Termination of oligomerization

It is a well-established characteristic of polymerization reactions that a monomer changes to a polymer via an activated intermediate.3 Chemical activation of a monomer requires (1) a redistribution of electron densities in the bonds of the monomeric molecule (the intramolecular effect) and/or (2) a change of properties of existing (loose) intermolecular bonds between the reacting sites (intermolecular effect an alteration of mutual orientation may suffice). For practical purposes, 

Woodward and R. Hoffmann, The Conservation of Orbital Symmetry. Academic Press, New York, 1970 (b) E. Heilbronner and H. Bock, Das HMO-Modell und seine Anwendung. Verlag Chemie, Weinheim, 1968. 

Marvel, An Introduction to the Organio Chemistry of High Polymers. Wiley, New York, 1959. 

High concentrations favor oligomerization, whereas at a lower concentration, an intramolecular reaction or depolymerization may compete with polymerization.4 In fact, many cases are known where a monomer and its oligomers exist in equilibrium under various conditions. 

Further discussion of the termination of oligomerization, especially for monomers having two (or more) functional groups, will be found in the next section. 

---

In this equation, Mp is the monomer concentration within forming particles, pa is the adsorption rate of oligomeric radicals by the forming particles, Vp is the volume fraction of forming particles within the system, and kp and k, are the rate constants of propagation and termination, respectively. 

The reaction of the monomeric metallo-ester with the monomer in THF allows the study of the initial stages of polymerization 50). The course of the reaction is followed by gas-chromatographic and GPC analyses of the terminated products. Oligomerization of the monomeric metallo-ester proceeds very fast. In a fraction of a second all the available monomer is consumed. In contrast, the disproportionation of the independently studied 67) dimeric metallo-esters, proceeds very slowly in a time scale of 10s s. Both reactions are described by the general scheme 67). 

The precursors for >C6 acids (namely, >C6 oi-dicarbonyls) are assumed to form by aldol condensation of ot-dicarbonyl compounds with other carbonyl compounds (route 7), and termination of this oligomerization is... 

Interestingly, one can use a Cys-Gly-Gly linker at the N- or C-terminal of the polypeptide chain in the design of disulfide-bridged coiled coils. The advantage of this approach is that the Cys-Gly-Gly linker allows complete flexibility of the polypeptide chains to adopt their most stable conformation, which includes different oligomerization states, while maintaining the polypeptide chains in a parallel manner. 49 In addition, the Cys-Gly-Gly linker eliminates the monomer-dimer equilibrium and the peptide concentration effect on stability, which is observed in two-stranded coiled-coil formation of noncovalent linked polypeptides. 49 861... 

Chojnowski and coworkers have examined the acid-catalysed disproportionation of oligomeric dimethylsiloxanols in dioxane in the presence of water2115. The results are interpreted in terms of a rate-determining cleavage of the terminal siloxane unit by water, followed by condensation of the resultant dimethylsilanediol (10) with the remaining dimethylsiloxanols (equations 7 and 8). 

---