Cationic polymerization of α‐pinene with aluminium‐based binary catalysts, 2. Survey of catalyst systems

A series of metal halides (AlCl 3 , AlBr 3 , AlEtCl 2 , BF 3 · OEt 2 , SnCl 4 , TiCl 4 , and WCl 6 ) were employed in conjunction with antimony trichloride (SbCl 3 ) for the cationic polymerization of α‐pinene in toluene at −15°C. The combinations of aluminium halides (AlCl 3 , AlBr 3 , and AlEtCl 2 ) with SbCl 3 gave high molecular weight oligomers ( M̄ n ≥ 700) in good yield (>80 wt.‐%) with low dimer contents (5–17 wt.%). The other metal halides, both in the absence and presence of SbCl 3 , gave large amounts (20–40 wt.‐%) of dimers along with low molecular weight oligomers. 1 H NMR analysis of the obtained oligomers showed that they consist of two isomerized repeating units, one ( 1 ) unsaturated (with a cyclohexene ring) and the other ( 2 ) saturated (with a bornyl structure). The oligomers obtained with aluminium halides/SbCl 3 contained higher amounts of unsaturated units 1 than those with aluminium halides alone. The higher the molecular weight, the larger the content of 1 . Isomerization of α‐pinene slightly occurred in the course of the polymerization, and the structure of the isomers depended on the catalyst systems. These results show that the aluminium halide/SbCl 3 binary systems promote the ring‐opening of the cyclobutane unit in α‐pinene to give higher molecular weight oligomers.