Ring Forming Polymerization
Propagation reactions of unconjugated dienes can proceed by an intra-intermolecular process. This usually results in ring formation or in cyclopolymerization. It can be illustrated as follows:
where, X can designate either a carbon or a heteroatom. An example of such a polymerization is a free-radical polymerization of quaternary diethyldiallylamine [98]:
Another example is a polymerization of 2,6-disubstituted 1,6-heptadiene [103]:
The intra-intermolecular propagations can result in ring structures of various sizes. For instance, three-membered rings can form from transanular polymerizations of bicycloheptadiene [100, 101]:
Four-membered rings form in free-radical polymerization of perfluoro-1,4-pentadiene [103]. The size of the ring that forms depends mainly on the number of atoms between the double bonds:
Formation of many five-membered rings is also known. One example is a polymerization of 2,3 dicarboxymethyl-1,6 hexadiene [98]:
where, R=COOCH3. The polymer that forms, shown above, is cross-linked, but spectroscopic analysis shows that 90% of the monomer placement is through ring formation [104]. Formations of six-membered rings are also well documented. Two examples were shown above in the polymerization of a quaternary diethyldiallylamine and in the polymerization of 2,6 disubstitued, 1,6-heptadiene. Many other 1,6 heptadienes yield linear polymers containing six-membered rings [105]. This tendency to propagate intra-intermolecularly by the unconjugated dienes is greater than can be expected from purely statistical predictions [106]. Butler suggested that this results from interactions between the olefinic bonds [107–109]. Ultraviolet absorption spectra of several unconjugated diolefins does show bathochromic shifts in the absorption maxima relative to the values calculated from Woodward’s rule [102, 104]. This supports Butler’s explanation [107, 109]. A RAFT type of controlled/living polymerization (see Sect. 3.14.4) was applied to cyclopolymerization of t-butyl (hydroxyethyl) acrylate ether dimer [108]. The polymerizations, carried out in xylene at 70C, yielded polymers with six-membered tetrahydropyran repeat units:
