Streamlining the Synthesis of Macrocyclic Polystyrene by a Series of Atom Transfer Reactions

Polymers, which are everywhere, continue to be an area of study in both industry as well as in academia. Depending on the architecture, polymers can possess any number of different properties. Cyclic polymers possess particularly interesting properties owing to the unique continuous topology which lacks chain ends, but are historically challenging to make in high yields, where one way to synthesize macrocycles is by subjecting linear polymers to a ring closing reaction. In this work, monobrominated polystyrene (PSBr) was synthesized by atom transfer radical polymerization (ATRP). The PSBr precursor polymers were then employed directly in both an atom transfer radical coupling (ATRC) reaction and a radical trap-assisted atom transfer radical coupling (RTA-ATRC) reaction to afford polystyrene dimers. The radical trap 2-methyl-2-nitrosopropane (MNP) in the RTA-ATRC reaction proved to be essential to couple PS chains in high yields without continued polymerization of the unreacted monomer still present since the PSBr polymers were not purified prior to being used in the coupling reaction. The streamlined process was then extended to the formation of macrocycles in a RTA-ATRC reaction starting with dibrominated polystyrene (BrPSBr) precursors prepared by ATRP. Following polymerization, the ATRP phase was directly crossed over to a RTA-ATRC reaction in a one pot, two step sequence. The number of macrocycles increased as the total reaction volume was increased, where the G values in all cases were ~ 0.8, evidencing cyclic product. When 300 mL THF solvent was used, ~ 45 % cyclic product was formed in the coupling phase from the unpurified BrPSBr polymers. When the more active ligand tris[2-(dimethylamino)ethyl]amine (Me6TREN) was used in the RTA-ATRC phase of the reaction sequence in combination with pseudo-high dilution conditions, the total amount of cyclic product obtained increased to 75 %. To study the effects of solvent on the cyclization reaction, BrPSBr polymers with a molecular weight of ~ 6200 g/mol were prepared by ATRP and subsequently purified and isolated. The highest yield of cyclic product at ~ 65 % was achieved when a 50:50 ratio of the theta solvent cyclohexane and the good PS solvent THF was used in the cyclization reaction. When the same 50:50 solvent mixture was used with unpurified BrPSBr polymers with a molecular weight of ~ 2500 g/mol with all other reaction parameters the same as that which afforded 75 % cyclic product, the result was a significant decrease to only 45 % macrocyclic product formation