End-biting

The ratio of back- and end-biting reactions depends on the reaction conditions, and may differ considerably in polymerizations of various monomers. In cationic polymerization, methyloxirane produces mainly a mixture of cyclic tetramers [335] and chloromethyloxirane yields both dimers and tetramers [336]. Under similar conditions, 1,3-dioxolane or 1,3-dioxepane yield a number of cyclic derivatives [337], the distribution of cyclics being in agreement with the Jacobson-Stockmayer theory [338],... 

The ratio of linear and cyclic macromolecules will be sharply affected by a change in the kinetics and mechanism of propagation, e. g. by transition from growth on activated chain ends to polymerization with activated monomer. An inactive macromolecule cannot produce macrocycles, neither by back- nor by end-biting reactions [339],... 

In the polymerization of 1,3-dioxolane initiated with protonic acid, another reaction pathway for cyclization is becoming possible [133]. Initiation with protonic acids introduces HO- head group, considerably more nucleophilic than acetal oxygens along the chain. Thus, end-to-end closure reaction (end-biting) proceeds extensively, especially at the early stages of reaction, when the distance between both ends of the macromolecule is still relatively short ... 

The presence of macrocycles of various size in heterochain polymers prepared by polycondensation or polymerization is a well known phenomenon This is due to reactions discussed in the preceding sections of this review and can be simply presented as a result of back-biting (including end-biting) reactions (the influence of end-biting will be discussed separately) ... 

In the polymerization of several cyclic acetals and ethers autoacceleration was observed. There might be a number of reasons for this behaviour, and some of these have already been discussed, namely the preinitiation equilibria and the inequality ki enhanced reactivity of the hydroxy end group in polyacetals toward active species (in comparison with acetal groups) is another, not yet considered, reason for induction periods. When the pol5mierization d ee increases with conversion the proportion of the active tertiary oxonium ions also increases, at the cost of the less reactive secondary oxonium ions (cf. Ref. 164), because the b k-biting or intermolec-ular transfer become more important than the end-biting. Thus, there b no need to make speculative assumptions about the nature of the active species and to propose the two stage polymerization of acetals in order to explain the induction... 

Termination and Transfer Processes Macrocyclization. End-Biting and Back-Biting... 

The importance of simultaneous end-biting and back-biting, according to the neral mechanism of propagation accompanied by transfer due to cyclization ... 

Dimers, trimers and tetramers form either separately or as members of a continuous spectrum of cyclic oligomers. These products are formed either by end-to-end cyclization (end-biting) or by back-biting reactions involving tertiary onium ions as discussed further in this section ... 

Fig. 3.4. Schematic representation of a change of concentration of a given cyclic oligomer for backbiting (lower curve) and end-biting (upper curve) schemes during polymerization. Concentration...

The last three examples in Table 7.1 are concerned with the distributions of some cyclic sulfides formed in polycondensation reactions of dithiols with dibromides. The FAB spectra have a sufficient number of peaks, and therefore the distribuhon laws can be derived with some confidence. Poly(tri-methylene sulfide) and poly(m-phenylene sulfide) appear to yield mixtures of cyclic oligomers corresponding to JS equilibrium conditions. However, MS data for poly(hexame ylene sulfide) show a better fit for the n distribution law, indicating the possible control of an end-biting reaction. ... 

A special case of kinetic enhancement in macrocycles is observed when a zwitterionic polymerization of lactide was carried out. Waymouth and co-workers have observed formation of cyclic polymer of up to 3 x 10" and dispersity below 1.3 when polymerization was initiated by heterocyclic carbene (l,3-dimesitylimidazol-2-ylidene). It stems probably from propagation being much faster than cyclization and end-biting being much faster than backbiting. The latter can be explained by the reaction of oppositely charged chain-ends, located close to each other more frequently than can be expected for neutral chains (Scheme 12). 

On the other hand, it has been found over the past 20 years that the highest molar masses of polycondensates reported in the literature were obtained from non-stoichiometric 02 + 2 polycondensations. This discrepancy results firom the fact in Flory s theory (and thus in all textbooks based on it) the role of end-biting cyclization is ignored. Polycondensations of a-b monomers exclusively yield oligomers and polymers terminated by one a and on b function, an optimum scenario for efficient cyclization, unless the growing chains are extremely stiff, but soluble or meltable. In the case of an 2 + 2 polycondensation only 50 % of aU... 

The predominance of diol terminated linear chains reduces the influence of end-biting , but the formation of cyclics by back-biting cannot be suppressed. The role of ring-chain equilibria and the extraction and characterization of cyclic oligoesters is discussed in Chap. 5. However, it was also demonstrated by means of MALDI-TOF mass spectrometry that high molar mass PBT contains cyclic polymers with masses at least up to 12 kDa and not only cyclic oligomers [8]. 

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