ISSN:
0025-116X
Keywords:
Chemistry
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
Enantioasymmetry was studied for polymerizations of methylthiirane initiated by bis(isopropyl-S-cysteinato)cadmium and run in bulk. The ratio of rates of consumption of the two antipodes is independent of their concentrations. The enantioasymmetric constant kR, measuring the preferential choice of the R-antipode, is depending on both the relative number of the two types of active sites and on their reactivities. An effect on enantioasymmetry of the molecular weight of the growing chains bearing the active sites, is occurring with two distinct stages: The first one is observed, with an enantioasymmetric constant kR = 1,5, from the appearance of stereospecificity up to molecular weights of 35000 (for the main chain) and the second one with kR = 1,07 for molecular weights higher than 35000. In the first stage, the cysteinato end group of the oligomer chain remains linked to cadmium (by cadmium-nitrogen complexation) and the two active sites are diastereoisomers having different reactivities. In the second stage, the macrocyclic oligomer becomes too large and cadmium-nitrogen interaction is suppressed. The active sites are now enantiomers having the same reactivity. In this case, enantioasymmetry depends only on the unbalance between the amounts of the two types of active sites. The maximum size the macrocyclic oligomer can reach depends on the initiator concentration.
Additional Material:
1 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/macp.1993.021940323
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