Polymers, kinetic inhibition

A kinetic model based on homogeneous polymerization was developed to describe the polymerization in CO2 [51, 54]. A model based on the reaction scheme in Fig. 3 adequately described the polymerization rates and the poly-dispersity of the polymer. Monomer inhibition was incorporated into the model to account for the observed deviation from first-order kinetics. However, imperfect mixing of the higher viscosity medium is an alternative explanation. It was concluded that termination was by combination, for three reasons. First, there was no existing literature to support termination by disproportionation for PVDF. Second, the polydispersity was approximately 1.5 at low monomer concentrations. Third, NMR studies showed no evidence of unsaturation. 

It was demonstrated that a stereocenter positioned far away from the reactive isocyanide group as in monomer 82b (Chart 14) can still induce chirality in the main chain of polyisocyanides, resulting in the formation of an excess of one particular helix.224-226 Kinetic control over the helix sense in the polymerization of 82b was confirmed by the noncooperative transfer of chirality from the monomer to the macromolecule e.g., a linear relation was found between the ee present in the isocyanide and the optical activity in the polymers formed.227 The kinetic inhibition of the growth of one particular handedness using (5)-2-isocyanovaleric acid184 (vide supra) is used to force 82b and 82c into a macromolecular helicity with a screw sense opposite that of the one preferred... 

A kinetic inhibition, for example, can occur such that already formed polymer blocks the propagation path of a growing chain because all monomer in this region has already been consumed. The free radicals remain intact, but the polymerization is stopped. 

Liquid-phase chlorination of butadiene in hydroxyhc or other polar solvents can be quite compHcated in kinetics and lead to extensive formation of by-products that involve the solvent. In nonpolar solvents the reaction can be either free radical or polar in nature (20). The free-radical process results in excessive losses to tetrachlorobutanes if near-stoichiometric ratios of reactants ate used or polymer if excess of butadiene is used. The "ionic" reaction, if a small amount of air is used to inhibit free radicals, can be quite slow in a highly purified system but is accelerated by small traces of practically any polar impurity. Pyridine, dipolar aptotic solvents, and oil-soluble ammonium chlorides have been used to improve the reaction (21). As a commercial process, the use of a solvent requites that the products must be separated from solvent as well as from each other and the excess butadiene which is used, but high yields of the desired products can be obtained without formation of polymer at higher butadiene to chlorine ratio. 

The kinetic and inhibition methods have considerable limitations when applied to both two-component and one-component catalysts (160a). The most reliable results can be obtained by the methods based on tagging the growing polymer chains. Therefore, it is advisable to note some details of this method. 

Most kinetic treatments of the photo-oxidation of solid polymers and their stabilization are based on the tacit assumption that the system behaves in the same way as a fluid liquid. Inherent in this approach is the assumption of a completely random distribution of all species such as free radicals, additives and oxidation products. In all cases this assumption may be erroneous and has important consequences which can explain inhibition by the relatively slow radical scavenging processes (reactions 7 and 9) discussed in the previous section. 

Like the oxidation of hydrocarbons, the autocatalytic oxidation of polymers is induced by radicals produced by the decomposition of the hydroperoxyl groups. The rate constants of POOH decomposition can be determined from the induction period of polymer-inhibited oxidation, as well as from the kinetics of polymer autoxidation and oxygen uptake. The initial period of polymer oxidation obeys the parabolic equation [12]... 

Results have generally been disappointing. It can be difficult to remove the TSA from the polymer, but a more fundamental problem concerns the efficiency of the catalysis observed. The most efficient systems catalyze the hydrolysis of carboxylate and reactive phosphate esters with Michaelis-Menten kinetics and accelerations (koAJKM)/kunoJ approaching 103,1661 but the prospects for useful catalysis of more complex reactions look unpromising. Apart from the usual difficulties the active sites produced are relatively inflexible, and the balance between substrate binding and product inhibition is particularly acute. 

A model has been developed to describe the penetration of polydimethylsi-loxane (PDMS) into silica agglomerates [120]. The kinetics of this process depend on agglomerate size and porosity, together with fluid viscosity. Shearing experiments demonstrated that rupture and erosion break-up mechanisms occurred, and that agglomerates which were penetrated by polymer were less readily dispersed than dry clusters. This was attributed to the formation of a network between sihca aggregates and penetrated PDMS, which could deform prior to rupture, thereby inhibiting dispersion. 

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