Polymerization cage effects

Because of its tunable density and low viscosity, synthetic organic chemists are beginning to utilize supercritical C02 as a medium for exploring reaction mechanisms and solvent cage effects [10,11]. Asymmetric catalysis represents an area in which supercritical C02 may be useful as a solvent [12]. For polymerization reactions, in particular, the solvency of C02 as a medium and the plasticization effects of C02 on the resulting polymeric products represent the properties of central importance. These significant properties of C02 are explored in detail below. When all of these factors are combined with the fact that C02 may obviate the use of much more expensive and hazardous solvents,... 

We have also investigated the kinetics of free radical initiation using azobisisobutyronitrile (AIBN) as the initiator [24]. Using high pressure ultraviolet spectroscopy, it was shown that AIBN decomposes slower in C02 than in a traditional hydrocarbon liquid solvent such as benzene, but with much greater efficiency due to the decreased solvent cage effect in the low viscosity supercritical medium. The conclusion of this work was that C02 is inert to free radicals and therefore represents an excellent solvent for conducting free radical polymerizations. 

The mechanism of antioxidant action on the oxidation of carbon-chain polymers is practically the same as that of hydrocarbon oxidation (see Chapters 14 and 15 and monographs [29 10]). The peculiarities lie in the specificity of diffusion and the cage effect in polymers. As described earlier, the reaction of peroxyl radicals with phenol occurs more slowly in the polymer matrix than in the liquid phase. This is due to the influence of the polymeric rigid cage on a bimolecular reaction (see earlier). The values of rate constants of macromolecular peroxyl radicals with phenols are collected in Table 19.7. 

In grafting undecylenic acid, a monomer cage effect has been observed. The kinetic law follows a 1.5th power of the monomer concentration (46). The overall initial rate of polymerization Rp can be described as... 

The most commonly used water-soluble initiator is the potassium, ammonium, or sodium salt of peroxodisulfates. Redox initiators (Fe2+salt/peroxodisul-fate, etc.) are used for polymerization at low temperatures. Oil-soluble initiators, such as azo compounds, benzoyl peroxides, etc., are also used in emulsion polymerization. They are, however, less efficient than water-soluble peroxodisulfates. This results from the immobilization of oil-soluble initiator in polymer matrix, the cage effect, the induced decomposition of initiator in the particle interior, and the deactivation of radicals during des orption/re-entry events [14, 15]. 

The initiation step could also be positively affected by the above-mentioned transport properties, as the efficiency factor f assumes higher values with respect to conventional liquid solvents due to the diminished solvent cage effect One further advantage is constituted by the tunability of the compressibility-dependent properties such as density, dielectric constant, heat capacity, and viscosity, all of which offer additional possibilities to modify the performances of the polymerization process. This aspect could be particularly relevant in the case of copolymerization reactions, where the reactivity ratios of the two monomers, and ultimately the final composition of the copolymer, could be controlled by modifying the pressure of the reaction system. 

Inisurfs, Transurfs and Surfmers may be used to reduce/avoid the use of conventional surfactants in emulsion polymerization. However, when Inisurfs and Transurfs are used, the stability of the system cannot be adjusted without affecting either the polymerization rate (Inisurfs) or the molecular weight distribution (Transurfs). Furthermore, the efficiency rate of Inisurfs is low due to the cage effect. It is therefore not obvious yet that these classes will become commercially significant. 

However, the introduction of the solvent into the polymerization medium poses new problems. The solvents must be pure, without inhibiting and transfer agents. Every solvent takes part in the polymerization process its effect is almost never limited to the mere physical dilution of the monomer. It solvates the active centres it participates in processes connected with energy and impulse transfer often it serves as a transfer agent (so that the degrees of polymerization of solution-polymerized products are usually lower compared with bulk-polymerized polymers) it may form complexes with some component of the system it modifies initiation efficiency by the cage effect etc. 

The cage effect has a considerable influence on the course of radical polymerizations. It is held responsible for many kinetic anomalies. 

Cage effects also account for the fact that not all the radicals produced from the decomposition of initiators such as azobisisobutyronitrile (AIBN) are effective in initiating radical polymerizations. In the somewhat simplified reaction Scheme (5-168) depicting the thermolysis of AIBN, two types of cyanopropyl radicals are shown, one still within the solvent cage, whereas the others have reached their statistical separation... 

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. 

The ceiling temperature T can be considered the upper temperature at which a pyrolytic process will reach equilibrium. It may be seen, therefore, as a recommended temperature for pyrolysis. However, in practice, the application for macromolecules of the above relations is not straightforward. The theory was developed for ideal systems (sometimes in gas phase), and although in principle this theory should hold true for any system, its application to condensed phases or polymeric materials may be accompanied by effects difficult to account for (phase change, melting, cage effect [2], etc.). The reaction rate could also be low at calculated Tq values. For this reason, temperatures 50° C or 100° C higher than Tq must frequently be used as practical values of the temperature used in pyrolysis. 

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

Neglecting Chain transfer, derive an expression for the rate of polymerization showing the cage effect. 

Comparison of Eq. (P6.7.11) with Eq. (6.24) shows that the polymerization rate is reduced by the cage effect. 

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. 

We have also studied the kinetics of free radical initiation in CO2 using azobis(isobutyronitrile) (AIBN) as an initiator [35]. These experiments were accomplished using high pressure UV spectroscopy, and illustrated that AIBN decomposes more slowly in CO2 than in traditional hydrocarbon solvents, yet the initiator efficiency is much greater in CO2 due to the reduced solvent cage effect in the low viscosity supercritical medium. The main conclusion drawn from this work was that CO2 can therefore be employed effectively as a solvent for free radical polymerizations and remains an inert solvent even in the presence of highly electrophilic hydrocarbon radicals. 

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]. 

Once the initiating radical is formed, there is competition between addition to the monomer and all other possible secondary reactions. A secondary reaction, such as a recombination of fragments, can be caused by the cage effect of the solvent molecules. Other reactions can take place between a radical and a parent initiator molecule. This can lead to formation of different initiating species. It can, however, also be a dead end as far as the polymerization reaction is concerned. 

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. 

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