Conventional Free-radical Polymerization

That the Poisson distribution results in a narrower distribution of molecular weights than is obtained with termination is shown by Fig. 6.11. Here N /N is plotted as a function of n for F= 50, for living polymers as given by Eq. (6.109). and for conventional free-radical polymerization as given by Eq. (6.77). This same point is made by considering the ratio M /M for the case of living polymers. This ratio may be shown to equal... 

Copolymers of VDC can also be prepared by methods other than conventional free-radical polymerization. Copolymers have been formed by irradiation and with various organometaHic and coordination complex catalysts (28,44,50—53). Graft copolymers have also been described (54—58). 

As discussed in Section 7.3, conventional free radical polymerization is a widely used technique that is relatively easy to employ. However, it does have its limitations. It is often difficult to obtain predetermined polymer architectures with precise and narrow molecular weight distributions. Transition metal-mediated living radical polymerization is a recently developed method that has been developed to overcome these limitations [53, 54]. It permits the synthesis of polymers with varied architectures (for example, blocks, stars, and combs) and with predetermined end groups (e.g., rotaxanes, biomolecules, and dyes). 

Radiation-Induced Polymerization. Polymerization induced by irradiation is initiated by free radicals and by ionic species. On very pure vinyl monomers, D. J. Metz demonstrated that ionic polymerization can become the dominating process. In Chapter 12 he postulates a kinetic scheme starting with the formation of ions, followed by a propagation step via carbonium ions and chain transfer to the vinyl monomer. C. Schneider studied the polymerization of styrene and a-methylstyrene by pulse radiolysis in aqueous medium and found results similar to those obtained in conventional free-radical polymerization. She attributes this to a growing polymeric benzyl type radical which is formed partially through electron capture by the styrene molecule, followed by rapid protonation in the side chain and partially by the addition of H and OH to the double vinyl bond. A. S. Chawla and L. E. St. Pierre report on the solid state polymerization of hexamethylcyclotrisiloxane by high energy radiation of the monomer crystals. 

Owing to the insolubility of the polar monomer-zinc chloride complex, handling of the reaction mixture is difficult. However, a second patent (73) describes an improved process wherein the polar monomer is utilized in considerable excess with no effect on the polar monomer content of the resulting copolymer, in contrast to the results from a conventional free radical polymerization. This is consistent with the mechanism shown in Reaction 23 and essentially eliminates the participation of a polar monomer-complexed polar monomer complex. 

Although ATRP behaves differently from conventional free radical polymerization, the fundamental reactions involved are very similar and include initiation, propagation, transfer and termination (see Scheme 6). Since chain termination does not occur in a truly living polymerization, the living character of the chains in ATRP derives from the fact that chain propagation is first order with respect to radical concentration and irreversible bi-molecular termination is second order. As such, the concentration of the radicals is kept very low, the rate of bi-molecular termination is greatly reduced, and typically less than 10% of all of the chains will terminate. Unlike conventional free radical polymerization, where the rate is dictated by a steady state between the initiation and termination rates, the rate and concentration of propagating radicals in ATRP is controlled entirely by the equilibrium between activation and deactivation [255]. 

If reactions 1 to 3 in Scheme 7 are considered, there is no reason to assume that addition of a RAFT agent to a conventional free radical polymerization will have an effect on the polymerization rate, since the equilibrium concentration of propagating radicals will not be affected. However, it has been found that considerable retardation does take place in RAFT polymerization [275-281]. The intermediate radical was postulated to be the reason for the significant retardation of the polymerization rate. Two explanations for retardation have been put forward ... 

Another example of the macroinitiator approach to making block copolymers is shown in Scheme 8.4. Since methyl methacrylate (MMA) polymerization cannot effectively be initiated by TEMPO-based alkoxyamine initiators, a poly(methyl methacrylate) macroinitiator (XIII) was prepared using conventional free radical polymerization [16]. However, the azo initiator was functionalized with a TEMPO-based alkoxyamine. Since the main mechanism of termination during bulk MMA polymerization is by radical coupling, most of the MMA polymer chain-ends are functionalized with alkoxyamine groups. 

Which monomer yields high-molecular-weight polymers in conventional free radical polymerizations Explain the difference. 

In conventional free radical polymerization, the initiation, propagation, and termination are kinetically coupled. Consequently, the increase of initiation rate increases the overall polymerization rate but reduces the degree of polymerization. In contrast to this situation (kinetically coupled initiation, propagation, and termination), the formation of chemically reactive species is not the initiation of a subsequent polymerization. Under such an activation/deactivation decoupled reaction system, the mechanism for how chemically reactive species are created and how these species react to form solid material deposition cannot be viewed in analogy to polymerization. 

In contrast to the above situations, parylene polymer deposition has very poor adhesion to a smooth surface substrate but can penetrate deep into small cavities. para-Xylylene prefers to react with another para-xylylene or its derivatives. Although it has the feature of difunctional free radical, it is rather stable and does not initiate polymerization of other monomers for conventional free radical polymerization. In spite of numerous attempts, the polymerization of various vinyl monomers initiated by para-xylylene or copolymerization of vinyl monomers with /7ura-xylylene has been elusive. 

Metal-catalyzed living radical polymerizations may be carried out either in solution or in the bulk. Importantly, unlike conventional free radical polymerization, the Trommsdrof or gel effect is absent in these living processes in the bulk.238 For the solution processes, nonpolar or less polar solvents are employed, such as toluene, xylene, and benzene. Polar solvents are sometimes employed for not only solubilizing the monomers, the produced polymers, and... 

Figure 6.27 Mechanism of RAFT process. Polymerizations can be carried out in bulk, solution, emulsion or suspension, using azo or peroxy initiators as in conventional free-radical polymerization. The moiety S=C(X)S- remains as the end group.

The success of the living-radical polymerization field will be defined on the basis of the commercialization of any of these processes [48], It is believed that the strength of the living-radical polymerization systems lies in their ability to make polymers of novel architecture, for example, block copolymers. However, very little work has been done to look at the properties of materials prepared by these processes. It remains to be seen whether block copolymers, prepared by living-radical polymerization processes, have any performance advantages over random copolymers prepared by conventional free-radical polymerization. 

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