Seven experimental criteria for living polymerizations are critically reviewed. The limitations of the use of linear plots of Mn (or xn) versus conversion and linear plots of the number of polymer chains versus conversion are illustrated by an example of alkyllithium-initiated polymerization of styrene by incremental additions of styrene with deliberate termination (5% per incremental monomer addition). The experimental criteria of using narrow molecular weight distribution and the formation of block copolymers are illustrated with practical examples and size exclusion chromatography data. The use of the terms 'living polymerization with reversible termination' and 'living polymerization with reversible chain transfer' are proposed. scope and limitations of the experimental criteria for living polymerization are critically reviewed. In particular, the limitations of the use of linear plots of number average molecular weight, A, (or X,, the number average degree of polymerization) versus conversion and plots of the number of polymer molecules versus conversion as critieria for living polymerization systems will be demonstrated.
EXPERIMENTAL
Chemicals and solventsStyrene, butadiene and benzene were carefully purified as described previously." sec-Butanol (Aldrich, 99Y0) was transferred into a graduated cylinder with a high-vacuum stopcock, degassed on the vacuum line and diluted with benzene. sec-Butyllithium (Lithium Corporation, 12 w t % in cyclohexane) was analyzed by the double titration method with 1,2-dibrom0ethane.~~
PolymerizationsAnionic polymerizations with .wc-butyllithium a s initiator were carried out in benzene solution a t 25°C in allglass, sealed reactors using breakseals or high-vacuum