Pronounced Effects of Temperature and Monomer Concentration on Isotactic Specificity of Triphenylmethyl Methacrylate Polymerization through Free Radical Mechanism. Thermodynamic versus Kinetic Control of Propagation Stereochemistry

Abstract

The stereochemical control of free radical reaction is generally more difficult than that of the other types of reactions. 1 This is the case not only for the organic synthesis but also for macromolecular chemistry; only few examples are known as stereoregular free radical polymerization in solution. 2 -5 Further, the stereospecificity of free radical polymerization in solution relies mainly on the structure of starting monomer, which makes it difficult to obtain polymers with widely varying stereoregularity from a certain monomer.In the present study, we found remarkable effects of reaction temperature, monomer concentration in feed ([M] 0 ), and solvent on tacticity in the free radical polymerization of triphenylmethyl methacrylate (TrMA) and succeeded in the synthesis of the polymers having triad isotacticity (mm) ranging from 64% to 99%. 6 -8 This communication describes the stereochemistry of TrMA polymerization in comparison with that of the polymerizations of methyl methacrylate (MMA) and 1-phenyldibenzosuberyl methacrylate (PDBSMA), a TrMA analogue, which we have reported to give a highly isotactic polymer by radical polymerization. 3 TrMA is known to give an isotactic polymer by radical polymerization though the triad isotacticity has been reported to be only 64%. 2 The isospecificity in this polymerization can be ascribed to a helical conformation of the growing radical because TrMA leads to an optically active, almost perfectly isotactic, and purely single-handed helical polymer by asymmetric anionic polymerization 9 -11 as well as the other bulky methacrylates. 12 The conditions and results of polymerization ofTrMA, PDBSMA, and MMA in toluene are summarized in Table I. It can roughly be said that a lower temperature and a higher [M] 0 give a higher yield and a higher degree 556 of polymerization (DP) of the products in the polymerizations of the three monomers. A higher propagation rate at a higher [M] 0 and lower chances of depolymerization at a lower temperature may be responsible for these observations.The polymers having the wide range of tacticity were obtained by changing reaction temperature and [M] 0 in TrMA polymerization. At a higher polymerization temperature and at a lower [M] 0 , a higher isotacticity was achieved; the mm content of the polymer exceeded 98% under the conditions of runs 4 and 7 (Figure 1). In contrast to the TrMA polymerization, almost perfectly isotactic polymers were obtained regardless of [M] 0 and temperature in the polymerization of PDBSMA (runs 8-11) 13 and the stereochemistry of MMA polymerization was not affected by [M] 0 (runs 12-14). Reaction temperature is known to affect stereochemistry of MMA polymerization to a small extent; rr triad content decreases only by 5% with increasing reaction temperature from 30°C to 70oC. 14 We propose the following models of propagating species in order to understand the unusual results. ( 1) There are, at least, two types of helical propagating radicals having different possibilities of meso monomer addition. (2) The two m...

Stereospecific radical polymerization of fluoroalkyl acrylates

“…9 In this work, as shown in Table V, a marked monomer concentration effect on the stereochemistry was also found in the polymerization of HFiPA in THF. In the temperature range of 0 ϳ Ϫ98°C, the r-specificity was higher at a lower monomer concentration.…”

“…The concentration effects may be explained by postulating the existence of conformers for the growing species, which have different stereospecificities of monomer addition as in the case of the triphenylmethyl methacrylate polymerization. 9 The simplest assumption may be as follows. There are two interchangeable species in the reaction system: one has a higher r-specificity and the other has a lower r-specificity.…”

Synthesis and post-polymerization reaction of end-clickable stereoregular polymethacrylates via termination of stereospecific living anionic polymerization

“…25 Therefore, numerous efforts have been paid for the control of stereoregularity of PMMA. In radical polymerization, highly isotactic PMMAs (mm content > 99.9%) was synthesized via the polymerization of bulky monomers such as 1-phenylsuberyl methacrylate and the post-polymerization modification for isotactic polymer, [26][27][28] and polymerization of MMA in fluoroalkyl alcohols for syndiotactic polymer (rr content = 93%). 29,30 Furthermore, the simultaneous control of stereoregularity and molecular weight has been achieved by living radical polymerization at low temperature (~ -78 °C) in fluorous alcohols, where st-PMMA (rr content ~ 85%) could be synthesized.…”

Stereogradient Polymers Formed by Controlled/Living Radical Polymerization of Bulky Methacrylate Monomers

“…[9] In particular, the isotacticity increases with the decreasing monomer concentration, which suggests the possibility of preparing stereogradient polymers by giving the chains a living nature (Scheme 1).…”