Polymerization reactions step-change

The theory of radiation-induced grafting has received extensive treatment [21,131,132]. The typical steps involved in free-radical polymerization are also applicable to graft polymerization including initiation, propagation, and chain transfer [133]. However, the complicating role of diffusion prevents any simple correlation of individual rate constants to the overall reaction rates. Changes in temperamre, for example, increase the rate of monomer diffusion and monomer... 

Radical polymerizations have three important reaction steps in common chain initiation, chain propagation, and chain termination. For the termination of chain radicals several mechanisms are possible. Since the lifetime of a radical is usually less than 1 s, radicals are continuously generated and terminated. Each propagating radical can add a finite number of monomers between its initiation and termination. If a divinyl monomer is in the monomer mixture, the reaction kinetics changes drastically. In this case, a dead polymer chain may grow again as a macroradical, when its pendant vinyl groups react with radicals, and the size of the macromolecule increases until it extends over the whole available volume. 

Consider the following mechanism for step-change polymerization of monomer M (Px) to P2, P3,..., Pr,. The mechanism corresponds to a complex series-parallel scheme series with respect to the growing polymer, and parallel with respect to M. Each step is a second-order elementary reaction, and the rate constant k (defined for each step)1 is the same for all steps. 

Some redox polymerizations involve a change in the termination step from the usual bimolecular reaction to monomolecular termination involving the reaction between the propagating radicals and a component of the redox system. This leads to kinetics that are appreciably different from those previously encountered. Thus, in the alcohol-Ce4+ system (Eq. 3-39), termination occurs according to... 

From this equation, it is clear that concentration of the solvent, S, influences a number of sites on the template which are occupied by the monomer, M. As the result of monomer units association with the template, the orientation of the substrate takes place and some special type of structure can be created. The structures, in which the monomer is aligned in a regular manner on the polymer template, were described by Chapiro in the case of polymerization of acrylic acid and acrylonitrile and details are described below. The ordered structure increases concentration of monomer at the reaction site, affects distances between pre-oriented monomer molecules, and changes a steric hindrance. This change in structure leads to the change in the kinetics of the polymerization reaction and it is responsible for stereo-control of the propagation step. 

Bryant has calculated the changes in free energy for various reaction steps of the polymerization of tetrafluoroethylene. He concluded (1) that the initiation and propagation are about twice as favorable for tetrafluoroethylene as the analogous reactions for ethylene, (2) that termination by combination is more favorable than disproportionation, and (3) that chain-transfer to monomer and to polymer are less likely than the combination of radicals. 

In the chain propagation reaction a 71-bond is changed into a 0-bond by addition of an adjacent monomer molecule to the intermediates. In this way in every reaction step about 0.9 eV are released. The resulting energy level scheme of the polymerization reaction is shown in Fig. 26. It represents the energetic positions of the resulting DR or AC intermediates characterized by the general notation M and the transient pair states M M and The addition reaction steps may be induced optically... 

With the continual discovery of new reactions leading to the formation of polymers, and with the increased understanding of the mechanisms of those reactions, it is natural that certain definitions of descriptive terms will change with time. The term polycondensation is used here to denote those polymerization reactions which proceed by a propagation mechanism in which an active polymerization site disappears every time one monomer equivalent reacts. This may be illustrated for one propagation step in polyesterification by... 

Figure 18. Degree of polymerization with time in response to step change in reaction temperature in controlled semibatch reactor. Key ------, WADP -------,...

These observations, added to the fact that addition of polar solvents to polymerization reactions of butadiene initiated by organolithium reagents in hydrocarbon solvents can change the stereochemistry from one of predominantly rtr-1,4 to that of 1,2 and trans-, , suggests that the lithium may form a tt complex with the olefin, and if this is sufficiently strong, orient the monomer prior to the incorporation step. 

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