Cp2ZrHCl induced catalytic chain scission of diene-based polymers under mild conditions: Influence of chemical environment around C=C bonds

Controlled chain scissions of diene-based polymers, including cis-1,4-polybutadiene (cPB), cis-1,4-polyisoprene (cPIP), trans-1,4-polyisoprene (tPIP), 1,4-polychloroprene (PCP) and polycyclooctene (PCOE), were carried out under mild conditions via the hydrozirconation between C=C bonds and bis(cyclopentadienyl) zirconium hydrochloride (Cp2ZrHCl). The molecular weights (Mn) of the chain-scission products mainly depended on the loadings of Cp2ZrHCl and reaction time regardless of reaction media and temperature; however, cis/trans configuration ratio of C=C bonds in chain-scission products was influenced by reaction media, temperature and time. By means of NMR spectroscopy, chain-scission mechanism was confirmed to be β-alkyl elimination reaction. Deuterium (2H) tracing experiments showed that the hydrogenation of C=C bonds was a detrimental competing reaction to β-alkyl elimination. Furthermore, the hydrogenation was strongly influenced by steric and electronic effects around C=C bonds. However, chemical environments around C=C bonds did not affect chain-end functionalization of products, as chain-end alkylzirconium complexes generated from β-alkyl elimination were facilely quenched by electrophiles to obtain chain-end functionalized products. More importantly, under the assistance of hydrogen (H2), highly-efficient catalytic chain scission was achieved. Even more, cross-linked rubbers (such as cross-linked cPIP) could be also cleaved catalytically into soluble oligomers.