4,4â²-Azobis(4-cyanopentanoic acid) (ACPA) is a free radical initiator used in polymer synthesis. ACPA is a water-soluble initiator used in both heterogeneous and homogeneous free-radical polymerizations. It is used as an initiator in reversible additionâÂÂfragmentation chain transfer polymerization (RAFT). When heated to decomposition, c. 70 ðC, it releases N<sub>2</sub> and produces 2 equivalents of reactive radicals capable of initiating polymerization.