J. Am. Chem. Soc. 1997, 119, 10987-10991
10987
Free Radical Initiation Mechanisms in the Polymerization of Methyl Methacrylate and Styrene with 1,1,3,3-Tetramethylbutyl Peroxypivalate: Addition of Neopentyl Radicals Tomoyuki Nakamura,†,‡ W. Ken Busfield,† Ian D. Jenkins,*,† Ezio Rizzardo,§ San H. Thang,§ and Shuji Suyama‡ Contribution from the Faculty of Science and Technology, Griffith UniVersity, Nathan, Queensland 4111, Australia, DiVision of Molecular Science, CSIRO, PriVate Bag 10, Clayton South MDC, Victoria 3169, Australia, and Fine Chemicals and Polymers Research Laboratory, NOF Corporation, Taketoyo-cho, Chita-gun, Aichi 470-23, Japan ReceiVed July 7, 1997X
Abstract: The reactions of 1,1,3,3-tetramethylbutyl (tert-octyl) peroxypivalate (1) with methyl methacrylate (MMA) and styrene in the presence of the free radical scavenger 1,1,3,3-tetramethyl-2,3-dihydro-1H-isoindol-2-yloxyl (2) have been studied at 60 °C. tert-Butyl and tert-octyloxyl radicals (3) were generated from the thermolysis of 1. The predominant unimolecular reactions of 3, that is, β-scission to form neopentyl radicals (14b) and a 1,5-H shift to form 4-hydroxy-2,2,4-trimethylpentyl radicals (14c), were observed in both monomer systems. The resulting alkyl radicals underwent selective addition to the two monomers. The relative reactivities of the alkyl radicals toward addition to the monomers were obtained from competitive addition/trapping reactions. The absolute rate constants for the addition of alkyl radicals 14b and 14c to the two monomers at 60 °C were estimated to be 9.5 × 105 and 2.6 × 105 M-1 s-1 to MMA and 4.5 × 105 and 0.7 × 105 M-1 s-1 to styrene, respectively. The low reactivities of 3 and 14c toward addition to MMA and styrene were attributed to steric effects. Steric effects were also responsible for the low rate of the 1,5-H shift in 3.
Introduction The work described in this paper is part of an ongoing investigation of the reaction of a combination of tert-alkoxyl radicals and alkyl radicals, generated by the thermolysis of tertalkyl peroxypivalates, with commercially important vinyl and acrylic monomers. Previous papers have described the reactions of tert-butyl,1,2 tert-pentyl,2,3 and tert-hexyl2-4 peroxypivalates with methyl methacrylate (MMA) and styrene; this paper reports the results of the reactions of 1,1,3,3-tetramethylbutyl (tert-octyl) peroxypivalate (1) with MMA and styrene. The radical trapping technique, employing 1,1,3,3-tetramethyl-2,3-dihydro-1H-isoindol-2-yloxyl (2) as a radical scavenger, has been employed as
described previously.1-5 tert-Octyl peroxyesters are known to be more reactive (their half-lives are shorter)6,7than tert-butyl, †
Griffith University. NOF Corp. § CSIRO. * Corresponding author. Telephone: (07) 3875 7476. FAX: (07) 3875 7656. E-mail:
[email protected]. X Abstract published in AdVance ACS Abstracts, October 15, 1997. (1) Nakamura, T.; Busfield, W. K.; Jenkins, I. D.; Rizzardo, E.; Thang, S. H.; Suyama, S. J. Am. Chem. Soc. 1996, 118, 10824. (2) Nakamura, T.; Busfield, W. K.; Jenkins, I. D.; Rizzardo, E.; Thang, S. H.; Suyama, S. J. Org. Chem. 1997, 62, 5578-5582. (3) Nakamura, T.; Busfield, W. K.; Jenkins, I. D.; Rizzardo, E.; Thang, S. H.; Suyama, S. Macromolecules 1997, 30, 2843-2847. (4) Nakamura, T.; Busfield, W. K.; Jenkins, I. D.; Rizzardo, E.; Thang, S. H.; Suyama, S. Macromolecules 1996, 29, 8975. ‡
S0002-7863(97)02240-3 CCC: $14.00
tert-pentyl, and tert-hexyl analogues, and they are widely used to initiate free radical polymerization of common monomers such as (meth)acrylates, styrene, vinyl chloride, and so on.8,9 tert-Alkyl peroxypivalates are known to undergo a concerted two-bond scission.10 In previous work,1-4 we have shown (i) that the thermolysis of tert-alkyl peroxypivalates is not affected by the presence of 2 and it generates an equimolar amount of tert-butyl and tert-alkoxyl radicals in the monomer and (ii) that tert-butyl radicals are immediately trapped by 2 to form alkoxyamine 4 or undergo competitive (tail) addition to MMA (or S) followed by trapping to give alkoxyamine 5 (or 6) (Scheme 1). (5) (a) Rizzardo, E.; Solomon, D. H. Polym. Bull. 1979, 1, 529-534. (b) Moad, G.; Solomon, D. H. In ComprehensiVe Polymer Science; Eastmond, G. C., Ed.; Pergamon: London, 1989; Vol. 3, pp 116-117. (c) Bottle, S. E.; Busfield, W. K.; Heiland, K.; Jenkins, I. D.; Meutermans, W.; Monteiro, M. In Progress in Pacific Polymer Science 3; Ghiggino, K. P., Ed.; Springer-Verlag Berlin: Heidelberg, 1994; pp 85-97. (d) Busfield, W. K.; Grice, I. D.; Jenkins, I. D. Aust. J. Chem. 1995, 48, 625-634. (e) Moad, G.; Solomon, D. H. In The Chemistry of Free Radical Polymerization; Pergamon: London, 1995; pp 120-122 and references contained therein. (6) The half-lives of 1, tert-butyl, tert-pentyl, and tert-hexyl peroxypivalates at 60 °C in cumene have been reported to be 3.1, 6.5, 5.5, and 5.7 h, respectively. Komai, T.; Matsuyama, K.; Matsushima, M. Bull. Chem. Soc. Jpn. 1988, 61, 1641-1646. (7) Stromberg, S. E. In Plastic Handbook; The staff of Modern Plastics Magazine, Ed.; Mcgraw-Hill: New York, 1994; pp 111-113. (8) Sheppard, C. S. In Encyclopedia of Polymer Science and Engineering, 2nd ed.; Klingsberg, A., Piccininni, R. M., Salvatore, A., Baldwin, T., Eds.; Wiley-Interscience: New York, 1988; Vol. 11, pp 1-21. (9) (a) Kato, M.; Abe, I.; Aoshima, K. Jpn. Kokai Tokkyo Koho, 86/ 51012; Chem. Abstr., 1986, 105, 61117. (b) Suyama, S.; Ishigaki, H. Jpn. Kokai Tokkyo Koho, 92/7301; Chem. Abstr., 1992, 116, 195108. (c) Suyama, S.; Ishigaki, H. Jpn. Kokai Tokkyo Koho, 92/213302; Chem. Abstr., 1993, 118, 81643. (d) Suyama, S.; Nakamura, T.; Ishigaki, H. Jpn. Kokai Tokkyo Koho, 4/292081. (10) (a) Bartlett, P. D.; Simons, D. M. J. Am. Chem. Soc. 1960, 82, 17531756. (b) Koenig, T.; Wolf, R. J. Am. Chem. Soc. 1967, 89, 2948-2952. (c) Lorand, J. P.; Chodroff, S. D.; Wallace, R. W. J. Am. Chem. Soc. 1968, 90, 5266-5267. (d) Pryor, W. A.; Morkved, E. H.; Bickley, H. T. J. Org. Chem. 1972, 37, 1999-2005.
© 1997 American Chemical Society
10988 J. Am. Chem. Soc., Vol. 119, No. 45, 1997 Scheme 1
Nakamura et al. Chart 1. Products in the Reactions of 1 with (a) MMA and (b) Styrene in the Presence of Nitroxide 2
Results Following the thermolysis of 1 (0.040 M) in the presence of trap 2 (0.040 M) in neat monomer at 60 °C in Vacuo for 1 h,11 most of the excess monomer was removed at reduced pressure and the residue was then analyzed by reverse-phase HPLC, HPLC-MS, and NMR. Alkoxyamines (the reaction products) were formed in the relative percentage yields shown in Chart 1, while the various reactions of tert-octyloxyl radicals (3) in monomer are outlined in Scheme 2. Alkoxyamines 7-9 and 12 arise from the direct reaction of 3 with monomers followed by trapping. As expected, in the reaction with MMA, the hydrogen-abstraction product 8 was obtained as well as the addition product 7 (eqs 1 and 2). Another H-abstraction product 9 was also detected in a trace amount (
