Living/controlled free-radical polymerization

The formation of living polymer obeys a completely different mechanism than faced in radial polymerization. With living polymers, highly uniform materials can be synthesized. Living polymerization includes not only anionic polymerization but also atom transfer radical polymerization, living controlled free radical polymerization. 

A, Theis, M. H. Stenzel, T. P. Davis, C. Barner-Kowollik, Obtaining Chain Length Dependent Termination Rate Coefficients via Thermally Initiated RAFT Experiments Current Status and Future Chal-langes, in Acs Symposium Series on LIving/Controlled Free Radical Polymerization, K. Matyjaszewski, Ed. ACS Press Washington, D.C., 2006 944, 486-500. 

The three main requirements for an ideal SNR-mediated living/controlled free-radical polymerization are (i) essentially simultaneous initiation (ii) reversible reaction of SNR given in Eq. (11.10) and (iii) no important degree of irreversible termination, if any. Under these conditions, one would expect the system to be described by a simple kinetic scheme, as described below. 

As a result, a living /controlled free-radical polymerization via an atom transfer radical polymerization (ATRP) mechanism has been described for methyl 1-bicyclobutane carboxylate, providing decently narrow molecular weight distributions and adequate control of the molecular weights [4, 75]. An A-B polystyrene diblock copolymer was also obtained by the same authors, using the bromo-terminatedpoly(bicyclobutanecarboxylate) as a macroinitiator. Bicyclo[n.l.0]alkanes with n> 1 have also been polymerized (Table 13.6). 

Phosphoranyl radicals can be involved [77] in RAFT processes [78] (reversible addition fragmentation transfer) used to control free radical polymerizations [79]. We have shown [77] that tetrathiophosphoric acid esters are able to afford controlled/living polymerizations when they are used as RAFT agents. This result can be explained by addition of polymer radicals to the P=S bond followed by the selective p-fragmentation of the ensuing phosphoranyl radicals to release the polymer chain and to regenerate the RAFT agent (Scheme 41). 

The case of chain transfer to initiator may be exploited in special forms of controlled free-radical polymerization, e.g. with sulfur-containing initiators that are discussed later within the topic of living Ifee-radical polymerizations. 

Over the past 15 years, new free radical polymerization techniques have been developed which allow significantly improved control over polymer structure at the molecular level. By using these techniques, customized polymeric materials can be produced which are not possible using conventional methods of the past. These new techniques are typically termed living or controlled free radical polymerization. There is some debate over the semantic use of these terms [2,3], but the term living radical polymerization (LRP) will be used here for simplicity. 

B. Charleux, S. R. A. Marque, D. Bertin, S. Magnet, L Couvreur, Living character of polymer chains prepared via nitroxide-mediated controlled free-radical polymerization of methyl methacrylate in the presence of a small amount of styrene at low temperature, Macromolecules 2006,... 

NMP is another controlled free radical polymerization technique that can be used to polymerize styrene [98-100]. Weimer et al. investigated surface-initiated PS-MMT nanocomposites using MMT modified with a nitroxyl-mediated living... 

A new strategy has been proposed for the one-step synthesis of block copolymers, based on living/controlled free-radical process. It involves the use of an asymmetric difimctional initiator that is able to start simultaneous polymerization of two comonomers by different polymerization chemistries in such a way that this initiator remains attached to each type of the growing chain (Mecerreyes et al., 1998). The implementation of one-step synthesis is not simple, however. The two catalysts must be tolerant to each other as also to the two comonomers and the reaction temperature must be closely controlled. Living radical polymerization and ROP by coordination and insertion can meet these requirements. 

Considerable progress has been made in the synthesis of tailor-made block copolymers, and to some extend also to the corresponding graft copolymers, by living ionic polymerization, controlled free-radical polymerization and quite recently by non-covalent coupling techniques. Numerous examples of linear water-soluble A-B and A-B-A structures were described in addition to the possibility of functionalization of these copolymers with specific groups, such as reactive double bonds, ionic groups, fluorescence labels, either at the chain ends and/or at the junction of the blocks. 

Architectures such as block copolymers or star polymers, which could be obtained until now only by living ionic polymerizations, are now accessible by controlled free radical polymerization (see Chapter 9). 

Significant improvement in controlled polymerizations of a variety monomers, including styrene, acrylates, acrylamide, acrylonitrile, 1,3-dienes, and maleic anhydride has been achieved when alkoxyamines have been used as initiators for living, free radical polymerization.(696c, 697) Alkoxyamines can be easily synthesized in situ by the double addition of free radicals, generated by thermal decomposition of an azo-initiator, such as 2,2 -azo-h/.s-/.so-butyronitrile (AIBN), to nitrones (Scheme 2.206). 

He also prepared a poly(styrene-g-styrene) polymer by this technique [114], The lack of crosslinking in these systems is indeed proof of the control achieved with this technique. An eight-arm star polystyrene has also been prepared starting from a calixarene derivative under ATRP conditions [115]. On the other hand, Sawamoto and his coworkers used multifunctional chloroacetate initiator sites and mediation with Ru2+ complexes for the living free-radical polymerization of star poly(methylmethacrylate) [116,117]. More recent work by Hedrick et al. [84] has demonstrated major progress in the use of dendritic initiators [98] in combination with ATRP and other methodologies to produce a variety of structure controlled, starlike poly(methylmethacrylate). 

Living free-radical polymerization has recently attracted considerable attention since it enables the preparation of polymers with well-controlled composition and molecular architecture previously the exclusive domain of ionic polymerizations, using very robust conditions akin to those of a simple radical polymerization [77 - 86]. In one of the implementations, the grafting is achieved by employing the terminal nitroxide moieties of a monolith prepared in the presence of a stable free radical such as 2,2,5,5-tetramethyl-l-pyperidinyloxy (TEMPO). In this way, the monolith is prepared first and its dormant free-... 

While in most of the reports on SIP free radical polymerization is utihzed, the restricted synthetic possibihties and lack of control of the polymerization in terms of the achievable variation of the polymer brush architecture limited its use. The alternatives for the preparation of weU-defined brush systems were hving ionic polymerizations. Recently, controlled radical polymerization techniques has been developed and almost immediately apphed in SIP to prepare stracturally weU-de-fined brush systems. This includes living radical polymerization using nitroxide species such as 2,2,6,6-tetramethyl-4-piperidin-l-oxyl (TEMPO) [285], reversible addition fragmentation chain transfer (RAFT) polymerization mainly utilizing dithio-carbamates as iniferters (iniferter describes a molecule that functions as an initiator, chain transfer agent and terminator during polymerization) [286], as well as atom transfer radical polymerization (ATRP) were the free radical is formed by a reversible reduction-oxidation process of added metal complexes [287]. All techniques rely on the principle to drastically reduce the number of free radicals by the formation of a dormant species in equilibrium to an active free radical. By this the characteristic side reactions of free radicals are effectively suppressed. 

Husseman M, Malmstrom EE, McNamara M, Mate M, Mecerreyes D, Benoit DG, Hedrick JL, Mansky P, Huang E, RusseU TP, Hawker CJ (1999) Controlled synthesis of polymer brushes by Living free radical polymerization techniques. Macromolecules 32 1424-1431... 

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