Radical polymerization general features

The use of porphyrinic ligands in polymeric systems allows their unique physio-chemical features to be integrated into two (2D)- or three-dimensional (3D) structures. As such, porphyrin or pc macrocycles have been extensively used to prepare polymers, usually via a radical polymerization reaction (85,86) and more recently via iterative Diels-Alder reactions (87-89). The resulting polymers have interesting materials and biological applications. For example, certain pc-based polymers have higher intrinsic conductivities and better catalytic activity than their parent monomers (90-92). The first example of a /jz-based polymer was reported in 1999 by Montalban et al. (36). These polymers were prepared by a ROMP of a norbor-nadiene substituted pz (Scheme 7, 34). This pz was the first example of polymerization of a porphyrinic macrocycle by a ROMP reaction, and it represents a new general route for the synthesis of polymeric porphyrinic-type macrocycles. 

In polymerizing systems, the resonance-stabilised fragments could act mainly as terminating species, but it remains a fact nonetheless, that their formation, simultaneous with a reactive, initiating radical, is a general feature of the known, efficient, photoinitiator systems presumably because their relative stability facilitates dissociation. This may help to explain why benzil... 

One of the characteristic features of this type of polymerization is the relatively low molecular weight of the products. Catalysts such as NaH and BuLi are generally employed in these reactions (equations 144 and 145). In contradistinction to radical polymerizations, addition of methyl at the second bridgehead position increases both the yields and the degree of polymerization . ... 

General Features of Catalytic Chain Transfer 1.1. Free-Radical Polymerization... 

General Features of Atom Transfer Radical Polymerization (ATRP)... 

In CCT a metalloradical reversibly abstracts H from the chain-carrying radical and starts a new chain. Early work on CCT during radical polymerizations employed cobalt porphyrins during the polymerization of methyl methacrylate, and was carried out in the USSR (Smirnov, Marchenko in 1975 Enikolopyan in 1977). Gridnev discovered in Moscow in 1979 that cobaloximes were effective CCT catalysts, then moved to the US in 1992 (Wayland laboratory. University of Pennsylvania) and joined DuPont in 1994. The basic features of CCT have been described in a series of patents (at first Russian, then largely DuPont) that appeared in the 1980s [71], and in a comprehensive review that appeared in 2001 [72]. The mechanism in Scheme 1.8 has become generally accepted, and CCT has been successfully applied to other monomers (styrene, methacrylonitrile) and comonomers. 

The kinetics of DMDAACh radical polymerization in water solutions has the same quahtative features, as radical polymerization kinetics of other polymers [1]. In particular, this is expressed in sigmoid shape of kinetic curve Q, , which in general case is divided into three sections initial, of autoacceleration and finish polymerization [20]. For the curve the traditional approaches number, considered in detail in works [1, 20], is used. However, the indicated approaches do not take into account the stmcture of the main element of polymers synthesis in solutions — macromolecular coil and, hence, its possible variations in synthesis process. At present the theory of irreversible aggregation, linked closely to fractal... 

Glossary of terms relating to kinetics, thermodynamics, and mechanisms of polymerization. The document represents, in alphabetical order, recommended definitions of terms related to polymerization processes, principally to kinetics, thermodynamics, and mechanisms of polymerization. Since polymerization processes have specific features, the definitions presented in this document differ in some instances from the general definitions given in the Gold Book. The document on reversible deactivation radical polymerization is a logical extension of this glossary. 

An important feature of chain transfer is that the hydrogen abstraction can occur anywhere along a polymer chain. As such, chain transfer generally leads to branching. So, contrary to initial expectation, radical polymerization of a simple monomer like styrene can lead to a branched polymer. Branching substantially affects the material properties of a polymer, discouraging crystallinity and thus lowering T. ... 

The list of monomers compatible with anionic polymerization overlaps the radical list (Figure 13.16) considerably, but the unique features of anionic polymerization mean that the same polymer can be a different material. For example, one important feature of anionic polymerizations is that they show a tendency to produce stereoregular polymers, in contrast to radical polymerizations. As such, polystyrene produced by anionic polymerization is more crystalline than polystyrene produced by radical polymerization. This is just another example of how one polymer— polystyrene—can represent several different materials. Note also that because carbanions generally do not abstract protons from C-H bonds, chain transfer (and branching) is typically not a problem in anionic polymerizations. 

From a kinetic point of view, termination is, beyond any doubt, the most complicated reaction taking place in a simple bulk or solution free-radical polymerization. Interesting reviews and summaries on this and closely related areas have been written by North [11], Kamachi [12], Mahabadi [13], Mita and Horie [14], O Driscoll [15], Hamielec [16] and Litvinenko and Kaminsky [17]. The cause of this high degree of complexity can be attributed to the diffusion-controlled nature of this process, a feature that is nowadays generally accepted [11-17] (see also section 2.3). Diffusion control means that the observed rate coefficient of termination, kobs, in the reaction... 

Radicals are produced in a special reaction for starting a radical polymerization. Because free radicals are reactive intermediates that possess only very limited lifetimes, radicals are generally produced in the presence of a monomer that is to be polymerized. They react very rapidly with the monomer present. The rate of the reaction of initially formed free radicals with the monomer (the initiation step) is high compared with the rate of radical formation hence, the latter process is rate determining. Therefore, radical generation by respective initiators is a very characteristic and important feature of radical initiation. 

Although the subject of this book is controlled/living radical polymerization, this chapter will discuss those fundamental features common to all living polymerizations, and the discussion will generally concern a generic active species that could be anionic, cationic, or free radical. In some cases, features that are particularly relevant to a spedfic type of living radical polymerization will be addressed. Several excellent reviews specifically directed to controlled/living radical polymerizations have been published by Matyjaszewski. ... 

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