Uncontrolled radical polymerization

In contrast to ionic chain polymerizations, free radical polymerizations offer a facile route to copolymers ([9] p. 459). The ability of monomers to undergo copolymerization is described by the reactivity ratios, which have been tabulated for many monomer systems for a tabulation of reactivity ratios, see Section 11/154 in Brandrup and Immergut [14]. These tabulations must be used with care, however, as reactivity ratios are not always calculated in an optimum manner [15]. Systems in which one reactivity ratio is much greater than one (1) and the other is much less than one indicate poor copolymerization. Such systems form a mixture of homopolymers rather than a copolymer. Uncontrolled phase separation may take place, and mechanical properties can suffer. An important ramification of the ease of forming copolymers will be discussed in Section 3.1. 

The previous sections show that certain ionic liquids, namely the chloroalumi-nate(III) ionic liquids, are capable of acting both as catalyst and as solvent for the polymerization of certain olefins, although in a somewhat uncontrolled manner, and that other ionic liquids, namely the non-chloroaluminate(III) ionic liquids, are capable of acting as solvents for free radical polymerization processes. In attempts to carry out polymerization reactions in a more controlled manner, several studies have used dissolved transition metal catalysts in ambient-temperature ionic liquids and have investigated the compatibility of the catalyst towards a range of polymerization systems. 

For the evaluation of the obtained GC data, the decrease of MMA concentration is plotted vs. time. From the plot the apparent rate of conversion can be determined. Also, the degree of conversion can be calculated for each data pointThe resulting plot shows until what time the process occurs in a controlled manner and where the uncontrolled free radical polymerization sets in. 

Radicals add to unsaturated bonds to form new radicals, which then undergo addition to other unsaturated bonds to generate further radicals. This reaction sequence, when it occurs iteratively, ultimately leads to the production of polymers. Yet the typical radical polymerization sequence also features the essence of radical-induced multicomponent assembling reactions, assuming, of course, that the individual steps occur in a controlled manner with respect to the sequence and the number of components. The key question then becomes how does one control radical addition reactions such that they can be useful multicomponent reactions Among the possibilities are kinetics, radical polar effects, quenching of the radicals by a one-electron transfer and an efficient radical chain system based on the judicious choice of a radical mediator. This chapter presents a variety of different answers to the question. Each example supports the view that a multicomponent coupling reaction is preferable to uncontrolled radical polymerization reactions, which can decrease the overall efficiency of the process. 

The use of polysilanes as photoinitiators of radical polymerization was one of the hrst means whereby they were incorporated within block copolymer structures [38 0], albeit in an uncontrolled fashion. However the resulting block copolymer structures were poorly defined and interest in them principally lay in their application as compatibilisers for polystyrene (PS) and polymethylphenylsilane blends PMPS. The earliest synthetic strategies for relatively well-defined copolymers based on polysilanes exploited the condensation of the chain ends of polysilanes prepared by Wurtz-type syntheses with those of a second prepolymer that was to constitute the other component block. Typically, a mixture of AB and ABA block copolymers in which the A block was polystyrene (PS) and the B block was polymethylphenylsilane (PMPS) was prepared by reaction of anionically active chains ends of polystyrene (e.g. polystyryl lithium) with Si-X (X=Br, Cl) chain ends of a,co-dihalo-polymethylphenylsilane an example of which is shown in Fig. 2 [43,44,45]. Similar strategies were subsequently used to prepare an AB/ABA copolymer mixture in which the A block was poly(methyl methacrylate) (PMMA) [46] and also a multi- block copolymer of PMPS and polyisoprene (PI) [47]. 

Each of the above methods for controlling the radical polymerization of vinyl monomers has its strengths and weaknesses. For example, the rates in ATRP can be easily adjusted through both the amount and activity of the transition metal complexes (both activator and deactivator). Faster rates in RAFT require larger amounts of initiators, i.e., more uncontrolled chains, while faster NMP requires less persistent radicals, which may result in more termination higher polydis-persities. At the same time, transition metal complexes, although not attached to the polymer chains, require removal and can potentially be recycled. 

Various heterogeneous polymerization reactions of hydrophilic or water-soluble monomers in the presence of either difnnctional or multifunctional cross-linkers have been mostly utilized to prepare weU-defined synthetic nanogels. They include precipitation, inverse (mini)emulsion, and inverse micio ulsion polymerization utilizing an uncontrolled free radical polymerization process. 

The above two examples of the polymerization of styrene contrast the prototypical controlled and uncontrolled polymerizations. Controlled polymerizations offer simple molar mass control, the ability to define the polymer end groups, and give polymer samples with narrow molar mass distributions. Molar mass definition in uncontrolled polymerizations is more difficult, polymer end groups are determined by inherent termination (and transfer) reactions, and the molar mass distributions are typically broader. There is much contemporary interest in developing polymerization reactions that are controlled because of the precision with which macromolecules can be designed. Many chapters are dedicated to these endeavors with controlled radical polymerization receiving the most attention recently. 

FIGURE 12.13 M versus conversion, /from ACOMP for several BA polymerization reactions by RAFT ( 1 ) and a free radical polymerization reaction ( 5). For high [DoPAT]/[AIBN] ( 1-3), the reactions exhibit typical CRP behavior with nearly linear increase of mass versus /. The downward curvature in reaction 4 indicates significant deviations from the ideal living mechanism. Reaction 5 shows classical uncontrolled radical polymerization behavior of with conversion. Reprinted (adapted) with permission from Alb AM, Serelis AK, Reed WF. Kinetic trends in RAFT homopolymerization from online monitoring. Macromolecules 2008 41 332-338. 2008 American Chemical Society. 

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