Thermally and photochemically initiated thiol-ene click reactions using thiol- and allyl- end functionalized linear polystyrenes with various enes (allyl bromide, methyl acrylate, and methyl methacrylate) and thiol (3-mercaptopropionic acid) have been investigated. Allyl- and thiol-end-capped polystyrenes with controlled molecular weight and low polydispersity were prepared by atom transfer radical polymerization (ATRP) of styrene using functional initiator and end group modification approaches, respectively. Thiol-ene reactions can be initiated by both cleavage type photoinitiators such as (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (TMDPO) and 2,2-dimethoxy-2-phenyl acetophenone (EIMPA) and H-abstraction type photoinitiators such as benzophenone (BP), thioxanthone (TX), camphorquinone (CO), and classical thermal initiator, 2,2'-azobis(isobutyronitrile) (AIBN) at 80 degrees C. The kinetics of the reactions was monitored online with a real time ATR-FTIR monitoring system and the conversions were determined by H-1 NMR spectroscopy. A comparison of click efficiencies of the studied initiator systems was performed. Compare to the thermal initiators and H-abstraction type photoinitiators, cleavage type photoinitiators were found to induce thiol-ene click reactions with higher efficiency.