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Important, yet unexplored effects of chemically distinct initiator fragments incorporated at chain ends in linear polymer are investigated in depth. Polystyrene (PS) samples of a wide range of molecular weight (MW) were synthesized by conventional free radical polymerization and controlled radical polymerization using seven different initiators and compared with anionically polymerized PS. The initiator fragments incorporated during polymerization have major consequences on the glass transition temperature (Tg) and dynamic fragility of low MW PS. For example, with ~4 kg/mol PS, the Tg onset value and fragility can be tuned from ~334 K and ~65, respectively, with dodecanethiol and hydrogen atom chain ends to ~367 K and ~130, respectively, with cyanopentanoic acid chain ends. A similar high Tg and high fragility were measured with isobutyric acid/SG1 nitroxide chain ends. These remarkable effects, with a greater than 30 K difference in Tg and a factor of 2 difference in fragility, indicate that chain ends in low MW PS homopolymer play an "outsize" role in comparison to comonomer units in perturbing properties that are sensitive to the density of chain ends. The Tg results also provide further direct evidence against any correlation between the MW at which the Tg-MW dependence saturates and entanglement MW. Instead, the perturbation of Tg by the combined effects of a reduction in MW (increase in chain-end density) and chain-end structure correlates one-to-one within error with the perturbation of fragility. These results suggest that the susceptibility of fragility to be perturbed is key to the susceptibility of Tg to be perturbed.

L. Zhang, J.A. Marsiglio, T. Lan, J.M. Torkelson, "Dramatic Tunability of the Glass Transition Temperature and Fragility of Low Molecular Weight Polystyrene by Initiator Fragments Located at Chain Ends," Macromolecules, 49, 2387-2398 (2016). [Link]