Chem 421: Introduction to Polymer Chemistry

Free Radical Vinyl Chain Polymerization

Equivalent views of a carbon-carbon double bond:

The essence of chain polymerizations:

Each addition reproduces the reactive group. (True for every kind of chain polymerization, not just free radical.)

Three components of chain polymerizations:

• Initiation
• Propagation
• Termination

All chain polymerizations (of any kind) have at least the first two components; most have all three. The exceptions are living polymerizations that lack termination.

Thermodynamics (General for All Chain Polymerizations)

Vinyl polymerizations work in general because converting a double bond into two single bonds is exothermic. However, in more highly substituted systems, there is steric crowding in the polymer chains, and this decreases the driving force. (See the table below).

As temperature increases, at some point the entropy contribution overwhelms the enthalpy contribution. Above this temperature, depolymerization is faster than polymerzation. The polymer "unzips" back to monomer. The temperature at which this occurs is known as the ceiling temperature, T_c. The value of T_c varies widely with structure.

	Enthalpy (kcal/mol)	Tc (°C)
	-93	400
	-73	310
	-35	61
	-163	1000

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