Free Radical Polymerization - Monomers and Propagation

Monomers for Free Radical Polymerization

	Unsubstituted (ethylene) Works fine.
	Monosubstituted Works fine.
	1,1-Disubstituted Usually works.
	1,2-Disubstituted Seldom works.
	Trisubstituted Almost never works.
	Tetrasubstituted Almost never works.

The only exceptions to the unreactivity of tri- and tetra-substituted vinyl monomers are those with fluorine, like tetrafluoroethylene (CF₂=CF₂). The main cause of this reactivity pattern is the steric size of the substituents.

Problem with Alkyl Groups

Example for propylene:

Proplyene FR polymerization

The allyl radical (lower structure) is very stable, and forms preferentially. Attempted free radical polymerization of propylene fails.

But methyl methacrylate works fine:

MMA FR polymerization

A delicate balance is in effect here. Apparently, the resonance effect of the ester group tips the balance in favor of propagation versus hydrogen abstraction.

Mode of Addition

Monosubstituted and 1,1-disubstituted vinyl gruops present a choice to the incoming radical (either an initiator fragment or the propagating chain end).

Modes of addition

Attack almost always occurs at the least substituted carbon atom, mostly for steric reasons, but also because of the stabilizing effect of the substituent(s). The result is that head-to-tail addition predominates. The exceptions are small substituents like chlorine or especially fluorine.