Ru-based metathesis catalysts employed in cyclo-polymerization (CP) of 1,6-heptadiyne derivatives have promoted regioselective alpha-addition to alkynes, forming various conjugated polyenes containing exclusively five-membered repeat units. Recently, we discovered that a new chelated Ru catalyst could promote regioselective beta-addition to produce analogous polyenes containing six-membered rings with moderate to good beta-selectivity. Since then, we have focused our research on pursuing more active and beta-selective regiocontrol to produce conjugated polymers with excellent beta-selectivity, with a much broader range of monomers. Herein, we demonstrate highly beta-selective CP by combining a new dithiolate-chelated Ru-based catalyst with weakly coordinating pyridine additives, which significantly enhance the conversion and beta-selectivity. An in-depth mechanistic investigation by H-1 NMR revealed a prominent role for the additives, which improve the stability of the propagating carbene.