RAFT Polymerization of <i>N</i>-Isopropylacrylamide in the Absence and Presence of Y(OTf)₃: Simultaneous Control of Molecular Weight and Tacticity

Abstract

The reversible addition−fragmentation chain transfer (RAFT) polymerization of N -isopropylacrylamide (NIPAM) was carried out successfully in the absence and presence of Lewis acid Y(OTf) 3 to synthesize controlled molecular weight atactic and isotactic poly(NIPAM)s using both 1-phenylethyl phenyldithioacetate (PEPD) and cumyl phenyldithioacetate (CPDT) as the RAFT agents. The polymerization rate is about 16 times faster in the presence of the Lewis acid than that in the absence and 1.4 times faster with PEPD than with CPDT as the RAFT agent. The polymer with a higher polydispersity was obtained when prepared in the presence of the Lewis acid than that in the absence. A longer induction period was observed using CPDT than PEPD. The chain-end structure of isotactic poly(NIPAM) was determined by 1 H NMR and MALDI−TOF mass spectrometry. The RAFT agent derived isotactic poly(NIPAM) was the main product as expected from the well-accepted mechanism of RAFT polymerization. Moreover, a series of stereoblock [atactic(a)- b -isotactic(i)] poly(NIPAM) with different block lengths were synthesized via a one-pot synthesis procedure: synthesis of the atactic block in the absence of the Lewis acid followed by the addition of the Lewis acid to synthesize the isotactic block. The longer is the isotactic block length, the higher is the meso dyad value of the stereoblock polymer as expected. We also successfully synthesized the diblock copolymers, a-poly(NIPAM)- b -polystyrene and i-poly(NIPAM)- b -polystyrene, starting with the atactic and isotactic poly(NIPAM) macro-RAFT agents, respectively.