Beziehungen zwischen Struktur und Polymerisierbarkeit von Monomeren

Abstract

The factors which determin the polymerisability of monomers are reviewed and discussed. Except for the case of 1,2-disubstituted ethylene derivatives nonpolymerisability is always due to an unfavorable position of the polymerisation equilibrium. The free energy of the ring opening polymerisations is chiefly determined by the ring strain in the monomer and the repulsion of hydrogene atoms in the monomer and polymer. Polymerisations of carbon heteroatom double bonds are only feasible when the bond energy of the formed σ-bond is higher than the energy of the opened π-bond, and when the linear polymer is more stable than the corresponding cyclic trimer and tetramer. A high bond energy difference favors the polymerisation of carbon double bonds. This is reflected by the ease with which polymerisations of vinyl compounds take place. Steric strain in the formed polymer lowers the ceiling temperature of 1,1-disubstituted ethylene derivatives, some of them being no more polymerisable. The inability of the 1,2-substituted isomers for the formation of high polymers is the result of the slowness of propagation due to low reactivity of the double bond and to steric hindrance in the transition state. Alkyl groups as β-substituents give rise to strong chain transfer. The lack of polymerisability found with tri- and tetrasubstituted ethylenes is understood in a similar manner. The exceptional behavior of fluorine as a substituent results from its smallness and its high elektronegativity.