Polymerization TEMPO

Schmidt-Naake et al. [266] did dynamic DSC-measurements concerning TEMPO polymerization of / -MeSt and /7-ClSt and could show the ability to polymerize in the presence of TEMPO. They also found the exothermal peak of the living polymerization in the same temperature range as of the thermal polymerization. 

Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

The identification of both phenylethyl and 1-phenyl-1,2,3,4-lelrahydronaphthalenyl end groups in polymerizations of styrene retarded by FeCl3/DMP provides the most compelling evidence for the Mayo mechanism.316 The 1-phenyl-1.2,3,4-tetrahydronaphthalenvl end group is also seen amongst other products in the TEMPO mediated polymerization of styrene,317318 However, the mechanism of formation of radicals 96 in this case involves reaction of the nitroxide with the Diels-AIder dimer (Scheme 3.63). The mechanism of nitroxide mediated polymerization is discussed further in Section 9.3.6. 

The majority of polymers formed by living radical polymerization (NMP, ATRP, RAFT) will possess labile functionality at chain ends. Recent studies have examined the thermal stability of polystyrene produced by NMP with TEMPO (Scheme 8.3),2021 ATRP and RAFT (Scheme 8.4).22 In each case, the end groups... 

The use of the disulfide (13), which can dissociate thermally to give a sulfur analog of TEMPO (Section 9.3.6.1), has also been explored for controlling S polymerization though poor results were obtained.40... 

Catala and coworkers167JuiS made the discovery that the rate of TEMPO-mediated polymerization of S is independent of the concentration of the alkoxyamine. This initially surprising result was soon confirmed by others.23 69 Gretza and Matyjaszewski169 showed that the rate of NMP is controlled by the rate of thermal initiation. With faster decomposing alkoxyamines (those based on the open-chain nitroxides) at lower polymerization temperatures, the rate of thermal initiation is lower such that the rate of polymerization becomes dependent on the alkoxyamine concentration, Irrespective of whether the alkoxyamine initiator is preformed or formed in situ, low dispersities require that the alkoxyamine initiator should have a short lifetime. The rate of initiation should be as fast as or faster than propagation under the polymerization conditions and lifetimes of the alkoxyamine initiators should be as short as or shorter than individual polymeric alkoxyamines. 

The thermal decomposition of the phenylelhyl alkoxyamine with TEMPO and the fraction of living ends in TEMPO-mediated S polymerization has been studied by Priddy and coworkers.143 179 They concluded that to achieve >90% living ends conversions and/or nitroxide concentrations should be chosen to give V/ less than 10000.143 However, disproportionation or elimination is most important during polymerizations of methacrylates and accounts for NMP being less successful with... 

NMP has mainly been used for S polymerization (9.3.6.5.1) and, to a lesser extent, acrylate (9.3.6.5.2) polymerization. The early and much current work has focused on the use of TEMPO and derivatives. The open chain nitroxides 86-91 ( fable 9.3) provide broader though still restricted utility. Some of the previously difficult monomers that have recently been tackled successfully include HEA,196 DM AM197 and A A198 199 with nitroxide 89. 

NMP with acrylates and acrylamides with TEMPO provides only very low conversions. Very low limiting conversions and broad dispersities were reported.2 Better results were obtained with DTBN (83),111 151 imidazoline (61-64)I3S and isoindoline (59) nitroxides.111 However, limiting conversions were still observed. The self-regulation provided in S polymerization by thermal initiation is absent and, as a consequence, polymerization proceeds until inhibited by the buildup of nitroxide. The final product is an alkoxyamine and NMP can be continued... 

Of the major methods for living radical polymerization, NMP appears the most successful for polymerization of the diene monomers. There are a number of reports on the use of NMP of diene monomers (B, I) with TEMPO,188,1103 861 4, cw and other nitroxides.127 High reaction temperatures (120-135 °C) were employed in all cases. The ratio of 1,2- 1,4-cis 1,4-trans structures obtained is similar to that observed in conventional radical polymerization (Section 4.3.2). 

Successful NMP in emulsion requires use of conditions where there is no discrete monomer droplet phase and a mechanism to remove any excess nitroxide formed in the particle phase as a consequence of the persistent radical effect. Szkurhan and Georges"18 precipitated an acetone solution of a low molecular weight TEMPO-tcrminated PS into an aqueous solution of PVA to form emulsion particles. These were swollen with monomer and polymerized at 135 °C to yield very low dispersity PS and a stable latex. Nicolas et at.219 performed emulsion NMP of BA at 90 °C making use of the water-soluble alkoxyamine 110 or the corresponding sodium salt both of which are based on the open-chain nitroxide 89. They obtained PBA with narrow molecular weight distribution as a stable latex at a relatively high solids level (26%). A low dispersity PBA-WocA-PS was also prepared,... 

NMP in miniemulsion has been more successful. In miniemulsion polymerization nuclealion lakes place directly in the monomer droplets that become the polymer particles. Particle sizes are small (<100 nm). Most w ork has used TEMPO and high reaction temperatures (120-140 °C) with S or BA as monomer. 

Various initiation strategies and surfactant/cosurfactant systems have been used. Early work involved in situ alkoxyamine formation with either oil soluble (BPO) or water soluble initiators (persulfate) and traditional surfactant and hydrophobic cosurfactants. Later work established that preformed polymer could perform the role of the cosurfactant and surfactant-free systems with persulfate initiation were also developed, l90 222,2i3 Oil soluble (PS capped with TEMPO,221 111,224 PBA capped with 89) and water soluble alkoxyamines (110, sodium salt""4) have also been used as initiators. Addition of ascorbic acid, which reduces the nitroxide which exits the particles to the corresponding hydroxylamine, gave enhanced rates and improved conversions in miniemulsion polymerization with TEMPO.225 Ascorbic acid is localized in the aqueous phase by solubility. 

Addition of TEMPO post-polymerization to a methacrylate polymerization provides an unsaturated chain end (Scheme 9.52)i07 sw presumably by disproportionation of the PMMA propagating radical with the nitroxide. For polymers based on monosubstituted monomers (PS,1 0" PBA59,[Pg.534]

Sodium 4-oxy-2,2,6,6-tetramethyl-l-piperidinyloxy, TEMPONa, was used as a bifunctional initiator for the synthesis of PEO-fc-PS block copolymers [133]. Initially the ROP of EO was performed in THF at 60 °C to provide narrow molecular weight distribution chains with terminal TEMPO moieties. Using these functionalized PEO chains the polymerization of styrene was... 

A combination of TEMPO living free radical (LFRP) and anionic polymerization was used for the synthesis of block-graft, block-brush, and graft-block-graft copolymers of styrene and isoprene [201]. The block-graft copolymers were synthesized by preparing a PS-fo-poly(styrene-co-p-chloromethylstyrene) by LFRP [Scheme 110 (1)], and the subsequent re-... 

Finally, the use of stable free radical polymerization techniques in supercritical C02 represents an exciting new topic of research. Work in this area by Odell and Hamer involves the use of reversibly terminating stable free radicals generated by systems such as benzoyl peroxide or AIBN and 2,2,6,6-tetramethyl-l-piperidinyloxy free radical (TEMPO) [94], In these experiments, styrene was polymerized at a temperature of 125 °C and a pressure of 240-275 bar C02. When the concentration of monomer was low (10% by volume) the low conversion of PS which was produced had a Mn of about 3000 g/mol and a narrow MWD (PDI < 1.3). NMR analysis showed that the precipitated PS chains are primarily TEMPO capped, and the polymer could be isolated and then subsequently extended by the addition of more styrene under an inert argon blanket. The authors also demonstrated that the chains could be extended... 

Living Radical Polymerization of Styrene with TEMPO... 

In 1993, Georges and coworkers [23,202,203] first succeeded in the synthesis of poly(St) with a narrow molecular weight distribution through the free-radical polymerization process of St. The polymerization was carried out in the presence of BPO and 2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPO) ... 

Table 4. Living Radical Polymerization of St with TEMPO and BPOa [202]...

The polymerization of St with 56 as the initiator is considered to proceed via a reaction mechanism in Eq. (56), being identical to the models in Eqs. (18) and (20). The structure of both chain ends of the resulting polymer was confirmed by NMR using the deuterated St as the monomer. The polymerization with BPO and TEMPO without isolation of the adduct would also proceed via a similar path. In the absence of BPO, it has been reported that the radicals produced by spontaneous initiation according to the Mayo mechanism react with TEMPO to yield the adducts, and then they initiate polymerization [206]. 

The living radical polymerization process is also valid for the polymerization of water-soluble monomers. The polymerization of sodium styrenesulfonate in aqueous ethylene glycol (80%) in the presence of TEMPO using potassium per-sulfate/sodium bisulfite as the initiator at 125 °C gave a water-soluble polymer with well-controlled molecular weight and its distribution [207]. 

As is expected from these results, it is very difficult to control the polymerization of monomers other than St, e.g., that of MMA, because of the too small dissociation energy of the chain end of poly(MMA). In fact, the polymerization of MMA in the presence of TEMPO yielded the polymer with constant Mn irrespective of conversion, and the Mw/Mn values are similar to those of conventional polymerizations [216]. The disproportionation of the propagating radical and TEMPO would also make the living radical polymerization of MMA difficult. In contrast, the controlled polymerization of MA, whose propagating radical is a secondary carbon radical,has recentlybeen reported [217]. Poly(MA) with a narrow molecular weight distribution and block copolymers were obtained. 

The polymerization kinetics have been intensively discussed for the living radical polymerization of St with the nitroxides,but some confusion on the interpretation and understanding of the reaction mechanism and the rate analysis were present [223,225-229]. Recently, Fukuda et al. [230-232] provided a clear answer to the questions of kinetic analysis during the polymerization of St with the poly(St)-TEMPO adduct (Mn=2.5X 103,MW/Mn=1.13) at 125 °C. They determined the TEMPO concentration during the polymerization and estimated the equilibrium constant of the dissociation of the dormant chain end to the radicals. The adduct P-N is in equilibrium to the propagating radical P and the nitroxyl radical N (Eqs. 60 and 61), and their concentrations are represented by Eqs. (62) and (63) in the derivative form. With the steady-state equations with regard to P and N , Eqs. (64) and (65) are introduced, respectively ... 

This equation is similar to that for the ordinary polymerization, indicating that Rp is independent of the concentration of P-N. In fact, the polymerization rate experimentally determined in the presence of P-N agreed with the rate of thermally initiated polymerization without any initiators. The production of the polymer induced a decrease in the Rvalue because of the gel effects, resulting in an increase in the rate. The suppressed gel effects in the presence of TEMPO have also been reported [233]. Catala et al. interpreted the independence of the polymerization rate from the nitroxide concentration with the terms of the association of domant species. However, there is no experimental evidence for the association [229,234,235]. 

Nitroxide attached to macromolecules also induces the living radical polymerization of St. Yoshida and Sugita [252] prepared a polymeric stable radical by the reaction of the living end of the polytetrahydrofuran prepared by cationic polymerization with 4-hydroxy-TEMPO and studied the living radical polymerization of St with the nitroxide-bearing polytetrahydrofuran chain. The nitroxides attached to the dendrimer have been synthesized (Eq. 69) to yield block copolymers consisting of a dendrimer and a linear polymer [250,253]. 

As the initiator, a common radical initiator and arenesulfonyl chloride are also used [286,287]. As shown in Table 6, this polymerization has a significantly large polymerization rate, and it is hardly disturbed by impurities such as alcohol and water [288]. ATRP with Cu complex was also applied to the polymerization of acrylates [289,290], methacrylates [290-297], and AN [298] as well as St [288, 297, 299]. Because of the suppressed bimolecular termination, hyperbranched polymers are readily prepared [292], being similar to the polymerization with TEMPO previously described. 

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