Structural control in ATRP synthesis of star polymers using the arm-first method

Star polymers were synthesized via atom transfer radical polymerization (ATRP) using the "arm-first" method. First, linear poly(tert-butyl acrylate) (polytBA) precursors were prepared, and subsequently divinylbenzene (DVB) was added as cross-linker at certain tBA conversion. Several experimental parameters, including the polytBA arm length, the amount of cross-linker, the monomer conversion at which DVB was added, and the chemical composition of the star core (fraction of tBA units in the star core), were systematically varied. For example, the degree of polymerization of the polytBA arm was typically 40 and was varied from 20 to 90. The molar ratio of DVB to the macroinitiator was varied from 1.5 to 15, and DVB was added at 60%, 80%, and 90% tBA conversions. The influence of these factors on the structures of the star polymers was studied. Employing shorter arm lengths and using more cross-linker produced star polymers in higher yield, with higher molecular weight, with more arms per star, and with a more compact structure. The addition of DVB at lower tBA conversion caused the incorporation of more monovinyl monomer into the star core, which decreased the cross-link density of the core, facilitated incorporation of more arms into each star molecule, and increased the star size and star yield.