Propagation and Termination Reaction
The chain radical formed in the initiation step is capable of adding successive monomers to propagate the chain:
R-(CH2CHX-)CH2CHX• + CH2=CHX -----> R-(CH2CHX--)x+1CH2CHCHX•
Termination
Propagation would continue until the supply of monomer was exhausted were it not the strong tendency of radicals to react in pairs to form a paired-electron covalen bond with loss of radical activity. This tendency is compensated for in radical polymerization by the small concentration of radical species compared to monomers.
The termination step can take place in two ways, combination or coupling:
-CH2CHX• + •CHXCH2- -----> -CH2CHX-CHXCH2-
or disproportionation:
-CH2CHX• + •CHXCH2- -------> -CH2CH2X + CH2XCH2-
in which hydrogen transfer result in the formation of two molecules with one saturated and one unsaturated end group. Each type of termination is known. For example, studies of the number of initiative fragments per molecule showed that polystyrene terminates predominantly by combination, whereas poly(methyl methacrylate) terminates entirely by disproportionation at polymerization temperatures above 60oC, and partly by each mechanism at lower temperatures. The termination reaction, controlled by diffusion in many cases, has been reviewed.
R-(CH2CHX-)CH2CHX• + CH2=CHX -----> R-(CH2CHX--)x+1CH2CHCHX•
Termination
Propagation would continue until the supply of monomer was exhausted were it not the strong tendency of radicals to react in pairs to form a paired-electron covalen bond with loss of radical activity. This tendency is compensated for in radical polymerization by the small concentration of radical species compared to monomers.
The termination step can take place in two ways, combination or coupling:
-CH2CHX• + •CHXCH2- -----> -CH2CHX-CHXCH2-
or disproportionation:
-CH2CHX• + •CHXCH2- -------> -CH2CH2X + CH2XCH2-
in which hydrogen transfer result in the formation of two molecules with one saturated and one unsaturated end group. Each type of termination is known. For example, studies of the number of initiative fragments per molecule showed that polystyrene terminates predominantly by combination, whereas poly(methyl methacrylate) terminates entirely by disproportionation at polymerization temperatures above 60oC, and partly by each mechanism at lower temperatures. The termination reaction, controlled by diffusion in many cases, has been reviewed.
