The Pivotal Role of Excess Nitroxide Radical in Living Free Radical Polymerizations with Narrow Polydispersity

Abstract

The pivotal role of the nitroxide concentration in bulk living polymerization of styrene was studied between 115 and 135 °C, using in situ electron spin resonance spectroscopy (ESR) to follow the concentration of the TEMPO stable free radical during the polymerization. Molecular weight and conversion were also followed on the same reaction mixtures using gel permeation chromatography and thermogravimetric analysis, respectively. While molecular weights were linear with conversion, to high conversion, there was an increase in the polymerization rate with time: nonideal behavior for a living polymerization. However, the TEMPO concentration also shows a slow decay as polymerization proceeds. Using the current mechanistic model, which predicts a polymerization rate inversely proportional to TEMPO concentration, this changing concentration was incorporated into the kinetic analysis. Except for low conversion in the lowest temperature polymerization, correction for the TEMPO concentration resulted in ideal, constant polymerization rate constants. While increasing the initial TEMPO concentration decreases the rate of polymerization dramatically, the corrected rate is independent of initial TEMPO concentration, again consistent with the current mechanism. From these corrected polymerization rates, the activation energy for the release of TEMPO from the growing chain end was estimated as 82 kJ/mol, considerably less than the previously observed value of 130 kJ/mol for the release of TEMPO from styrene 1-mers. Using TEMPO as a probe of irreversible chain termination, ESR shows that irreversible chain termination up to 75% conversion is limited to less than 2 chains in a hundred. It is concluded that the TEMPO-mediated polymerization is a living polymerization under the conditions of this study. To aid in the understanding of these living polymerizations that are based on reversible termination, a new term has been defined, the germination efficiency, which describes the yield of living chains in terms of the reversible terminating agent.

?Living? radical polymerization of methyl methacrylate with diethyl 2,3-dicyano-2,3-diphenylsuccinate as a thermal iniferter

“…16 Then DCDPS was synthesized by an oxidative coupling reaction of ethyl ␣-cyanophenylacetate with a Cu ϩϩ -TMEDA-O 2 system in chloroform. 17 Namely, in a 250-mL three-necked flash equipped with a magnetic stirrer bar, an oxygen buret and a reflux condenser were placed 9.0 g (47.6 mmol) of ethyl ␣-cyanophenylacetate in 150 mL of chloroform, 5.0 g (50.5 mmol) of CuCl and 6.0 mL (50.5 mmol) TMEDA (as catalyst), and 7.0 g of MgSO 4 (to remove water). At 50°C, the whole system was then flushed and filled with oxygen.…”

“…In 1995 Matyjaszewski 2 and Sawamoto 3 discovered a new system, halocompound/transition metal/organic ligand, for living radical polymerization of several vinyl monomers, that is, atom transfer radical polymerization (ATRP), which appears to be the most versatile approach to controlled radical polymerization. Another attractive approach is nitroxyl-mediated polymerization, 4 which enables good control of the polymerization of styrene, but it is not so efficient for (meth)acrylate monomers. Also, Otsu and his coworkers 5 reported that iniferters (initiator-transfer agent-terminators) can be used for "living" radical polymerization of vinyl monomers.…”

N-tert-Butyl-1-diethylphosphono-2,2-dimethylpropyl nitroxide as counter radical in the controlled free radical polymerization of styrene: kinetic aspects

“…Concerning the capture rate constant k c , the kinetics of nitroxide radical trapping, including solvent effects as well as structural effects, have been studied by Beeckwith et al 18,19) and Ingold et al 20) , and the rate constant k c appeared to be lower than the diffusion-controlled limit. In the scope of macromolecular alkoxyamines, Veregin et al 21) have studied the bulk polymerization of styrene with Tempo as counter radical and benzoyl peroxide as initiator. They have reported a value of K = 3.4 6 10 -11 mol N L -1 at 125 8C, with an activation energy E a = 77.5 kJ N mol -1 , for the equilibrium involving polystyryl radical and Tempo.…”

Synthesis of block and graft copolymers containing liquid-crystalline segments

“…Some works state that improved ef®ciency requires excess of initiator, 12 while others claim that the nitroxide must be in excess for better results. 13 The results obtained from the LC monomer homopolymerization indicate that both situations and equimolar amounts of initiator and nitroxide can be successfully used if the ratio between them is near unity.…”

“…GPC fractionating gave an LC homopolymer and a block copolymer fraction in a proportion of 1:5. The block copolymer fraction 13 C NMR spectrum showed the presence of a peak at 42 ppm, which was assigned to the CH 3 ÐN present in the thiuram end-groups. The composition was determined from the 1 H NMR spectrum, which showed a molar ratio between polystyrene and LC segments of 1:9.…”