The phenylacetylene derivatives (4-decyloxyphenyl)acetylene (M1), (4-decyloxy-2-methylphenyl)acetylene (M2), and (4-decyloxy-2,6-dimethylphenyl)acetylene (M3) were polymerized by the well-defined Schrock-type initiator Mo[N-2,6-4-1-Pr2C6H3)(CHCMe2Ph)[OCMe(CF3)(2)](2) (I1) and by the ill-defined quaternary system MoOCl4-n-Bu4Sn-EtOH-quinuclidine (1:1:2:1) (I2). Comparison of the compatibility of the initiators with the different monomers revealed a correlation of the size of the ortho-substituents and the polymerizability of the monomers. M1 and M2 readily polymerized employing I1, but conversion of the sterically demanding monomer M3 remained incomplete. However, the use of I2 led to high monomer conversions and polymer yields only in case of M2 and M3. The steric bulkiness of the ortho-substituents also decisively affected the maximum effective conjugation length (N-eff) of the polymers and hence their absorption maximum (lambda(max)) as well as their solution stability as shown by UV-vis and GPC studies, respectively. (C) 2004 Wiley Periodicals, Inc.