Chain transfer reaction to terminal olefins in the polymerization of 1‐phenyl‐1‐propyne with Nb and Ta catalysts

<jats:title>Abstract</jats:title><jats:p>Effects of various olefins as chain transfer agents were studied in the polymerization of 1‐phenyl‐1‐propyne catalyzed by NbCl<jats:sub>5</jats:sub>—Bu<jats:sub>4</jats:sub>Sn (1:2) (Bu: butyl) and TaCl<jats:sub>5</jats:sub>—Bu<jats:sub>4</jats:sub>Sn (1:2). In the Nb‐catalyzed polymerizations, Si‐containing terminal olefins, especially butyldimethylvinylsilane and dimethylphenylvinylsilane, proved to greatly lower the polymer molecular weight; e.g., the number‐average molecular weight (<jats:italic>M̄</jats:italic><jats:sub>n</jats:sub>) of the polymer formed without an olefin was 2,0 · 10<jats:sup>5</jats:sup>, whereas it decreased to 7 · 10<jats:sup>3</jats:sup> − 1 · 10<jats:sup>4</jats:sup>, i.e., below 1/20 in the presence of the Si‐containing olefins (1/5 equivalent to the monomer). In the Ta‐catalyzed polymerization, in contrast, hydrocarbon terminal olefins like 1‐octene were effective; e.g., while the <jats:italic>M̄</jats:italic><jats:sub>n</jats:sub> of the polymer reached 4,0 · 10<jats:sup>5</jats:sup> without an olefin, the <jats:italic>M̄</jats:italic><jats:sub>n</jats:sub> was no more than 9 · 10<jats:sup>3</jats:sup> − 2 · 10<jats:sup>4</jats:sup>, i.e., smaller than 1/25 in the presence of olefins. Poly(1‐phenyl‐1‐propyne) obtained using the Nb catalyst in the presence of butyldimethylvinylsilane possesses the terminal butyldimethylsilyl group derived from the added olefin. Thus, suitable olefins are useful to control the molecular weight of poly(1‐phenyl‐1‐propyne) by chain transfer, which verifies the metal‐carbene mechanism for the polymerization of 1‐phenyl‐1‐propyne by group 5 transition metal catalysts.</jats:p>