Chain initiation cage effect

The efficiency of the intitiator is a measure of the extent to which the number of radicals formed reflects the number of polymer chains formed. Typical initiator efficiencies for vinyl polymerisations lie between 0.6 and 1.0. Clearly the efficiency cannot exceed 1.0 but it may fall below this figure for a number of reasons, the most important being the tendency of the newly generated free radicals to recombine before they have time to move apart. This phenomenon is called the cage effect . 

The least known of these reactions is chain initiation together with the efficiency of the primary radicals. This group consists of at least three elementary reactions. The so-called cage effect certainly plays an important role. On the other hand, the cage effect in its classical form cannot explain all phenomena sufficiently. 

It is also possible that stretched (high pH) versus coiled (low pH) polymer chains may exhibit different photophysics. This was commented by Chou and Jellinek in their early report on PMAA photochemistry, and a detailed TREPR study of pH-dependent cage effect manipulation using initiator-labeled PMAA was reported by Maliakal et al. " It has been reported by Mittal and coworkers that for small molecule analogs of PAA and PMAA, the quantum yield for Norrish I cleavage drops to zero for the completely deprotonated structure. For PMAA, we also observe a pH dependence in the TREPR signal intensities (data not shown). The data in Fig. 14.7 are the hrst experimental evidence for such an effect in PAA radicals. 

Because of their different hydrophilicities, the two free radicals formed at the same time can separate from each other quickly which can eliminate the cage effect. In a micelle, the local BA concentration may be quite high. Once a micelle is initiated, a number of BA molecules may be added quickly. As a result, some short BA blocks would be incorporated into a poly(MAETAC) chain to form something like multi-block copoly(MAETAC-BA), as shown in Fig. 19 [170]. Surfactant should stabilize the BA blocks so that the block copolymer remains in the aqueous phase. 

Other advantages of C02-based polymerizations are that there is no chain transfer to the solvent, and that the production of unstable end groups can be dramatically reduced. Guan et al [4] studied the decomposition of 2,2 -azobis-(isobutyronitrile) (AIBN) in SCCO2. It was found that initiator efficiencies greater than 80% were possible due to the low viscosity of CO2 and negligible solvent cage effects. Additionally, analysis of the decomposition products showed that there was no chain transfer to CO2. 

An interesting result was the way in which the viscosity, r/, had another effect, namely to control the diffusion of the free radicals out of the initial cage in which they were formed. Geminate recombination of radicals is therefore important and the formation of chain-carrying radicals was dependent on Also of interest is the observation that the decomposition... 

Problem 6.8 Consider the following scheme of reactions for free-radical chain polymerization initiated by thermal homolysis of initiator with cage effect [6] ... 

Practical free-radical polymerizations often deviate from Eq. (6.126) because the assumptions made in the ideal kinetic scheme are not fuUy satisfied by the actual reaction conditions or because some of these assumptions are not valid. For example, according to the ideal kinetic scheme that leads to Eq. (6.126), the initiation rate (Rf) and initiator efficiency (/) are independent of monomer concentration in the reaction mixture and primary radicals (i.e., radicals derived directly from the initiator) do not terminate kinetic chains, thoughrifi reality R may depend on [M], as in the case of cage effect (see Problem 6.7) and, at high initiation rates, some of the primary radicals may terminate kinetic chains (see Problem 6.25). Moreover, whereas in the ideal kinetic scheme, both kp and kt are assumed to be independent of the size of the growing chain radical, in reahty k[ may be size-dependent and diffusion-controlled, as discussed later. 

The factor f, called initiator efficiency, takes into account that not all the primary radicals R effectively initiate polymer chains some can be lost due to the so-called cage effect. This implies secondary reactions of the radicals within a cage of solvent surrounding the initiator [5] (the effect can be more pronounced at high conversions/viscosities due to diffusion limitations). The values of / usually lie in the range 0.3-0.8. 

When water-soluble initiators are used, most of the authors concluded that acrylamide polymerization proceeds within the monomer droplets, irrespective of the nature of the organic phase (aromatic or aliphatic) [28,30-34], Both monomer and initiator reside in the dispersed droplets and each particle acts as a small batch reactor. The process is essentially a suspension polymerization and therefore the kinetics resemble those for solution polymerization. Note that a prefix micro has been added in some cases to this type of polymerization (microsuspension) to emphasize the smallness of the reactor (d 1 pm) and the possibility of interfacial reactions [33]. A square root dependence of the polymerization rate, / p, on initiator concentration, [I] was often observed, in good accord with solution polymerization [28,32-34]. Higher orders were also found which were attributed to chain transfer to the emulsifier [30]. The reaction order with respect to monomer was found to vary from 1 [2832] to 1.7 [3031]> Orders higher than 1 are common for acrylamide polymerization in homogeneous aqueous solution and are explained by the occurrence of a cage effect [35]. 

The polymerization rate is directly proportional to the monomer concentration for ideal free radical polymerization kinetics. Deviations from this first-order kinetics can be caused by a whole series of effects which must be checked by separate kinetic experiments. These effects include cage effects during initiator free radical formation, solvation of or complex formation by the initiator free radicals, termination of the kinetic chain by primary free radicals, diffusion controlled termination reactions, and transfer reactions with reduction in the degree of polymerization. Deviations from the square root dependence on initiator concentration are to be primarily expected for termination by primary free radicals and for transfer reactions with reduction in the degree of polymerization. 

Cage effects tend to decrease the efficiency of utilization of free radicals produced for the initiation of chain reactions. A quantitative treatment appears in sight but will not be discussed here. 

The initiator radicals initially formed in solution are held together briefly in a cage of solvent molecules. This cage effect causes radical molecules to recombine and slows down their diffusion through the solvent. Therefore, the rate of initiation (R.) depends on the rate of decomposition of the initiator (k ) and on the fraction e of the initiator radicals that escape the solvent cage affecting the efficiency of the chain initiation reaction (20). 

The initiator decomposition rate must be reasonably constant during the polymerization reaction. The cage effect (recombination of initiator radicals before starting a polymer chain) should be small, which is generally more the case with azo compounds than with peroxides. 

Chain mobility has important implications for bimolecular reaction efficiency. Bimolecular reactions depend on (i) the frequency of encounter and (ii) the concentrations of the reactive species. Upon bond cleavage, macroradicals are formed in pairs and can initiate degradative reactions only if they are sufficiently flexible to move apart. If the macroradicals are held rigid, as in thermosets or below the Tg, they may recombine by the cage effect without any detectable molecular change. 

Secondary cage recombination of peroxy radicals [698]. In a solid polymer, a pair of polymer peroxy radicals (POO 2) is trapped in the polymer matrix. When a radical pair, produced by photoinitiation, escapes the initial cage, the probability of its recombination remains high even after several propagation steps. This phenomenon, known as secondary cage recombination, has a pronounced effect on the kinetics of oxidation and on the distribution of kinetic chain lengths in the oxidation process. 

The radical polymerization has a long history. Certainly the major credit in this area of polymer chemistry should be given to Hermann Staudinger (1881, Worms, to 1965, Freiburg). Since then all the elementary reactions, namely, initiation (including cage effect and related efficiency), chain propagation, chain transfer (to monomer, polymer, solvent). 

This reaction takes place at 60°C in a benzene solution, but not all the radicals produced may go on to initiate chain growth. Secondary reactions can occur between the radicals produced because of the confining effect of solvent molecules (the cage effect). Primary recombination can occur when the two benzoyloxy radicals produced are unable to diffuse away from each other fast enough... 

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