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

Abstract

SynopsisThe anionic polymerization of 1.3-cyclohexadiene (1.3-CHD) was investigated in temperatures that ranged from 25 to -77OC. Initiation by lithium naphthalene (N;,Li+) in tetrahydrofuran at -2OOC yields polymers with fairly narrow molecular weight distribution. The of t.hese polymers so prepared is ca. 20,000. Polymerization of 1.3-CHD conducted a t room temperature is accompanied by the dehydrogenation and disproportionation of the monomer, especially when NT,K+ acts as initiator.The mechanism of the initiation of the polymerization of 1.3-CHD by N:,Li+ was elucidated and the rate constants at -2OOC 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 ( A , , ,-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.Oligomers are formed when hexamethylphosphoramide is used as a solvent.

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“…However, in contrast to conventional dienes (e.g., butadiene, isoprene), the polymerization of 1,3‐cyclohexadiene (1,3‐CHD) has sometimes been reported to be difficult under various conditions, including ionic polymerization, radical polymerization, and coordination polymerization. The polymers obtained under these conditions were of low molecular weight or in low yield, and the molar ratio of 1,2‐addition (the 1,2‐CHD unit) and 1,4‐addition (the 1,4‐CHD unit) of the polymer chain could not be controlled 1–5, 31–36…”

Salt‐water recycling for brine production at road‐salt‐storage facilities

“…Side reaction 1 is chain termination via hydride abstraction, and 2 is chain transfer to monomer through proton abstraction. Side reaction 2 is producing CHDLi, which can reinitiate the polymerization, can be transformed into benzene through hydride abstraction, or can be isomerized into 1,4‐cyclohexadiene 24. DABCO is a polar additive that accelerates the polymerization and so minimizes the side reactions, and thus leading to polymers with controllable molecular weights and low polydispersity indices.…”

“…PCHE has the highest glass‐transition temperature ( T g ) among all hydrocarbon polymers. It has also low specific gravity, high heat resistance, high flexural modulus, high transparency, low dielectric constant, low water absorption, good UV resistance, and a refractive index similar to that of glass 24…”

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“…Control of molecular weight, molecular (13) 4 + n (14) weight distribution, chain-end functionality, and the ability to prepare block copolymers were reported; however, the block cop'Jlymers have somewhat broadened molecular weight distributions. McGrath and coworkers have improved on these methods, especially for preparation of polymers from higher molecular weight alkyl methacrylates and for block copolymer synthesis by developing an efficient monomer purification method using trialkylaluminum 17 reagents.…”