acrylic light pipe as described in bigley et al ., u . s . pat . nos . 5 , 406 , 641 and 5 , 485 , 541 , has adequate thermal stability for many purposes . it has been found that improved thermal stability , as reflected in color formation , can be imparted by adjusting the polymerization conditions to produce the uncured core polymer of the core / clad construction with a much reduced terminal vinyl content , preferably below 0 . 5 vinyl groups / 1000 monomer units .