Cyclization versus polymerization

Polycondensation chemistry results in the formation of both cyclic and linear oligomers [19]. In the case of ADMET polymerization, cyclic polymers or oligomers are formed through an intramolecular backbiting reaction between 

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From the preceding discussion, it can be seen that appropriate conditions must be maintained to maximize the 1 1 or 2 2 cyclization reactions. Cyclocondensations of two or four substrates (1 1 or 2 2 cyclizations) are favored in high dilution and with template cations. Often internal hydrogen bonding helps to favor the cyclic reaction product. A more complete discussion of cyclization versus polymerization in these polycondensation reactions in high-dilution conditions has been published (Fastrez, 1987). 

Another factor in step-growth polymerizations is cyclization versus linear polymerization.1516 Since ADMET is a step-growth polymerization, most reactions are carried out in the bulk using high concentrations of the reactant in order to suppress most cyclic formation. A small percentage of cyclic species is always present but is dependent upon thermodynamic factors, typical of any polycondensation reaction. 

CYCLIZATION VERSUS LINEAR POLYMERIZATION 2-5a Possible Cyclization Reactions... 

Polymerization versus cyclization varies as a function of concentration... 

Because the monomers themselves act as diluents in the crosslinking (co)polym-erization process and the monomers and (co)polymer concentration depend on the duration of polymerization, the question of how to find the critical conversion at which phase separation takes place (described as function phase equilibrium versus conversion) becomes very important. Apart from the effect of phase separation, the diluent produces other effects such as lowering the glass transition temperature and the reaction rate, promoting the cyclization reaction, and changing the conformation of chains when it is removed from the network. 

The general equation for determination of the equilibrium cyclization constants for the composed cyclics is in fact the one shown previously in Section 4.04.2 (eqn [24]), because the equilibrium in polymerizations of such monomers is the same as in copolymerization of corresponding simpler monomers (e.g., 1,3,5-trioxepane polymerization versus 1 1 copolymerization of 1,3-dioxolane and formaldehyde). 

Polymerization of a series of dienes of formula [106] has been studied (/i, 16, 93). The polymers that were obtained consisted of both soluble and cross-linked, insoluble fractions. Quantitative infrared and bromination experiments indicated that the soluble polymers possessed cyclic as well as open-chain structures. The percent of cyclization depended on the size of the incipient cycle and corresponded roughly to what is obtained in other cyclization reactions. The results are summarized in the accompanying table. A similar effect of ring size versus tendency toward cyclopolymerization was observed for monomers of structures [107] (92). 

Fig. 7.6 Average number degree of polymerization of the reaction mixture versus conversion for different cyclization rates P according to Eq. (7.14). Reproduced from Ref. [45, Fig. 3] with permission (Copyright Wiley-VCH Verlag GmbH Co, KGaA)...

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