11120
J. Phys. Chem. B 2007, 111, 11120-11126
Reversible Addition-Fragmentation Chain Transfer Polymerization of N-Isopropylacrylamide: A Comparison between a Conventional and a Fast Initiator Hassen Bouche´ kif and Ravin Narain* Department of Chemistry and Biochemistry, Biomolecular Sciences Program, Laurentian UniVersity, 935 Ramsey Lake Rd, Sudbury, ON, P3E 2C6, Canada ReceiVed: June 5, 2007; In Final Form: July 28, 2007
The reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide (NIPAM) was studied to determine the reasons for deviation of experimental molecular weights to lower molecular weight at high monomer conversion when S-1-dodecyl-S-(R,R′-dimethyl-R′′-acetic acid)trithiocarbonate (CTAm) and S,S-bis(R,R′-dimethyl-R′′-acetic acid)trithiocarbonate (CTAd) were used as RAFT agents at 65 °C. For this purpose, experiments were performed in N,N′-dimethylformamide (DMF) at the NIPAM/ CTA ratio of 200 with initiators capable of yielding fast and slow initiation, respectively by photochemical and thermal process, either at ambient temperature or at 65 °C. When the polymerization of NIPAM was conducted under these conditions with Irgacure-2959 (IRGC) as photoinitiator, a continuing supply of primary radicals by incremental initiator addition was required to achieve reasonably high conversion. This effect was also apparent by the loss of linearity of the first-order kinetic plot with a conventional initiator (4,4azobis(4-cyanovaleric acid) (ACVA) as azo-initiator, 10h (t1/2) decomposition at 65 °C) indicating that steadystate concentration of the macroradical decreases significantly with the initiator consumption. Nevertheless, polymers with predictable number-average molecular weight Mn (i.e., based on [monomer]/([CTA] + [initiator]) ratio) and narrow polydispersities were obtained (PDIs , 1.2) with CTAm indicating that the process of chain growth was controlled. When CTAd was used, instead of CTAm, the polymers obtained were characterized by a larger polydispersity (1.2 < PDIs < 1.3). The so-called “living steady-state concentration” in chain equilibration together with the linear dependence of Mn vs conversion was observed only when the 200/1 NIPAM/CTA mixture in DMF was subjected to a permanent photoirradiation at 65 °C. With ACVA, the deviation of the experimentally measured molecular weights at high conversion was accounted for by the simultaneous self-initiated polymerization of NIPAM with the controlled process in the presence of CTA at 65 °C. Similar drift from the linear dependence Mn vs conversion was also observed at 65 °C when a significant number of low molecular weight polymer chains were generated intentionally by photodecomposition of IRGC.
Introduction The reversible addition-fragmentation chain transfer (RAFT) process1 has emerged as an important radical polymerization technique for producing polymers of controlled architecture (e.g., block and star) for a wide range of monomer (e.g., Nisopropyacrylamide (NIPAM)2-16) and experimental conditions (e.g., water, organic solvent, and bulk). The mechanism given in the Scheme 1 relies on the chain transfer of growing radical, Pn•, to the RAFT agent, R-X, followed by the fragmentation of the intermediate, Pn-X•-R, to liberate the living group, R•, which then reinitiates the polymerization. Once the RAFT agent has been consumed, chain equilibration is established between active and dormant species and, in the presence of a suitable RAFT agent, polymers with predictable number average molecular weights (Mn) and narrow polydispersities (PDIs) 78%) when the RAFT polymerization of NIPAM in DMF, using trithiocarbonyl ester (CTAm and CTAd) in conjunction with ACVA, was carried out at 65 °C. Acknowledgment. The authors of this paper would like to thank Natural Sciences and Engineering Research Council (NSERC) and Ontario Centres of Excellence (OCE) Emerging Materials and Knowledge for financial support of this research.
Figure 7. Mn(GPC) (number-average molecular weight) (O, 9, 2) and Mw/Mn(GPC) (molecular weight distribution) (b, 0, ∆) vs nNIPAM/(nCTAm + nIRGC) × conversion dependences for the PNIPAM chain extension polymerization studies by incremental photoinitiator addition technique at 65 °C temperature in the presence of S-1-dodecyl-S-(R,R′-dimethylR′′-acetic acid)trithiocarbonate (CTAm). The initial concentrations were the following: [NIPAM]0 ) 2.95 M and [CTAm]0 ) 14.75 mM. For a time of photoirradiation of 7 min (0, 9, ∆, 2) and 30 min (b, O) at 350 nm (1.2 KW) the concentrations of IRGC were the following: S13, 495.6 (737) µg (µM) (first), 495.6 (763) µg (µM) (second), 264 (421) µg (µM) (third), 132 (218) µg (µM) (fourth), 264 (453) µg (µM) (fifth) (b, O); S11, 264 (393) µg (µM) (first), 264 (406) µg (µM) (second), 264(421) µg (µM) (third), 264 (436) µg (µM) (fourth), 396.5 (681) µg (µM) (fifth), 660 (1.178) µg (µM) (sixth) (∆, 2); S12, 660 (982) µg (µM) (first), 396.5 (610) µg (µM) (second), 395.6 (632) µg (µM) (third) (0, 9). For experimental details see Table 6 (series S11 and S12) in the Supporting Information. Targeted Mn ) 22 600 g/mol; (s) is the theoretical dependence of Mn vs nNIPAM/(nCTAm + nIRGC) × conversion calculated on the bases of moles of NIPAM converted into polymers for 100% CTAm and IRGC efficiency. (For GPC RI traces see Figure S6 (series S11) and Figure S9 (series S12) in the Supporting Information.)
as increment, Figure 7 series S12, yields 56.1% and 70.3% conversion, respectively, and the resulting polymer showed Mn(GPC) slightly inferior to Mn(th) (Mn(GPC) ) 12 875 g/mol and PDI ) 1.149 for Mn(th) ) 14 330 g/mol). A further addition of
Supporting Information Available: Tables and GPC RI traces of PNIPAM obtained in DMF; at different times using CTAm (series S1, Table 1, Figure S1) and CTAd (series S2, Table 2, Figure S2) as CTA agents in conjunction with ACVA with [NIPAM]0 ) 2.95 M, [CTA]0 ) 14.75 M, [ACVA]0 ) 2.95 mM, and T ) 65 °C; at different times using CTAm (series S3, Table 3, Figure S3) and CTAd (series S4, Table 3, Figure S4) as CTA agents in conjunction with IRGC with [NIPAM]0 ) 2.95 M, [CTA]0 ) 14.75 M, [ACVA]0 ) 2.95 mM, and T ) 65 °C; at different times without CTA agent (series S5, Table 4, Figure S5) with [NIPAM]0 ) 2.95 M and [ACVA]0 ) 2.95 mM, and T ) 65 °C; in incremental IRGC addition (series S6S9, Table 5, Figures S6-8) at ambient temperature with [NIPAM]0 ) 2.95 M and [CTAm]0 ) 14.75 M; in incremental IRGC addition (series S6-S9, Table 6, Figures S6-8) at 65 °C with [NIPAM]0 ) 2.95 M and [CTAm]0 ) 14.75 M. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Le, T. P. T.; Moad, G.; Rizzardo, E.; Thang, S. H. PCT Int. Appl. WO 98/01478 A1, 1998; Chem. Abstr. 1998, 128, 115390. Anderson, A. G.; Gridnev, A.; Moad, G.; Rizzardo, E.; Thang, S. H. PCT Int. Appl. WO 98/30601 A1, 1998. Chiefair, J.; Mayadunne, R. T. A.; Moad, G.; Rizzardo, E.; Thang, S. H. PCT Int. Appl. WO 99/31144 A1, 1999. Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, J.; Jeffery, J.; Le, T. P. T.; Mayadunne, R. T. A.; Meijs, G. F.; Moad, C. L.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1998, 31, 5559-5562. Rhodia chimie, ins.: Corpart, P.; Charmot, D.; Zard, S.; Frank, X.; Bouhadir, G. PCT. Int. Appl. WO 99/ 35177 A1, 1999. (2) Ganachaud, F.; Monteiro, M. J.; Gilbert R. G.; Dourges, M.-A.; Thang. S.; Rizzardo E. Macromolecules 2000, 33, 638. (3) Schilli, C. M.; Lanzendoerfer, M. G.; Muller; A. H. E. Macromolecules 2002, 35, 6819-682.
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