Nitroxide-mediated polymerization initiating systems

Controlled Radical Polymerization (CRP) is the most recently developed polymerization technology for the preparation of well defined functional materials. Three recently developed CRP processes are based upon forming a dynamic equilibrium between active and dormant species that provides a slower more controlled chain growth than conventional radical polymerization. Nitroxide Mediated Polymerization (NMP), Atom Transfer Radical Polymerization (ATRP) and Reversible Addition Fragmentation Transfer (RAFT) have been developed, and improved, over the past two decades, to provide control over radical polymerization processes. This chapter discusses the patents issued on ATRP initiation procedures, new functional materials prepared by CRP, and discusses recent improvements in all three CRP processes. However the ultimate measure of success for any CRP system is the preparation of conunercially viable products using acceptable economical manufacturing procedures. 

Controlled radical polymerization techniques are suitable for synthesizing polymers with a high level of architectural control. Notably, they not only allow a copolymerization with functional monomers (as shown previously for free-radical polymerization), but also a simple functionalization of the chain end by the initiator. Miniemulsion systems were found suitable for conducting controlled radical polymerizations [58-61], including atom transfer radical polymerization (ATRP), RAFT, degenerative iodine transfer [58], and nitroxide-mediated polymerization (NMP). Recently, the details of ATRP in miniemulsion were described in several reviews [62, 63], while the kinetics of RAFT polymerization in miniemulsion was discussed by Tobita [64]. Consequently, no detailed descriptions of the process wiU be provided at this point. 

Figure 2 Activation-deactivation equilibrium in nitroxide-mediated polymerization, (a) Bicomponent initiating system and (b) monocomponent initiating system.

For SCVP of styrenic inimers, the mechanism includes cationic (14 [18], 19 [29]), atom transfer radical (15 [22, 27]), nitroxide-mediated radical (16 [21]), anionic (20 [19]), photo-initiated radical (17 [2], 18 [52-55]), and ruthenium-catalyzed coordinative (21 [56]) polymerization systems. Another example in-... 

The last decades have witnessed the emergence of new living Vcontrolled polymerizations based on radical chemistry [81, 82]. Two main approaches have been investigated the first involves mediation of the free radical process by stable nitroxyl radicals, such as TEMPO while the second relies upon a Kharash-type reaction mediated by metal complexes such as copper(I) bromide ligated with 2,2 -bipyridine. In the latter case, the polymerization is initiated by alkyl halides or arenesulfonyl halides. Nitroxide-based initiators are efficient for styrene and styrene derivatives, while the metal-mediated polymerization system, the so called ATRP (Atom Transfer Radical Polymerization) seems the most robust since it can be successfully applied to the living Vcontrolled polymerization of styrenes, acrylates, methacrylates, acrylonitrile, and isobutene. Significantly, both TEMPO and metal-mediated polymerization systems allow molec-... 

Hawker CJ, Barclay GG, Orellana A et al. (1996) Initiating systems for nitroxide-mediated living free radical polymerizations synthesis and evaluation. Macromolecules 29 5245-5254... 

Apart from ATRP, the concept of dual initiation was also applied to other (controlled) polymerization techniques. Nitroxide-mediated living free radical polymerization (LFRP) is one example reported by van As et al. and has the advantage that no further metal catalyst is required [43], Employing initiator NMP-1, a PCL macroinitiator was obtained and subsequent polymerization of styrene produced a block copolymer (Scheme 4). With this system, it was for the first time possible to successfully conduct a one-pot chemoenzymatic cascade polymerization from a mixture containing NMP-1, CL, and styrene. Since the activation temperature of NMP is around 100 °C, no radical polymerization will occur at the reaction temperature of the enzymatic ROP. The two reactions could thus be thermally separated by first carrying out the enzymatic polymerization at low temperature and then raising the temperature to around 100 °C to initiate the NMP. Moreover, it was shown that this approach is compatible with the stereoselective polymerization of 4-MeCL for the synthesis of chiral block copolymers. 

For styrene-based random copolymers, functional groups can be introduced into the polymer chains via copolymerization with functional styrene derivatives, because the electronic effects of the substituents are small in the metal-catalyzed polymerizations in comparison to the ionic counterparts. Random copolymer R-6 is of this category, synthesized from styrene and />acetoxystyrene.372 It can be transformed into styrene// -vinylphenol copolymers by hydrolysis.380 The benzyl acetate and the benzyl ether groups randomly distributed in R-7 and R-8 were transformed into benzyl bromide, which can initiate the controlled radical polymerizations of styrene in the presence of copper catalysts to give graft copolymers.209 Epoxy groups can be introduced, as in R-9, by the copper-catalyzed copolymerizations without loss of epoxy functions, while the nitroxide-mediated systems suffer from side reactions due to the high-temperature reaction.317... 

Alternative bimolecular methods have been reported that involve mixing appropriate ratios of monomer with free-radical initiators (such as benzoyl peroxide) and an excess of the nitroxide stable free-radical moiety. Such bimolecular methods do not afford the same degree of control of molecular weight and polydispersity since the stoichiometry of the mediating system cannot be accurately dehned, which is a crucial factor in these controlled polymerization systems. A wide variety of unimolecular nitroxide based initiator systems have been described in the literature with those based upon the 2,2,6,6-tetramethylpiperidinyl-l-oxy (TEMPO) group proving to be the most commonly used. 

The unsuccessful attempts to copolymerize St with EPSt using a nitroxide-mediated reaction led Jones et al. to investigate ATRP as an alternative [113]. They used the CuBr/bpy catalyst system in conjunction with methyl 4-(bro-momethyljbenzoate as the initiator. The copolymers had monomodal GPC curves and narrow molecular weight distributions (Mw/Mn<1.25) when the monomer feed contained <10% of the epoxystyrene and the polymerization temperature was kept at 100 °C. This is in contrast to the results obtained from the TEMPO-mediated polymerization where all the copolymers had bimodal GPC traces and broad molecular weight distributions [113]. 

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