Control of the Termination Mechanism in Radical Polymerization by Viscosity: Selective Disproportionation in Viscous Media

Abstract

The termination mechanism of radical polymerization, that is, disproportionation (Disp) versus combination (Comb), determines the chain length and end-group structure of the resulting polymer as well as polymer properties, and yet factors governing the mechanism are still unclear. Furthermore, no attempts have been made to control the mechanism. Here, the effects of temperature and viscosity on the termination of methyl methacrylate (MMA) and styrene (St) polymerization were elucidated by using small molecular model-radicals and the corresponding polymer radicals in various solvents. The results showed that Disp was preferred over Comb if the temperature was decreased and the viscosity of the media was increased for all the radicals examined. Although the temperature effect on the Disp/Comb selectivity is counterintuitive because Disp should be favored entropically over Comb considering the decrease in the number of polymer chains in Comb, the results clearly showed that the observed inverse temperature effect was a result of the viscosity effect. Disp was favored over Comb at lower temperatures and in more viscous solvents because the transition state leading to Disp is more flexible than that for Comb. Because of the significant viscosity effect, Disp selectively occurred in highly viscous solvents; the Disp/Comb selectivity was 97/3 in both MMA and St termination. For the first time, the termination mechanism was intentionally controlled and such a high Disp selectivity was observed. In particular, the termination mechanism in St is described as Comb in textbooks, but nearly complete inversion of the selectivity from Comb to Disp is realized by simply changing the viscosity of the media.

The Effect of Viscosity on the Diffusion and Termination Reaction of Organic Radical Pairs

“…[18,19] They reacted with each other,y ielding the Disp products 9c and 10 c and/ort he Comb product 11 c (Scheme 2, path Ai llustrating the case of methyl methacrylate (MMA) termination). [17] In contrast, Matyjaszewski and Poli have proposed that the tellanyl-radical-mediated chain-transfer pathway is responsible for the formation of Disp products in our study from the mar-ginal Disp/Comb selectivity on the viscosity by using V-601. [16] 2) The termination of methacrylate and styrene polymerizations are significantly changed by the viscosity of the reaction media, and selective Disp was observedi nh ighly viscous media in both cases.…”

“…[15][16][17] Polymerend radicals, such as 7c,w ith controlled molecular weight are generated from the corresponding "living" polymer having the organotellurium end group 8c. [15][16][17] Polymerend radicals, such as 7c,w ith controlled molecular weight are generated from the corresponding "living" polymer having the organotellurium end group 8c.…”

In Situ Incorporation of Praziquantel in Polymer Microparticles through Suspension Polymerization for Treatment of Schistosomiasis

“…The obtained results indicate that addition of the comonomers shifted the distributions toward smaller molar masses, showing the influence of the comonomer on the properties of the final polymer. As MMA termination occurs mainly through disproportionation and monomer conversions were high in all cases and were not affected significantly by addition of the cationic comonomers, this fact may be related to increased chain transfer to the cationic comonomers, which can be favored when the lateral group present in the main chain is bulky, making more difficult the approach of the larger comonomer molecules to the reactive centers due to steric hindrance . This can possibly explain, as shown in Figure 15 , why the addition of praziquantel exerted less significant influence on the molar mass distributions of the obtained copolymers.…”

Catalyzed Radical Termination in the Presence of Tellanyl Radicals

“…The % Disp and Comb obtained in all our experiments are collected in the Supporting Information (Table S1) and shown graphically in Figure and Figure S12. Examination of these results shows quite clearly that the temperature and the solvent nature have little effect, if any, on the relative proportions of Disp and Comb, thus questioning the validity of the previously proposed “advanced collision model” …”

“…radicals, it was surprising to find that acrylate radicals resulting from a pMA−TePh macroinitiator terminate almost exclusively (99 %) through Disp, in contrast with several other previous and subsequent reports. In the most recent contribution, it was shown that the product distribution depends on the reaction medium, a greater viscosity resulting in an increase of the Disp/Comb ratio. For example, 59 % Disp was observed for the methacrylate unimer model compound Me 2 C(TeMe)CO 2 Me at relatively low viscosity ( η =1.1 mPas), whereas this amount increased significantly to 94 % at η =84 mPas at room temperature.…”