by two sets of harmonic oscillators with the same frequencies but different equilibrium positions corresponding to the initial and final electronic states [26, 203]. The reaction coordinate, defined as the energy difference between the reactant and the product states, is a linear combination of the oscillator coordinates, that is, it is a linear combination of harmonic functions and is, therefore, Gaussian. The mean value is = — , for state 1 and = , for state 2, respectively. We can represent Xi(r) and X2 t) in terms of a single Gaussian stochastic process x(t) with zero mean as follows ... 

This relationship is in agreement with the known dependence of rate on the square root of initiator concentration, and the rate acceleration tvithin the particles due to hindrance of termination due to the gel effect The effect of a on rate has been noted for styrene polymerizations in mixed solvents. Polymerization proceeds faster in the polar water/ethanol system than in either the less polar etiianol [28] or meihotgrethanol/etiianol blends [46] owing to the greater partitioning of the ncm-polar styrene into the particle. 

Subsequently, Overberger and coworkers proposed that insensitivity of reactivity ratios of these experimental parameters resulted from preferential solvation of the propagating carbenium ion by the most polar component present in the system (either nitrobenzene from the mixed solvent system they employed or chlorostyrene monomer). The active center in m-DIPB/m-DMB copolymerizations could be preferentially solvated by free m-DMB thus screening any effect of changing bulk solvent. However, Overberger, Ekrig, and Tanner 1 noted no dependence of reactivity ratios on initiation system whereas m-DMB/ m-DIPB copolymerizations are very dependent on the nature of the initiation system. Obviously, a more detailed study is required. 

Additionally, it has been noted that Tetralin operates via hydride transfer, at least in its reduction of quinones. Thus it has been shown that Tetralin readily donates hydrogen to electron-poor systems, such as quinones at 50°-160°C. The reaction is accelerated by electron-withdrawing substituents on the H-acceptor and polar solvents, and is unaffected by free radical initiators (6). These observations are consistent with hydride transfer, as is the more recent finding of a tritium isotope effect for the reaction (7). 

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