Polymerization conventional radical

Most radicals are transient species. They (e.%. 1-10) decay by self-reaction with rates at or close to the diffusion-controlled limit (Section 1.4). This situation also pertains in conventional radical polymerization. Certain radicals, however, have thermodynamic stability, kinetic stability (persistence) or both that is conferred by appropriate substitution. Some well-known examples of stable radicals are diphenylpicrylhydrazyl (DPPH), nitroxides such as 2,2,6,6-tetramethylpiperidin-A -oxyl (TEMPO), triphenylniethyl radical (13) and galvinoxyl (14). Some examples of carbon-centered radicals which are persistent but which do not have intrinsic thermodynamic stability are shown in Section 1.4.3.2. These radicals (DPPH, TEMPO, 13, 14) are comparatively stable in isolation as solids or in solution and either do not react or react very slowly with compounds usually thought of as substrates for radical reactions. They may, nonetheless, react with less stable radicals at close to diffusion controlled rates. In polymer synthesis these species find use as inhibitors (to stabilize monomers against polymerization or to quench radical reactions - Section 5,3.1) and as reversible termination agents (in living radical polymerization - Section 9.3). 

Metal complex-organic halide redox initiation is the basis of ATRP. Further discussion of systems in this context will be found in Section 9.4, The kinetics and mechanism of redox and photoredox systems involving transition metal complexes in conventional radical polymerization have been reviewed by Bam ford. 

In conventional radical polymerization, the chain length distribution of propagating species is broad and new short chains are formed continually by initiation. As has been stated above, the population balance means that, termination, most frequently, involves the reaction of a shorter, more mobile, chain with a longer, less mobile, chain. In living radical polymerizations, the chain lengths of most propagating species are similar (i.e. i j) and increase with conversion. Ideally, in ATRP and NMP no new chains are fonned. In practice,... 

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. 

End-functional polymers, including telechelic and other di-end functional polymers, can be produced by conventional radical polymerization with the aid of functional initiators (Section 7,5.1), chain transfer agents (Section 7.5.2), monomers (Section 7.5.4) or inhibitors (Section 7.5.5). Recent advances in our understanding of radical polymerization offer greater control of these reactions and hence of the polymer functionality. Reviews on the synthesis of end-functional polymers include those by Colombani,188 Tezuka,1 9 Ebdon,190 Boutevin,191 Heitz,180 Nguyen and Marechal,192 Brosse et al.rm and French.194... 

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,... 

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. 

Figure 9.1 Predicted evolution of molecular weight (arbitrary units) with monomer conversion for a conventional radical polymerization with a constant rate of initiation (---------------) and a living polymerization (--).

Figure 9.2 Calculated (a) number and (b) GPC distributions for three polymers each with =100. The number distributions of chains formed by conventional radical polymerization with termination by disproportionation or chain transfer...

The kinetics and mechanism of living radical polymerization have been reviewed by Fischer,21 Fukuda et at.,22 and Goto and Fuktida.23 In conventional radical polymerization, new chains are continually formed through initiation w hile existing chains are destroyed by radical-radical termination. The steady state concentration of propagating radicals is 10"7 M and an individual chain will have a lifetime of only 1-10 s before termination within a total reaction lime that is... 

In conventional radical polymerization the rate of polymerization is described by eq. 5 (Section 5.2.1). As long as the rate of initiation remains constant, a plot of ln([M]0/[ Vf]t) vs time should provide a straight line. 

Of the major methods for living radical polymerization, NMP appears the most successful for polymerization of the diene monomers. There are a number of reports on the use of NMP of diene monomers (B, I) with TEMPO,188,1103 861 4, cw and other nitroxides.127 High reaction temperatures (120-135 °C) were employed in all cases. The ratio of 1,2- 1,4-cis 1,4-trans structures obtained is similar to that observed in conventional radical polymerization (Section 4.3.2). 

The molecular weight in reverse ATRP will depend on the concentration of the initiator (In) and the initiator efficiency (/) and ideally is given by eq. 11. Side reactions between the catalyst and the initiator and the radicals formed from the initiator may lead to efficiencies being lower than those observed in conventional radical polymerization. 

ATRP has been widely used for the polymerization of methacrylates. However, a very wide range of monomers, including most of those amenable to conventional radical polymerization, has been used in ATRP. ATRP has also been used in cyclopolymerization (e.g. of 16flm364) and ring opening polymerization or copolymerization e.g. of 16T 115 366 and 162 67). ... 

Various side reactions may complicate RAFT polymerization. Transfer to solvents, monomer and initiator occur as in conventional radical polymerization. Other potential side reactions involve the intermediate radicals 165 and 167. These radicals may couple with another radical (Q ) to form 271 or disproportionate with Q to form 270. They may also react with oxygen. The intermediate radicals 165 and 167 are not known to add monomer. 

RAFT polymerization can be performed simply by adding a chosen quantity of an appropriate RAFT agent to an otherwise conventional radical polymerization. Generally, the same monomers, initiators, solvents and temperatures are used. The only commonly encountered functionalities that appear incompatible with RAFT agents are primary and secondary amines and thiols. 

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. 

The grafting through approach involves copolymerization of macromonomers. NMP, ATRP and RAFT have each been used in this context. The polymerizations are subject to the same constraints as conventional radical polymerizations that involve macromonomers (Section 7.6.5). However, living radical copolymerization offers greater product uniformity and the possibility of blocks, gradients and other architectures. 

ATRP has also been used to synthesize maeromononicrs subsequently used to make graft copolymers by conventional radical polymerization. Thus, low molecular weight PBA formed by ATRP was converted in near quantitative yield to the methacrylate ester (351) or the corresponding acrylate ester.612... 

PMBV was synthesized by a conventional radical polymerization. The monomer unit compositions of the PMBV were 0.64, 0.25, and 0.11 unit mole fractions for MPC, BMA, and VPBA, respectively. The number-averaged molecular weight and weight-averaged molecular weight were 6.2 x 104 and 6.5 x 104, respectively. This PMBV was completely water-soluble due to hydrophilic MPC units in the polymer. Figure 1 shows the chemical structure of PMBV. 

We have repeated similar degrafting experiments for brush formation via ATRP. While there have been reports on degrafting using conventional radical polymerization [10,58], this discussion will be limited to brush formation by ATRP. In unpublished work [59], we immobilized an ATRP initiator, (1 l-(2-bromo-2-methyl)propionyloxy)undecyltrichlorosilane) on StOber silica and conducted a styrene polymerization. Degrafting of the PS brushes was conducted by etching of the silica cores with HE From TGA analysis of the immobilized initiator and the corresponding PS brush system, we determined that there are 4.8 initiator molecules/nm and / = 0.06. The initiator density corresponds well to the values of 2.4-5.0 reported by Patten and co-workers [56,57] for the immobilization of (2-(4-chloromethylphenyl)ethyl)dimethylethoxysilane on a similar support. 

The free-radical nature of ATRP is well established through a number of studies [Matyjaszewski et al., 2001], The effects of inhibitors and retarders, solvents, and chain-transfer agents are the same in ATRP as in conventional radical polymerization. The regio-selectivity, stereoselectivity, and copolymerization behaviors are also the same. 

Poly(4-vinylpyridine) was used also as a template for polymerization of maleic anhydride. Maleic anhydride is very difficult to polymerize by conventional radical polymerization, but in the presence of poly(4-vinylpyridine) in chloroform or in nitromethane, polymerization proceeds at room temperature just after mixing 0.5% solution of poly(4-vinylpyridine) with 1% solution of maleic anhydride. A yellow precipitate is obtained. The precipitate is a mixture of poly(maleic anhydride), poly(4-vinylpyridine), and unreacted maleic anhydride. In the absence of oxygen, polymerization is much slower. The reaction stops on the stage of donor-acceptor complex formation. 

Au NPs protected with a thermo-responsive polymer such as PNIPAM by the covalent grafting to technique with different end-functional PNIPAMs and various ratios between PNIPAM and HAuC14 has been studied. PNIPAM samples were synthesized through either conventional radical polymerization or living/controlled radical polymerization. With this approach, very small and quite monodisperse Au NPs are obtained with diameters ranging from 1.5 to 2.3 nm [94]. 

Conventional radical polymerization usually produces polymers with a broad distribution in DP. The polymers are mixtures of the instantaneous polymers with DPw/DPn of at least 1.5 for the termination by recombination or 2.0 either for the termination by disproportionation or for the chain transfer to small molecules. In this respect, any living polymerization with rapid initiation will afford polymers with a narrow DP distribution of the Poisson type. Ring-opening met-hathesis polymerization of norbornenyl-terminated macromonomers, 8, 15, and 16, appears promising in this regard [22,23]. 

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