Propagation structural isomerism

In this connection, a recently proposed theory (16) to explain the effect of lithium concentration on polyisoprene chain structure deserves mention. This theory is based on a proposed competition between the rates of chain propagation and isomerization of the chain end, which presumably changes from the cis-1,4 to the trans-1,4 configuration. Although this theory may have some merit, it cannot account for the resiilts demonstrated in Tables II, III and V above, i.e., the absence of any effect of temperature or degree of conversion, both of which would strongly affect the propagation rate, but would not be expected to influence the chain-end isomerization rate. It is far more likely, therefore, that the effects on chain structure described above are due to subtle effects of these reaction parameters on the structure and reactivity of the carbon-lithium bond complex at the active chain end. 

Thus propagation must be much faster than isomerization, and the product will be determined by thermodynamics, rather than by reaction kinetics. The net results of the two processes may be quite similar, however, in that polymers of unexpected structures may be obtained, and copolymers may be prepared by polymerization of a single monomer. 

This means that the isopropyl group stabilises the secondary ion sufficiently (compared to but-l-ene) for hydride transfer reactions to be suppressed, at least at low temperature. At about -120 °C a high polymer of structure (IV) is formed. This is a true phantom polymer, since there exists no corresponding monomer. Evidently, at the very low temperature the propagation reaction which would lead to structure (III) becomes much slower than the isomerization reaction ... 

A careful analysis based on these experimental results excluded a chain-propagation process [33a]. On account of the 3-position of the methylthio or methoxy substituent in the thiophene or pyrrole rings, three isomeric dimers may be formed. The main reaction path can be deduced from the mesomeric forms of the radical cations (2)". The two most important mesomeric structures are those with the unpaired electron in... 

Whereas the cationic polymerization of furfurylidene acetone 3a engenders crosslinked structures (25), the use of anionic initiators results in linear structures (26). However, the propagation is preceded by an isomerization of the active species which eliminates the steric hindrance to propagation arising from the 1,2-disubstitution in the monomer structure. A proton shift from the 4- to the 2-position places the negative charge at the extremity of the monomer unit and the incoming monomer can add onto this anion without major restrictions. The polymer structure thus obtained is ... 

Isomerization polymerizations are polyaddition reactions where the propagating species rearranges to energetically preferred structures prior to subsequent chain growth. 

The results of the study of the effect of synthesis conditions on the composition of poly(4-methyl-l-pentene) have shown that even under conditions most favorable for the successful competition of isomerization with propagation, i.e., —120° C, using EtAlCl2, in ethyl chloride, the polymer contains only 50% of the desired 1,4-structure. It appears that in the series (n— l)-methyl-l-alkenes as n increases the likelihood of obtaining completely isomerized products via cationic isomerization polymerization is decreased. This is supported qualitatively by results obtained in the cationic polymerization of 4-methyl-l-hexene, an (n—2)-methyl-1-alkene (17). 

Generally, metallocenes favor consecutive primary insertions as a consequence of their bent sandwich structures. Secondary insertion also occurs to an extent determined by the structure of the metallocene and the experimental conditions (especially temperature and monomer concentration). Secondary insertions cause an increased steric hindrance to the next primary insertion. The active center is blocked and therefore regarded as a resting state of the catalyst (138). The kinetic hindrance of chain propagation by another insertion favors chain termination and isomerization processes. One of the isomerization processes observed in metallocene-catalyzed polymerization of propylene leads to the formation of 1,3-enchained monomer units (Fig. 14) (139-142). The mechanism originally proposed to be of an elimination-isomerization-addition type is now thought to involve transition metal-mediated hydride shifts (143,144). 

In contrast, radical polymerization of 2,3-benzo-7-methylene-l,4,6,9-tetraoxa-spiro[4.4]nona-2-ene (138) gave only a vinyl polymer (139) with no occurrence of ring-opening isomerization, even in the solution polymerization at 165 °C. [105] It appears that the propagating radical having a spiro orthocarbonate structure is too stable to liberate the corresponding carbonyl compound, phenylene carbonate. 

The isomerized structure dominates even at - 100° C, but accounts for only 70% of the repeat units at - 130° C. Similar but more complicated structures are formed in 4-methyl-1-butene polymerizations by competing hydride and methide shifts [298]. Other monomers whose propagating carbenium ions isomerize include 5-methyl-l-hexene, 4,4-dimethyl-1-pen-tene and some terpenes [299]. 

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