Kinetics and mechanism of the anionic polymerization of cyclohexadienes initiated by naphthalene radical anions and dianions

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Abstract
The anionic polymerization of 1.3‐cyclohexadiene (1.3‐CHD) was investigated in temperatures that ranged from 25 to −77°C. Initiation by lithium naphthalene (N−·,Li+) in tetrahydrofuran at −20°C yields polymers with fairly narrow molecular weight distribution. The M̄w of these polymers so prepared is ca. 20,000. Polymerization of 1.3‐CHD conducted at room temperature is accompanied by the dehydrogenation and disproportionation of the monomer, especially when N−·,K+ acts as initiator. Oligomers are formed when hexamethylphosphoramide is used as a solvent. The mechanism of the initiation of the polymerization of 1.3‐CHD by N−·,Li+ was elucidated and the rate constants at −20°C in tetrahydrofuran of the elementary reactions were determined. It was established that the dianions formed by disproportionation of N−·,Li+ act as effective initiators for 1.3‐CHD. The adducts formed constitute the cyclohexanyl and naphthyl carbanionic groups. The former carbanions (λmax ∼ 275 nm) propagate the polymerization. The initially formed dimeric adducts are stabilized by the separation of the carbanionic end groups by the additional monomer units. Chain transfer to the monomer limits the growth of the polymers. The isomerization of the cyclohexadienyl anions, formed as result of chain transfer, may be followed by the elimination of lithium hydride. The latter reaction represents a termination step. Addition of 1.4‐CHD to the reaction mixture enhances the chain transfer and the termination.