Controlled/living radical well-defined polymers

The tendency of nitrones to react with radicals has been widely used in new synthetic routes to well-defined polymers with low polydispersity. The recent progress in controlled radical polymerization (CRP), mainly nitroxide-mediated polymerization (NMP) (695), is based on the direct transformation of nitrones to nitroxides and alkoxyamines in the polymerization medium (696, 697). In polymer chemistry, NMP has become popular as a method for preparing living polymers (698) under mild, chemoselective conditions with good control over both, the polydispersity and molecular weight. 

New approaches based on the introduction of reactive species into reaction mixtures that tend to cap the growing chains reversibly allow, in many cases, production of well-defined polymers and copolymers with narrow polydispersi-ties. Up to few years ago, such a possibility was unobtainable by a classical free radical process. The proposed principle of control of macroradical reactivity is very interesting conceptually, and represents a very powerful tool to prepare block copolymers with well-controlled structures. However, it is clear that the true living character as demonstrated by some anionic polymerizations is still not obtained and much more work needs to be done to understand and control this new process better. 

The rapid progress and proliferation of metal-catalyzed living radical polymerization has allowed a variety of vinyl monomers to be polymerized into well-defined polymers of controlled molecular weights and narrow MWDs. Most of them are conjugated monomers such as methacrylates, acrylates, styrenes, acrylonitrile, acrylamides, etc., except dienes, which possess not only alkyl substituents but also aprotic and protic functional groups. This fact attests to the versatility and flexibility of metal catalysis for precision polymerization. 

VAc has been successfully polymerized via controlled/ living radical polymerization techniques including nitroxide-mediated polymerization, organometallic-mediated polymerization, iodine-degenerative transfer polymerization, reversible radical addition-fragmentation chain transfer polymerization, and atom transfer radical polymerization. These methods can be used to prepare well-defined various polymer architectures based on PVAc and poly(vinyl alcohol). The copper halide/t is an active ATRP catalyst for VAc, providing a facile synthesis of PVAc and its block copolymers. Further developments of this catalyst will be the improvements of catalytic efficiency and polymerization control. 

Living radical polymerization (LRP) has attracted growing attention as a powerful synthetic tool for well-defined polymers 1,2). The basic concept of LRP is the reversible activation of the dormant species Polymer-X to the propagating radical Polymer (Scheme la) 1-3). A number of activation-deactivation cycles are requisite for good control of chain length distribution. 

Since the discovery of living polymerizations by Swarc in 1956 [1], the area of synthesis and application of well-defined polymer structures has been developed. The livingness of a polymerization is defined as the absence of termination and transfer reactions during the course of the polymerization. If there is also fast initiation and chain-end fidelity, which are prerequisites for the so-called controlled polymerization, well-defined polymers are obtained that have a narrow molar mass distribution as well as defined end groups. Such well-defined polymers can be prepared by various types of living and controlled polymerization techniques, including anionic polymerization [2], controlled radical polymerization [3-5], and cationic polymerization [6, 7]. 

Controlled/ living radical polymerization (CLRP) processes are well-established synthetic routes for the production of well-defined, low-molecular weight-dispersity polymers [99]. The types of CLRP processes (initiator-transfer agent-terminator (INIFERTER), atom transfer radical polymerization (ATRP), nitroxide-mediated radical (NMRP) polymerization, reversible addition-fragmentation transfer (RAFT)) and their characteristics are described in Section 3.8 of Chapter 3 and in Section 14.8 of Chapter 14. 

This is therefore the practical requirement for the synthesis of well-defined polymers, such that complete monomer conversion can be reached and the chain ends can be quantitatively functionalized. However, since chain breaking reactions are actually present, such systems are more appropriately labeled controlled polymerizations rather than living polymerizations. In fact, conditions have recently been established for controlled radical polymerizations, even though it is impossible to avoid bimolecu-lar termination [12-20]. The extent of the Tivingness or controllability of a polymerization can be ranked if the individual or relative rate constants of propagation, transfer and termination are known [10, 11]. 

The synthesis of well-defined macromolecules with controlled compositions, architectures, and functionalities has emerged as an important aspect of polymer science. The development of controlled/living radical polymerization (CRP) permitted achievement of well-defined polymers using a radical process easy to carry out. In the last 20 years, techniques such as atom transfer radical polymerization (ATRP), nitroxide mediated polymerization... 

As a unique method of controlled/living radical polymerization, ATRP has had a tremendous impact on the synthesis of macromolecules with well-defined compositions, architectures, and functionalities, including star- and comb-like polymers as well as branched, hyperbranched, dendritic, network, cyclic type structures and so forth. 

Von Werne, T., and Patten, T. E. (1999). Preparation of structurally well-defined polymer-nanoparticle hybrids with controlled/living radical polymerizations. J. Am. Chem. Soc., 121(32) 7409-7410. 

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