A method is described in which 14C‐labeled chain‐transfer agents are employed to measure chain‐transfer constants in anionic polymerization as low as 10−6. Each chain‐transfer step incorporates one molecule of the chain‐transfer agent into the polymer so that measurement of the activity and conversion allows evaluation of the chain‐transfer constant. This method is independent of the initiator concentration and efficiency, making the technique especially useful when problems with the initiator are encountered. The experimental procedure is described in detail for the case of chain transfer to toluene in the n‐butyllithium‐initiated polymerization of styrene, where CRH was found to be 5 × 10−6. A mathematical treatment is given showing the relationship between the degree of polymerization (DPn) and chain transfer.
Lithium-7 NMR spectroscopy was used to study the dilithium initiator (DLI) (1,3-phenylenebis-(3-methyl-l-[methylphenyl]pentylidene))dilithium (1). In hydrocarbon solution, 1 was found to exist predominantly as polyaggregated species, and all the aggregates exchanged rapidly on the NMR time scale at temperatures above Q O C . Addition of an equivalent per lithium of the tridentate ligand N,N,N',"',N"-pentamethyldiethylenetriamine (PMDETA) to 1 quantitatively converted the various polyaggregatss to a single new species which was assigned to be monomeric 1 with each lithium complexed by one PMDETA ligand. The early stages of isoprene polymerization initiated by both the monolithium initiator (MLI) (1,ldiphenyl-3-methylpenty1idene)lithium (2) and 1 were monitored by 'Li NMR. Several different poly-(isoprenyllithium) aggregates were visible in polyisoprene solutions initiated with 2, one of which exchanged much more rapidly with dimeric 2 than the others. Initiation with DLI 1 results in formation of a third type of species along with the two poly(is0prenyllithium) aggregates. This third species contains initiator living ends and is believed to be a mixed aggregate of both isoprenyllithium and initiator. Initiation by both 1 and 2 proceeds more rapidly than subsequent propagation. Under seeding reaction conditions, PMDETA has no obvious effect on the early stages of isoprene polymerization initiated by either 1 or 2.
IntroductionPreceding reports in this series1 on dilithium initiator (DLI) 1,3-phenylenebis(3-methyl-l-[methylphenyllpentylidene)dilithium (1) have demonstrated its effectiveness as initiator for anionic polymerization and utility in the preparation of triblock copolymers of styrene-butadiene-
Chromyl chloride in carbon tetrachloride solution reacts in the cold with cyclohexene, cyclopentene, and 1-hexene to give the various isomeric chlorohydrins as the major products. The major product from 2-methyl-1-pentene was 2-methylpentanal. Significant amounts of α-chloro carbonyl compounds and some α,β-unsaturated carbonyl compounds were also produced. The significance of these results is discussed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.