RAFT Polymerization of N-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 1H 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.

Living Radical Polymerization by the RAFT Process

“…RAFT polymerization can be conducted in the presence of Lewis acids.There are reports of attempted tacticity control of homopolymers [91,[168][169][170] (to enable the synthesis of stereoblock copolymers [171] ) and control of the alternating tendency for copolymerizations [172,173] through the use of Lewis acids as additives. For MMA polymerization, the addition of the Lewis acid scandium triflate, Sc(OTf) 3 , increases the fraction of isotactic triads and enhances the rate of polymerization in conventional radical [174] and RAFT processes.…”

Polypeptide Diblock Copolymers: Syntheses and Properties of Poly(N-isopropylacrylamide)-b-Polylysine

“…Below this critical temperature, it is in a hydrophilic state, and above this temperature it is in a hydrophobic state. 22 All the major CRP techniques, atom transfer radical polymerization (ATRP), 23 nitroxide-mediated polymerization (NMP), 24 and reversible addition-fragmentation chain transfer polymerization (RAFT), 25 have been successfully used to prepare poly(N-isopropylacrylamide) with low-to-moderate chain length and narrow molecular weight distribution.…”

Well-defined poly(N-isopropylacrylamide) with a bifunctional end-group: synthesis, characterization, and thermoresponsive properties

“…As a result, an ever-increasing interest has been focused on the phase transition behavior of PNIPAM in aqueous solutions during the past decades. Many efforts have been devoted to providing insights into the influence of the factors regarding concentration [6,10], cosolvents [11][12][13], molecular weight, polydispersity [14][15][16][17], and tacticity [18][19][20] as well as end-group [21][22][23][24][25][26] on the thermoresponsive properties of PNIPAM.…”