Defects in the main chain of phenyl-substituted poly(p-phenylene vinylene)s (Ph-PPVs) can have a profound influence on the operational Lifetime of LEDs, as demonstrated here. The formation of these tolane-bisbenzyl defects (shown in the Figure) is rationalized by examining the mechanism of polymerization, and monomers are designed that yield polymers with fewer defects and hence enhanced performance (see also cover).