Temperature Solvent Polarity Effects

Polar stilbenes with electron donor and acceptor groups at the 4- and 4 -positions. With these compounds a strong solvent polarity effect overlaps the temperature effect. 4,4 -NMS behaves as a group 2 compound in... 

Sipos, L., De, P., Eaust, R., 2003. Effect of temperature, solvent polarity, and nature of lewis acid on the rate constants in the carbocationic polymerization of isobutylene. Macromolecules 36 (22), 8282-8290. 

Effects of solvent polarity, counter-anion nucleophilidty, temperature, and monomer concentration on the carbenium ion polymerization chemistry have been extensively studied29,36 38,49. Based on previous knowledge26"29 Me3Al was chosen because with this coinitiator undesired chain transfer to monomer processes are absent. Preliminary experiments showed that Et3Al coinitiator did not yield PaMeSt, possibly because the nuc-leophilicity of the counter-anion Et3AlQe is too high and thus termination by hydrida-tion is faster than propagation36. ... 

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. 

On the basis of these studies we decided to carry out a series of AMI and IMA experiments (2) with the TMPCl/EtAlCl2/DtBP combination. Figures 1 and 2 show the results. The M versus Wp (g of poly(P-PIN) formed) plots and the N (number of moles of poly(P-PIN) formed) versus Wp plots (insets) indicate increasing deviation from the theoretical values (calculated for Ieff = 100%). According to these results chain transfer proceeds in these polymerizations, i.e., the systems are nonliving. Further experimentation would be necessary to develop satisfactory living conditions, in particular to investigate the effect of solvent polarity, temperature and electron donors on the mechanism. 

The susceptibility or mixing coefficients, pj and pj , depend upon the position of the substituent (indicated by the index, /) with respect to the reaction (or detector) center, the nature of the measurement at this center, and the conditions of solvent and temperature. It has been held that the p/scale of polar effects has wide general applicability (4), holding for substituents bonded to an sp or sp carbon atom (5) and, perhaps, to other elements (6). The or scale, however, has been thought to be more narrowly defined (7), holding with precision only for systems of analogous pi electronic frameworks (i.e., having a dependence on reaction type and conditions, as well as on position of substitution). 

Hot splitless WCOT 0.5 ppm (FID) without preconcentration Lower injection temperature than split Trace analysis Handles dirty samples Automation Flash vaporisation Optimisation required (splitless time, oven temperature, solvent) Limited number of solvents ( solvent effect ) Thermal degradation possible Discrimination possible Poor direct quantification Unsuitable for very polar substances... 

Effect of Solvent Polarity. Quasiliving polymerization of IBVE was also attempted in a nonpolar solvent, n-heptane, ap -50 and -70°C. Figure 5 gives results obtained at -70°C. The Mn versus Wj ve plots obtained at this temperature are strongly curved. Importantly, the N values were less than unity at the beginning of the reactions and they increased beyond unity with increasing WibVE Evidently initiation is slow in nonpolar media due to incomplete ionization of the initiator (i.e., N >1). 

In the foregoing discussions we have taken the DC as a measure of the polarity of a solvent, but it is of course no more than a rough make-shift, for lack of anything better. As to the effects of solvent polarity on kp, very much the same applies as was said about the effects of temperature. Attempts at correlating kp with solvent polarity (through the... 

The reaction constant p depends on nature of reaction, solvent and temperature. It is a measure of the susceptibility of the reaction to polar effect. A positive value of p for a reaction shows that the reaction is accelerated by electron withdrawing substituents (o = + 1.0). Thus a positive value of p indicates the reaction center has higher electron density in the transition state than in the starting material. While negative value of p indicates that the reaction center has a lower electron density in the transition state than in the starting material and the reaction is accelerated by electron donating... 

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