Atom transfer radical initiator efficiency

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-... 

Triphenylamine derivatives are known to be efficient hole transport materials and are widely used in organic light-emitting devices. Thelakkat et al. reported the synthesis of a 2,2-bipyridine ligand capped with polyfvinyl-triphenylamine) at both ends.97 The polymer chain was synthesized by the atom transfer radical polymerization of 4-bromostyrene using 4,4-bis (chloromethyl)bipyridine as the initiator (Scheme 18). The bromide groups were then replaced by diphenylamine in the presence of palladium catalyst. Polymer 33 was then obtained by the metalation reaction. 

Atom transfer radical polymerization (ATRP) was selected as an exemplary CRP technique to systematically study the kinetics and gelation behavior during the concurrent copolymerization of monovinyl monomers and divinyl cross-linkers (Scheme 2). The effect of different parameters on the experimental gelation was studied, including the initial molar ratio of cross-linker to initiator, the concentrations of reagents, the reactivity of vinyl groups present in the cross-linker, the efficiency of initiation, and the polydispersity of primary chains. Experimental gel points based on the conversions of monomer and/or cross-linker at the moment of gelation, were determined and compared with each other in order to understand the influence of each parameter on the experimental gel points. 

Similarly, atom transfer radical polymerization (ATRP) has been used by Matyjaszewski and others for the synthesis of polystyrene and polyacrylates witii controlled molecular weights. This process is bas on a Cu(I) assisted atom-transfer radical polymerization (ATRP) One of the end groups is de ed by the structure of the initiator, whereas the other one contains an alkyl halide, such as chloride or bromide that can be converted to other functional groups. Additionally, the radical intermediates of ATRP are tolerant to many function groups, which can not be used directly in anionic or cationic processes, such as hydroxyalkyl, epoxy, enabling the direct synthesis of well-defined glycidyl, hydroxyethyl(meth)acrylates and other functional monomers. Percec and Barboiu have prepared polystyrene derivatives with efficient control of chain-end chemistry by the use of functionalized arenesulfonyl chlorides. 

In contrast, here a bifunctional initiator is employed and the polymerization order of the two blocks is inverted In a first step, the styrene block is synthesized by atom transfer radical polymerization (ATRP) followed by the addition of lactide via the recently developed organocatalytic ring-opening polymerization, as depicted in Fig. 3.1 [4, 5]. This synthesis route reduces the involved steps and enables a simplified and time-efficient preparation of copolymers with different block compositions. Importantly, both polymerization techniques offer precise and robust control over the copolymer composition, which is an essential requirement to reliably target the double-gyroid s narrow location in phase space [6]. 

Li, W. Gao, H. Matyjaszewski, K. Influence of initiation efficiency and polydispersity of primary chains on gelation during atom transfer radical copolymerization of monomer and cross-linker. 

As mentioned above, the cleavage of Si-Si-bonds in polysilanes upon irradiation with UV-light is very efficient and results in the formation of silicon radicals. These radicals react with olefins to initiate radical polymerizations. Especially the polymerization of methylmethacrylate and styrene with a variety of polysilanes as photoinitiators has been studied in detail [104]. The advantage of this kind of initiation is the possibility to prepare polysilane-polyolefin hybrids [105]. All modifications of radical polymerizations, such as the atom transfer radical polymerization (ATRP), are possible with polysilanes as photoinitiators [106]. 

Tomlinson MR, Efimenko K, Genzer J (2006) Study of kinetics and macroinitiator efficiency in surface-initiated atom-transfer radical polymerization. Macromolecules 39(26) 9049-9056... 

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