Macroradical mobility

The predominant mode of polymerization is in the interior of the particles and this leads to a reduction of macroradical mobility, usually referred to as radical occlusion, and a marked autoacceleration of the polymerization rate. 

The chain termination rate varies inversely with the viscosity of the polymerization medium because of the Trommsdorff Effect (i.e., the reduction of the macroradical mobility with increasing reaction viscosity). This effect significantly influences reaction rate[ ,2, 10]. 

Norrish and Smith [29] and later Tromsdorff et al. [30] described a polymerization of methyl methacrylate, the rate of which increased from a certain conversion. The number of monomers of similar behaviour was extended by methyl acrylate [31 ], butyl acrylate [32] and other acrylates [33] and methacrylates [34], and vinyl acetate. The effect was explained by the reduction of the termination rate caused by hindered macroradical mobil-ity in viscous medium it was called the gel effect, or the Norrish-Tromsdorff effect. The gel effect is clearly manifested in radical polymerizations of weakly transferring monomers in bulk. It is significant also in the presence of a good solvent. The gel effect is suppressed by the presence of poor solvents++ and by... 

There are different variants of the DCR conception, differing one from another by the set of physical parameters, which are taken into account, and by the details of the analytical description of diffusion control on the rate of an elementary reaction. As a rule, the main attention focuses on bimolecular chain termination, the constant rate of which is considered as a function of macroradicals mobility, depending on their length [34-46], free volume [47-53] or characteristic viscosity [54-57] of the monomer/polymeric solution. In a range of studies [58-61] the initiation efficiency and the constant rate of the chain propagation are also used as a function of macroradicals mobility. 

Recombination reactions between two different macroradicals are readily observable in the condensed state where molecular mobility is restricted and the concentration of radicals is high. Its role in flow-induced degradation is probably negligible at the polymer concentration normally used in these experiments (< 100 ppm), the rate of radical formation is extremely small and the radicals are immediately separated by the velocity gradient at the very moment of their formation. Thus there is no cage effect, which otherwise could enhance the recombination efficiency. 

It can be seen from equation (2) that when y 0 the model falls into the classical expression for the rate of conversion of free radical polymerization. Equation (la) shows that this will be the case whenever all macroradicals have the same high mobility (i.e., as n tends to infinity) or when both entangled and non-entangled radicals have the same termination rate constant (i.e. a is equal to unity). 

As the polymerization reaction proceeds, scosity of the system increases, retarding the translational and/ or segmental diffusion of propagating polymer radicals. Bimolecular termination reactions subsequently become diffusion controlled. A reduction in termination results in an increase in free radical population, thus providing more sites for monomer incorporation. The gel effect is assumed not to affect the propagation rate constant since a macroradical can continue to react with the smaller, more mobile monomer molecule. Thus, an increase in the overall rate of polymerization and average degree of polymerization results. 

Therefore, concentrations of the polyvinyl monomer, the branching and network formation proceeds as shown in Figure 12. Mlcrogel-llke species are formed first which are highly internally crossllnked. The pendant double bonds in the interior are very Immobile so that their reactivity is strongly diffusion controlled and they almost cannot react at all, even with the monomers. Only the more mobile pendant double bonds in the periphery of these species can enter into reactions with macroradicals and participate in Interbinding of the species together. 

Another unique attribute of polymerizations of multifunctional monomers is the dominance of reaction diffusion as a termination mechanism [134,136, 143-146]. Reaction diffusion involves the mobility of radicals by propagation through unreacted functional groups. This termination mechanism is physically different from translation and segmental diffusion termination mechanisms which involve the diffusion of polymer macroradicals and chain segments to bring radicals within a reaction zone before terminating. Whereas normal termination mechanisms are related to the diffusion coefficient of the polymer, reaction diffusion must be considered differently. In essence, reaction diffusion is... 

Berlin and coworkers (5,56) desired to obtain a material with an increased mechanical strength. They carried out a plasticization of bulk ami emulsion polystyrene molecular weight 80000 and 200000 respectively at 150-160° C, with polyisobutylene, butyl rubber, polychloroprene, polybutadiene, styrene rubber (SKS-30) and nitrile rubber (SKN 18 and SKN 40). The best results were obtained with the blends polystyrene-styrene rubber and polystyrene-nitrile rubber. An increase of rubber content above 20-25% was not useful, as the strength properties were lowered. An increase in the content of the polar comonomer, acrylonitrile, prevents the reaction with polystyrene and decreases the probability of macroradical combination. This feature lowers the strength, see Fig. 14. It was also observed that certain dyes acts as macroradical acceptors, due to the mobile atoms of hydrogen of halogens in the dye, AX ... 

The rate constant of a transfer reaction will therefore be the higher, the weaker C-H bond is attacked by a peroxyl radical. From this it is obvious that the maximum rate of oxidation of polyethylene will increase with increasing number of tertiary hydrogens in the polymer [13]. Since the process includes the interaction of a macroradical with a macromolecule which both are of restricted translational mobility, the maximum rate of oxidation does not depend on the low content of reactive allyl hydrogens in polyethylene. 

Let us now examine the reactions of macroradicals with various agents. Doubtless, in this cases kinetic relationships derived for the same reactions but in gaseous and liquid state, should be corrected allowing for the specific character of the mobility of molecules in a solid body. When the reaction takes place in solid polymers, we deal with a highly hindered movement of the reacting compound molecules towards... 

Bolshakov et al. [44] observed a large difference in the reactivities of active centres in MMA and MA polymerizations at low temperatures (100-130 K). Substitution of the a proton with a —CH3 group results in steric hindrance of the centre and lower reactivity of the macroradical. At higher temperatures, steric hindrance is less severe, due to increased methyl mobility, and the reactivities of the radicals... 

In a Soviet study23,24 the mobility of cellulose macromolecular fragments was investigated by means of the paramagnetic label technique. Cellulose macroradicals serving as paramagnetic centres were obtained by irradiation of cellulose at —120° to —140 °C. The mobility of their fragments was found to increase sharply with the water content in the sample to reach a maximum at 10% water. It is this increase which appears to be responsible for the rapid decomposition of macroradicals in moist cellulose. 

The rate of propagation is affected much less than the rate of termination. The propagation reaction involves the reaction of a large radical with a small monomer whose diffusion rate is not changed significantly, whereas the termination process involves two macroradicals whose ends have reduced mobility, because motion of their centers of mass has become restrained. The net result in this case is an increase in the effective kp/k J ratio in Eq. (6-29) and an increase in the rate of polymerization. 

Stepwise decay was also observed when PMMA was irradiated in the presence of ethyl mercaptan (EtSH) [245]. The initial decay rate of the radicals measured at 150°K is proportional to the concentration of EtSH, indicating that the decaying pairs are mixed pairs formed by a radical from PMMA and a radical from EtSH. In fact, radiolysis of pure PMMA results in the formation of pairs of macroradicals. Some are due to main-chain scission, others to hydrogen abstraction from the polymer by CH 3 or CH30 radicals produced by side-chain scission. At 150°K, in pure irradiated polymethylmethacrylate, the mobility of the macroradicals is limited and their rate of decay comparatively low. In the presence of ethyl... 

As the polymerization proceeds with further consumption of the monomers, viscosity of the reaction mixture dramatically increases, thus additionally restricting the mobility of the growing chain s ends. For this reason, even at a high conversion of monomers, a certain part of pendant vinyl groups miss the chance of meeting a growing macroradical and remain unreacted in the final material. Thus, approximately 30% of DVB units have been found incorporated in a styrene—8% DVB network by one double bond only [32]. Even in the copolymer with l%p-DVB obtained at 70°C, 40% of DVB vinyl groups remained unreacted. This quantity dropped to 16% when the polymerization was finished at 95°C [33]. 

Telogens are well known as substances, some bonds of which dispose to homolytic cleavage on reacting with a radical. A growing macroradical clips off the mobile hydrogen atom from ct-carbon of an aromatic ethyl... 

Rate constants of macroradical decay in isotactic polypropylene chtinge in relation to the content of the crosslinks [74]. For the first stage of crosslink formation, the rate constant of radical decay decreases by about 1 order for higher conversions of ca osslinking the rate constant increases. The initial decrease of the rate constant seems to be associated with a reduced mobility of macromolecular segments, while the subsequent increase with a gradual reduction of polymer crystallinity. 

---