Conventional radical processes

Statistical copolymers are an important class of materials, as their properties can differ significantly from the corresponding homopolymers or block copolymers. The major problem, prior to the advent of CRP methods, was that with conventional radical processes, slow continuous initiation produced a compo-... 

Muller et al. used the MM technique to synthesize graft copolymers [318]. They copolymerized nBA with an co-methacryloyl-pMMA MM both via a conventional radical process and an ATRP reaction. Although the MM was con-... 

Scheme 48. Preparation of pNVP-g-pS by combining ATRP with a conventional radical process [317]...

During a nonliving polymerization, such as conventional radical processes, four distinct mechanistic steps occur. As shown in Scheme 2.19 for monosubstituted vinyl monomers, these include initiation, propagation, irreversible termination by coupling or disproportionation, and chain transfer [76]. 

In the last two decades, tremendous improvanents to conventional radical polymerizations have been made [10, 78, 79]. In general, converting nonliving systems into a controlled/ living polymerization requires suppression of irreversible chain termination (Scheme 2.19) and chain transfer. Additionally, improving the initiation process, which is typically inefficient for conventional radical processes, is also necessary to produce a controlled polymerization. 

ATRP is converted into a system which displays conventional RP characteristics upon the addition of octanethiol as a chain transfer reagent. Chain transfer in n-butyl acrylate (nBA) polymerization also resembles the conventional radical process. However, some differences between all CRP processes and conventiotral RP are observed " and can be explained by the coexistence of various competing eqrrilibtia. ... 

From this we can see that knowledge of k f and Rf in a conventional polymerization process readily yields a value of the ratio kp fkt. In order to obtain a value for kf wc require further information on kv. Analysis of / , data obtained under non-steady state conditions (when there is no continuous source of initiator radicals) yields the ratio kvlkx. Various non-stcady state methods have been developed including the rotating sector method, spatially intermittent polymerization and pulsed laser polymerization (PLP). The classical approach for deriving the individual values of kp and kt by combining values for kp kx. with kp/k, obtained in separate experiments can, however, be problematical because the values of kx are strongly dependent on the polymerization conditions (Section... 

In this chapter, we restrict discussion to approaches based on conventional radical polymerization. Living polymerization processes offer greater scope for controlling polymerization kinetics and the composition and architecture of the resultant polymer. These processes are discussed in Chapter 9. 

Living polymerization processes lend themselves to the synthesis of end functional polymers their use in this context is described in Chapter 9. In this section we limit discussion to processes based on conventional radical polymerization,... 

Dithiols and dienes may react spontaneously to afford dithiols or dienes depending on the monomer dithiol ratio.221 However, the precise mechanism of radical formation is not known. More commonly, pholoinilialion or conventional radical initiators are employed. The initiation process requires formation of a radical to abstract from thiol or add to the diene then propagation can occur according to the steps shown in Scheme 7.17 until termination occurs by radical-radical reaction. Termination is usually written as involving the monomer-derived radicals. The process is remarkably tolerant of oxygen and impurities. The kinetics of the tbiol-ene photopolymerizalion have been studied by Bowman and... 

Radical polymerization is often the preferred mechanism for forming polymers and most commercial polymer materials involve radical chemistry at some stage of their production cycle. From both economic and practical viewpoints, the advantages of radical over other forms of polymerization arc many (Chapter 1). However, one of the often-cited "problems" with radical polymerization is a perceived lack of control over the process the inability to precisely control molecular weight and distribution, limited capacity to make complex architectures and the range of undefined defect structures and other forms of "structure irregularity" that may be present in polymers prepared by this mechanism. Much research has been directed at providing answers for problems of this nature. In this, and in the subsequent chapter, we detail the current status of the efforts to redress these issues. In this chapter, wc focus on how to achieve control by appropriate selection of the reaction conditions in conventional radical polymerization. 

Most are proposed to involve the general ATRP mechanism. However, it should also be noted that the detailed mechanism has not been elucidated in all cases and not all need be radical processes in the conventional sense. Moreover, in many polymerizations, the active catalyst is formed in situ and its exact nature is not rigorously established. 

One might also anticipate that the influence of bootstrap effects (Section 8.3.1.2) would be quite different in living and non-living processes. 68 A comprehensive study of reactivity ratios in living and conventional radical polymerization may provide a test of the various hypotheses for the origin of this effect. 

Many block and graft copolymer syntheses involving transformation reactions have been described. These involve preparation of polymeric species by a mechanism that leaves a terminal functionality that allows polymerization to be continued by another mechanism. Such processes are discussed in Section 7.6.2 for cases where one of the steps involves conventional radical polymerization. In this section, we consider cases where at least one of the steps involves living radical polymerization. Numerous examples of converting a preformed end-functional polymer to a macroinitiator for NMP or ATRP or a macro-RAFT agent have been reported.554 The overall process, when it involves RAFT polymerization, is shown in Scheme 9.60. 

In addition to the two major processes, cross-linking and chain modification (or cyclization), chain scission doubtless occurs also to varying degrees during conventional vulcanizations. Processes of this nature are not difficult to envisage in the presence of free radicals. The radical intermediate (II) may, for example, undergo /3-fission as follows ... 

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