Enhanced glass transition temperature of low molecular weight poly(methyl methacrylate) by initiator fragments located at chain ends

We discovered important effects of chemically distinct initiator fragments incorporated at chain ends on the glass transition temperature (T_g) of poly(methyl methacrylate) (PMMA). Polymers of various molecular weight (MW) were synthesized via free radical polymerization (FRP) using 2,2’-azobis(2-methylpropionitrile) (AIBN) and benzoyl peroxide (BPO) as initiator. All samples have identical triad distribution with ∼58% syndiotactic content. The T_g values of ∼3 kg/mol PMMA possessing AIBN and BPO initiator fragments are 11–13 K higher than that of an anionic PMMA standard of similar MW. The elevated T_g values in low MW PMMA synthesized by FRP are attributed to attractive interactions arising from polar chain ends. The T_g-MW dependence becomes weaker at the lowest MWs examined in PMMA samples synthesized by FRP (with AIBN or BPO as initiator) and by anionic polymerization. The methodology of incorporating polar chain ends to increase the T_g of low MW polymers provides new avenues for material design.