The present invention relates to an initiator injection nozzle (1) for mixing an initiator with a process fluid. The initiator injection nozzle includes a body (10) having (i) a process fluid inlet port (11) to receive the process fluid and an outlet port (17); (ii) a process fluid flow passage (13) through which the process fluid traverses along a process fluid central flow axis (15) between the process fluid inlet port (11) and the outlet port (17), the process fluid flow passage 13 having a constricting portion (12), a throat (14) and an expanding portion (16); (iii) an initiator inlet (30) to receive the initiator and an initiator outlet (32); and (iv) an initiator fluid flow passage (34) through which the initiator traverses along an initiator central flow axis (35) between the initiator (inlet 30) and the initiator outlet (32), the process fluid flow passage (13) intersecting the initiator fluid flow passage (34) at the throat (14). At least one elongated vane (20) having a leading end (22) and a trailing end (26) is provided within the process fluid flow passage (13) and extends along the process fluid central flow axis (15), wherein the initiator outlet (32) is disposed on a surface (24) of the vane (20). The initiator injection nozzle has the initiator outlet (32) is disposed on a portion of an axial edge (24) of the vane (20).