Initiation mechanisms in radical polymerizations : Reaction of cumyloxy radicals with methyl...

Aust. J. Chem., 1982,35,2013-24

Initiation Mechanisms in Radical Polymerizations: Reaction of Cumyloxy Radicals with Methyl Methacrylate and Styrene

Ezio Rizzardo, Algirdas K. Serelis and David H. Solomon

Division of Applied Organic Chemistry, CSIRO, P.O. Box 4331, Melbourne, Vic. 3001.

Abstract

Cumyloxy (1-methyl-1-phenylethoxy) radicals have been generated by thermolysis (60") of dicumyl hyponitrite in methyl methacrylate and styrene. The carbon-centred radicals formed by interaction of cumyloxyl with the respective monomers were trapped as stable adducts of 1,1,3,3-tetramethylisoindolin-2-yloxyl. Extensive hydrogen atom abstraction and methyl radical generation as well as double-bond addition were observed in methyl methacrylate. Styrene underwent only double-bond addition by both cumyloxy and methyl radicals. Some possible implications of these results for polymer structure are discussed. A kinetic study of the decomposition of dicumyl hyponitrite in cyclohexane at various temperatures gave

for the rate constant. Rate constants for the addition of cumyloxyl to methyl methacrylate (k % 2 x lo4 dm3 mol-I s-') and styrene (k % 2 x lo5 dm3 mol-' s-') at 60" have been estimated.

Introduction

Recent work in these laboratories1-5 and elsewhere6-'' has demonstrated the existence of initiation pathways for radical polymerization other than the 'text-book' tail addition of initiator radical to monomer." The major successes in this area have been achieved through the use of radical-trapping reagents'-6* which capture the

* For general reviews, see refs 9,12.

Rizzardo, E., and Solomon, D. H., Polym. BUN., 1979, 1, 529. Moad, G., Rizzardo, E., and Solomon, D. H., J. Macromol. Sci., Chem., 1982, 17, 51. Griffiths, P. G., Rizzardo, E., and Solomon, D. H., J. Macromol. Sci., Chem., 1982, 17, 45. Griffiths, P. G., Rizzardo, E., and Solomon, D. H., Tetrahedron Lett., 1982, 23, 1309. Moad, G., Rizzardo, E., and Solomon, D. H., Macromolecules, 1982, 15, 909. Sato, T., and Otsu, T., Makromol. Chem., 1977, 178, 1941. Otsu, T., Tanaka, H., and Quach, L,, Makromol. Chem., 1980, 181, 1897. Kunitake, T., and Murakami, S., J. Polym. Sci., Polym. Chem. Ed., 1974, 12, 67. Perkins, M. J., Ado. Phys. Org. Chem., 1980, 17, 1.

lo Encina, M. V., Rivera, M., and Lissi, E. A., J. Polym. Sci., Polym. Chem. Ed., 1978, 16, 1709. 'l For example, Morrison, R. T., and Boyd, R. N., 'Organic Chemistry' 3rd Edn, pp. 206, 1032 (Allyn & Bacon: Boston 1973). l 2 Evans, C. A., Aldrichimica Acta, 1979, 12, 23; Janzen, E. G., Evans, C. A., and Davis, E. R., in 'Organic Free Radicals' (ACS Symposium Series 69) (Ed. W. A. Pryor) p. 433 (American Chemical Society: Washington 1978).

E. Rizzardo, A. K. Serelis and D. H. Solomon

carbon-centred radicals formed by reaction of an initiating alkoxy radical with monomer before any significant amount of propagation can occur. The C-nitroso spin traps most commonly e m p l ~ y e d ~ - ~ suffer from some severe limitations : nitroxide radical adducts of varying stability are formed, and these can only be examined by e.s.r. spectroscopy. The adducts cannot be separated, but must be analysed as mixtures, thus giving rise to overlapping e.s.r. spectra in which signals from minor components may be lost. Furthermore, e.s.r. spectra are seldom unambiguous, even for single radicals, and can only give information about the structure in the immediate vicinity of the unpaired electron.

The use of stable nitroxide radicals as radical traps,1-5,'3 on the other hand, leads to stable, non-radical adducts (alkoxy amines) which can be easily manipulated and investigated by use of the full range of spectroscopic techniques. The radical-trapping reagent 1,1,3,3-tetramethylisoindolin-2-yloxyl (1) which was developed in these l a b ~ r a t o r i e s ~ , ~ , ' ~ has proved particularly versatile in that it provides adducts which are usually crystalline, stable, and easily characterized spectroscopically. In addition, the u.v.-active chromophore permits ready detection and quantification of its adducts by h.p.1.c. analysis. Thus, for example, the reactions of t-butoxyl with methyl meth- ahylate were shown by radical-trapping experiments with isoindolinyloxyl (I) to include hydrogen atom abstraction from the ester methyl group as well as p-scission to, and subsequent reactions of, methyl r a d i ~ a l . ~ , ~ These pathways had escaped detection in the earlier work which relied on e.s.r. for product ana ly~ i s .~

The successful utilization of isoindolinyloxyl (I) in the study of the interaction of t - b u t o ~ y ~ - ~ and benzoyloxy5 radicals with typical vinyl and acrylic monomers prompted an extension of this investigation to other radical species which have commercial significance as polymerization initiators. This communication reports our results from the reaction of cumyloxy (I-methyl-1-phenylethoxy) radicals (2) with methyl methacrylate and styrene in the presence of isoindolinyloxyl (1).

l 3 Neiman, M. B., and Rozantsev, E. G., Bull. Acad. Sci. USSR, Dit.. Chem. Sci., 1964, 1095; Murayama, K., Morimura, S., and Yoshioka, T., Bull. Chem. Soc. Jpn, 1969, 42, 1640; Willson, R. L., Trans. Faraday Soc., 1971, 67, 3008; Howard, J. A., and Tait, J. C., J. Org. Chern., 1978, 43, 2493; Grattan, D. W., Carlsson, D. J., Howard, J. A., and Wiles, D. M., Can. J. Chem., 1979, 57, 2834; Kabun, S. M., and Buchachenko, A. L., BUN. Acad. Sci. USSR, Diu. Chem. Sci., 1966, 1430. l4 Griffiths, P. G., Moad, G., Rizzardo, E., and Solomon, D. H., Aust. J . Chem., in press.

Initiation Mechanisms in Radical Polymerizations 201 5

Generation of Cumyloxy Radicals

The two commercial sources of cumyloxyl (2), i.e. cumyl hydroperoxide (3) and dicumyl peroxide (16), were unsuitable for this study, the former because of the concurrent formation of hydroxy radicals, and the latter because of the high temperatures required to produce a reasonable rate of hom~lysis.~' A clean, low-temperature source of cumyloxyl(2) was therefore sought. Our initial efforts were directed towards preparation of the hypothetical dicumyl peroxalate (7), since analogous compounds, such as di-t-butyl peroxalate16 (9), meet the necessary criteria and have found wide- spread use as alkoxy radical However, all attempts to prepare (7) by reaction of oxalyl chloride with cumyl hydroperoxide (3) proved unsuccessful. The frequent observation of phenol and acetone formation, even in the presence of hin- dered amine bases, was indicative of the susceptibility of either the hydroperoxide (3) or the peroxalate (7) to ionic rearrangement.17 Dicumyl hyponitrite (6),18 on the other hand, could be easily prepared from silver hyponitrite* and a - b r o m ~ c u m e n e ~ ~ (4) by a procedure analogous to that used for the preparation of di-t-butyl hypo- nitrite23 (8).

The kinetics of decomposition of dicuinyl hyponitrite (6) were studied at various temperatures in cyclohexane by monitoring the disappearance of the azo absorption at 226 nm in the U.V. spectrum. Standard analysis procedures24 were used to verify that the reaction was first order with respect to dicumyl hyponitrite (6). The results are given in Table 1, and the activation parameters calculated from these rate data are: AH$ 113+4kJmol-l, AS: 39f2JK- 'mo l - l and A ( 7 . 7 + 0 . 4 ) ~ 1 0 ' ~ s - ~ . The absence of any significant solvent dependence or induced decomposition can be reasonably assumed by analogy with the known b e h a ~ i o u r ~ ~ of di-t-butyl hyponitrite (8), and our subsequent work has provided no indication to the contrary.

Finally, it is interesting to compare the rates of decomposition of di-t-butyl peroxalate (9) (k 1.7 x s- l ; tlI2 6.8 min)I6 and di-t-butyl hyponitrite (8) (k 2.0 x s- l ; t,,, 56 min, by i n t e r p ~ l a t i o n ) ~ ~ at 60". If this 10-fold reactivity difference can be extrapolated to the corresponding cumyl derivatives (7) and (6), then our results for the hyponitrite (6) (Table 1; tl12 7 .7 min at 60") suggest that the hypothetical

* This was prepared via sodium hyponitrite from hydroxylamine and pentyl nitrite.lgs20 In our hands, the yield of silver hyponitrite obtained by this method was comparable to that reported in a recent purportedly convenient synthesi~.~' The older method, however, has distinct advantages in its operational simplicity and lack of necessity for special precautions.

Is Brandrup, J., and Immergut, E. H., (Eds) 'Polymer Handbook' p. 11-20 (John Wiley: New York 1966). l6 Bartlett, P. D., Benzing, E. P., and Pincock, R. E., J. Am. Chem. Soc., 1960,82,1762; Sheldon, R. A,, and Kochi, J. K., J. Oug. Chem., 1970, 35, 1223. l 7 March, J., 'Advanced Organic Chemistry: Reactions, Mechanisms, and Structure' 2nd Edn, p. 1012 (McGraw-Hill, Kogakusha: Tokyo 1977). I s Howard, J. A., and Chemier, J. H. B., Can. J. Chem., 1976, 54, 382. l9 Jones, L. W., and Scott, A. W., J. Am. Chem. Soc., 1924, 46, 2172. 20 Scott, A. W., J. Am. Chem. Soc., 1927,49, 986. 21 Mendenhall, G. D., J. Am. Chem. Soc., 1974,96, 5000. 2 2 Belg. Pat. 645,807 (Chem. Abstr., 1965, 63, 9863d). 23 Kiefer, H., and Traylor, T. G., Tetuahedrort Lett., 1966, 6163. 24 Frost, A. A., and Pearson, R. G., 'Kinetics and Mechanism' 2nd Edn, pp. 12,41 (Wiley-Toppan: Tokyo 1961).


peroxalate (7) would have, for our purposes, an inconveniently high rate of decomposition.

Table 1. Rate constants, 104k (s-I), for the decomposition of dicumyl hyponitrfte (6) in cyclohexane at various temperatures

Temperature stability & 0 . lCC

Methyl Methacrylate

The decomposition of dicumyl hyponitrite (6) (0.020 M) in methyl methacrylate in the presence of a 10% excess of isoindolinyloxyl(1) (0.044 M) at 60' gave rise to a mixture of the six alkoxy amines (10)-(15) and dicumyl peroxide (16) in the yields (based on initiator) shown under the structural formulae, as well as acetophenone and cumyl alcohol (5) (in addition, there were noted a number of minor products, totalling < 0.5 %, which could not be isolated in sufficient quantities for identification). The pathways by which products (5) and (10)-(16), and acetophenone, arise are shown in Scheme 1, and represent four modes of reaction of cumyloxyl (2): addition to a double bond, hydrogen atom abstraction, / ? - s c i s ~ i o n , ~ ~ - ~ ~ and (cage) r ec~mbina t i on .~~

Of primary significance is the fact that the expected process, namely addition of cumyloxyl (2) to the double bond of methyl methacrylate, while being completely regiospecific to give radical (21) and hence alkoxy amine (15), accounts for only 51 % of the total consumption of cumyloxyl (2). The other products (lo), (12) and (13) (20.9 %) of the interaction of cumyloxyl with methyl methacrylate involve hydrogen abstraction from the allylic position to give (17) (18.8%) and from the ester methyl group to give (18) (2.1 %). This latter species (18) is highly reactive and propagates to form radical (19) in competition with direct trapping as adduct (12). The allylic radical (17), in contrast, is much less reactive towards monomer and forms only adduct (10). This, however, does not necessarily imply that (17) would fail to add to monomer in the absence of radical traps-the copolymerization of butadiene with methyl methacrylate3' and the initiation of polymerization by ally1 radical32 are but two examples of analogous processes which support this proposition. That the abstracting radical is cumyloxyl (2) and not some other species (e.g. methyl) is con- firmed by the close agreement between the combined yields of abstraction-derived products (lo), (12) and (13), and that of cumyl alcohol (5) (19.1 %).

2 s Walling, C., and Padwa, A,, J. Am. Chem. Soc., 1963,85, 1593. Kochi, J. K., J. Am. Chem. Soc., 1962,84, 1193.

27 Kochi, J. K., in 'Free Radicals' (Ed. J. K. Kochi) Vol. 11, p. 683 (John Wiley: New York 1973). 2 8 Walling, C., and Wagner, P. J., J. Am. Chem. Soc., 1964, 86, 3368. 29 Bacha, J. D., and Kochi, J. K., J. Org. Chem., 1965, 30, 3272. 30 Koenig, T., and Fischer, H., in 'Free Radicals' (Ed. J. K. Kochi) Vol. I, p. 157 (John Wiley: New York 1973); Koenig, T. W., in 'Organic Free Radicals' (ACS Symposium Series 69) (Ed. W. A. Pryor) p. 134 (American Chemical Society: Washington 1978). 31 Lewis, F. M., Walling, C., Cummings, W., Briggs, E. R., and Wenisch, W. J., J. Am. Chern. Soc., 1948, 70, 1527. 32 Cooper, W., J. Chem. Soc., 1951, 3106.

Initiation Mechanisms in Radical Polymerizations

Scheme 1. Decomposition of dicumyl hyponitrite (6) in methyl methacrylate.


The hydrogen abstracting ability of initiator radicals such as cumyloxyl toward methyl methacrylate has important implications with regard to the structure and properties of poly(methy1 methacrylate). Incorporation of residues derived from radicals such as (17) and (18) into a polymer produces unsaturated end-groups as reactive sites. These might be undesirable from one viewpoint, such as that of polymer stability or degradability, or, alternatively, they might prove beneficial for chemical manipulations, such as further polymerization leading to branched polymers. Further- more, abstraction is expected to occur from the ester methyl groups in the polymer itself,'' thereby providing another mechanism for chain branching. These aspects have been discussed more fully elsewheren3

The exclusive mode of p-scission of cumyloxyl is, as e ~ ~ e c t e d , ' ~ - ~ ' that leading to acetophenone and methyl radical (Scheme I), with no products derived from phenyl radicals being detected. Also noteworthy is the high reactivity of methyl radical toward methyl methacrylate, as indicated by the fact that a significant proportion of methyl radicals escape trapping by isoindolinyloxyl (1) by undergoing addition to monomer to give (14) (Scheme 1). The substantial extent (23 %) to which cumyloxyl undergoes p-scission in methyl methacrylate is not necessarily undesirable in the context of initiation of polymerization since the methyl radicals thus generated would ultimately become unreactive saturated polymer end-groups.

There is a general lack of data on rates of reaction of initiator radicals with monomer. The competition between P-scission of cumyloxyl and its reaction with methyl methacrylate provides a means of estimating the rates of the various processes described above (for a discussion of the concept of free-radical clocks, see the review by Griller and Ing01d~~).

Walling and Wagner have determined Arrhenius parameters (A log A - 5.04 and AE, 9.65) for the competition between p-scission and hydrogen abstraction from cyclohexane by t-butoxyl in Freon.28 Absolute values of logA 9 .1 and E, 6 .1 for the hydrogen abstraction34* enable calculation of a rate constant, k 6.4 x 103 s-', for p-scission of t-butoxyl at 60". Walling and Padwa have determined rates of p-scission for t-butoxyl and cumyloxyl (inter aka) relative to hydrogen abstraction from cyclohexane in carbon t e t r a~h lo r ide .~~ If the rates of hydrogen abstraction by these two species are assumed to be similar, a comparison of the relative (to hydrogen abstraction) p-scission rates reflects the ratio of the absolute rates. Thus, interpolation of the data of Walling and Padwa to 60" reveals a 15-fold greater rate of fragmentation for cumyloxyl, and therefore an absolute value of 9 .5 x lo4 s-' for k. By taking the concentration of methyl methacrylate at 60" as 9 1 M, the yields of products of reaction with cumyloxyl lead to the following rate constants: addition to monomer, k 2 x lo4

* While a number of estimates of these parameters are available, the above values were chosen because they were obtained by direct measurement of abstraction from cyclopentane. (It has pre- viously been established by competition experiments that abstractions from cyclohexane and cyclopentane occur at equal rates.35) Other recent work describing the direct measurement of hydrogen abstraction rates does not include determination of Arrhenius parameter^.^^ A more recent sum- mary of relevant work is given by Choo and B e n ~ o n . ~ ~

33 Griller, D., and Ingold, K. U., Acc. Chem. Res., 1980, 13, 317. 34 Wong, S. K., J. Am. Chem. Soc., 1979, 101, 1235. 3 5 Wagner, P., and Walling, C., J. Am. Chem. Soc., 1965, 87, 5179. 36 Paul, H., Small, R. D., and Scaiano, J. C., J. Am. Chem. Soc., 1978, 100, 4520. 37 Choo, K. Y., and Benson, S. W., Int. J. Chem. Kinet., 1981, 13, 833.


dm3 mol-I s-' : allylic hydrogen abstraction, 1c 9 x lo3 dm3 mol-I s-'; and ester hydrogen abstraction, k 1 x lo3 dm3 mol-' s-I. [The approximations inherent in these calculations are compounded by the dependence on solvent of rates of alkoxy radical fragmentations (but not, however, of hydrogen atom abstraction^).^^^^'^^^ Thus, Walling and Wagner found maximum p-scission in acetic acid and minimum in Freon-1 13 or carbon tetra~hloride.~' The rate variation factor was 10 for t-butoxyl, but only 2 for curnyloxyl. A solvent correction factor in the above estimates would therefore probably be in the range 1-5-2.1

A recent investigation in these laboratories of the reaction of t-butoxy radical with methyl methacrylate in the presence of isoindolinyloxyl (1) revealed similar patterns of reactivity to those exhibited by cumyloxyl (2).3 Thus, for example, the tail addition of t-butoxyl to methyl methacrylate to form (22) dominates hydrogen atom abstraction (from both the allylic and ester groups) by a factor of 1 .7, with allylic abstraction being preferred to ester abstraction in the ratio 7.2 : l.* The corresponding ratios for the products of reaction of cumyloxyl, namely addition (21) to abstraction (17)+(18), and allylic abstraction (17) to ester abstraction (18) (Scheme l), are 2.4 : 1 and 9.0 : 1 respectively. That such differences occur is not surprising; however, the small variation and the dependence of these processes on a 'complex interplay' of steric, electronic, polar, and thermodynamic factors preclude any cogent analysis. As might be expected, the partitioning between trapping by isoindolinyloxyl (1) and addition to monomer of methyl radical (2.5 : 1) and radical (18) (2.0 : 1) is identical, within experimental error, in both systems. The major difference between t-butoxyl and cumyloxyl is the extent of p-scission. The ratios of yields of /%scission products to addition plus abstraction products for cumyloxyl (1 : 3.1) and t-butoxyl (1 : 24) indicate an approximately 7.7-fold greater rate of fragmentation of cumyloxyl in methyl methacrylate. Any comparison with the 15-fold rate difference observed in carbon tetrachloride (see above) must be viewed in the light of the different extent to which changes in solvents affect the fragmentations of t-butoxyl and cumyloxyl (see above), and also of the different addition and abstraction rates of the two species toward methyl methacrylate.

Finally, it is interesting to consider some recent observations on the reactions of methyl methacrylate with benzoyloxy radicak3' In marked contrast to t-alkoxy radicals, only 0 .5 % of products derived from allylic hydrogen abstraction were found. Furthermore, it was uncertain whether benzoyloxyl or phenyl derived therefrom was the abstracting species. Also noteworthy was the occurrence of 7 % of head addition of benzoyloxyl to monomer.

* These ratios represent minor refinements of the originally published data3 made possible by the advent of superior analytical equipment. We thank Dr P. G. Griffiths for making the improved data available to us before publication. The other products from the reaction of t-butoxyl with methyl methacrylate were alkoxy amines (10)-(14), acetone, t-butyl alcohol and di-t-butyl peroxide.

Moad, G., Rizzardo, E., and Solomon, D. H., Malcuomol. Chem., in press; Cuthbertson, M. J., Moad, G., Rizzardo, E., and Solomon, D. H., Polym. Bull., 1982, 6, 647.


Styrene

Styrene proved a much more selective substrate than methyl methacrylate in its reactions with cumyloxyl(2). Thus, decomposition of dicumyl hyponitrite (6) (0,020 M) in styrene in the presence of a 10% excess of isoindolinyloxyl(1) (0.044 M) at 60" gave rise to the cumyloxyl adducts (24) (87.2%) and (25) (two diastereoisomers, 0 .3 and 0.4%) and the methyl adduct (23) (1.1 %), together with methoxyisoindoline (1 1) (4.7 %), dicumyl peroxide (16) (6.4 %) and acetophenone. Addition of cumyloxyl to styrene, manifest as (24) and (25), is completely regiospecific and represents the only detectable pathway for the direct interaction of cumyloxyl and styrene. The observation of products arising by propagation in styrene, i.e. (26) + (27) + (25) (Scheme 2), but not in methyl methacrylate, most probably reflects a lower rate of trapping of styryl radicals by isoindolinyloxyl (1) since the rate of propagation of styrene polymerization is slower than that of methyl metha~rylate.~' No products indicative of hydrogen abstraction, including cumyl alcohol (5 ) , or of aromatic substitution could be identified [trace amounts (<0 .1% each) of other minor products were detected, but the compounds could not be isolated in sufficient quantities for

cage / recombination

PhCOCH; + 'CH;

Scheme 2. Decomposition of dicumyl hyponitrite (6) in styrene

39 Billingham, N. C., and Jenkins, A. D., in 'Comprehensive Chemical Kinetics, Vol. 14A; Free Radical Polymerization' (Eds C. H. Bamford and C. F. H. Tipper) p. 419 (Elsevier: Amsterdam 1979).

Initiation Mechanisms in Radical Polyn~erizations

identification]. The absence of hydrogen abstraction cannot, however, be extrapolated to polymerization reactions, since in the presence of significant concentrations of polystyrene the saturated polymer backbone would provide a much more reactive substrate for hydrogen abstraction than monomer. Such reactions have indeed been observed between t-butoxy radicals and p~lystyrene?~

The partitioning of cumyloxyl between fragmentation and addition to styrene in a 1 : 15 ratio can be used to estimate the rate constant of the latter process as k 2 x 10' dm3 mol-' s-' at 60". The calculation is based on a styrene concentration of 8.37 M at 60" and is subject to the same limitations and approximations as out- lined for methyl methacrylate.

The reactions of t-butoxyl with styrene have also been examined by using this radical-trapping technique.' The results are qualitatively similar to those found in the present work, with the major difference being a fragmentation-to-addition ratio of 1 : 76. On the other hand, reaction of benzoyloxy radicals with styrene was found to exhibit markedly different ~elect ivi ty .~.~ These radicals undergo tail addition, head addition and aromatic substitution in the ratio 79 : 6 : 14, as well as fragmentation to phenyl radicals.

Conclusions

It is now evident that the initiation of free-radical polymerization may involve a complex series of competing reactions. Futhermore, in the reactions of free radicals with a given monomer, the nature of the products is very much dependent on the nature of the attacking radical. The various processes which occur are likely to be manifest as structural defects or irregularities in the polymer, with possible conse- quences in polymer properties.

Experimental Infrared spectra were determined as liquid films or KBr discs (solids) with a Unicam SPlOO or

a Perkin-Elmer 577 grating infrared spectrophotometer. N.m.r. spectra were recorded on a Varian T60 or a Bruker WM250 spectrometer with (D)chloroform as solvent and tetramethylsilane as internal standard. Chemical ionization mass spectra were obtained with a Finnigan 3300 mass spectrometer, methane being used as reagent gas. Ultraviolet spectra were measured with a Unicam SP1700 spectrophotometer.

Elemental analyses were carried out by the Australian Microanalytical Service, AMDEL, Melbourne.

Gas-liquid chromatographic analyses were carried out on a Pye 104 chromatograph equipped with a flame ionization detector. The column used was a SCOT Carbowax 20M, 34 m by 0.5 mm, 23,300 NEFF, and nitrogen was used as the carrier gas at 4.0 ml/min. H.p.1.c. was performed on a Dupont Instruments 850 liquid chromatograph equipped with variable-wavelength u.v. detector coupled to an LDC 308 computing integrator. Altex Ultrasphere ODS or Zorbax (Dupont) ODS columns were employed, and all analyses were carried out at 35'.

Melting points were measured with a Kofler hot stage, and are uncorrected, as are boiling points. Methyl methacrylate (c. 150 ml) was purified by washing with 5 % NaOH (2 x 50 ml), drying

(Na2S04) and distillation through a 60-cm vacuum-jacketed Vigreux column, b.p. 100". Styrene (c. 150 ml) was passed several times through 10-cm columns of Woelm neutral alumina (activity I), then distilled (b.p. 64'146 Ton). 1,1,3,3-Tetramethylisoindolin-2-yloxyl (I), m.p. 128-129", was prepared as described elsewhere?J4

40 Niki, E., Ohto, N., Kanauchi, T., and Kamiya, Y., Eur. Polym. J., 1980, 16, 559, and references cited therein.

E. Rizzardo, A. K. Serelis and D. H. Solon~on

The preparation described below was based on a Patent abstract." A solution of cumene (12.0 g, 0.10 mol) in carbon tetrachloride (120ml) containing azobisisobutyronitrile (trace) and N-bromo- succinimide (18.0 g, 0.101 mol) was lowered into an oil bath preheated to c. 140". Vigorous stirring was commenced and continued for 10 min after the onset of reflux, after which time the reaction mixture was cooled in an ice bath and filtered. The residue was washed with a little carbon tetrachloride, and the combined filtrates were concentrated (bath temperature < 35") and distilled to give the bromide (4) as a yellow oil (13.4 g, 6873, b.p. 63"/004 Torr (litS4l 45-46"/0.01 Torr).

Bis(I-methyl-1-phenyletlzyl) Hyponitrite (6)

Sodium hyponitrite was prepared by the method of Scottz0 and converted into silver hyponitrite as described by Jones and S ~ o t t . ' ~

Powdered silver hyponitrite (6.62 g, 24 mmol) was added in small portions to a stirred, ice- cooled solution of bromide (4) (11.94 g, 60 mmol) in pentane (25 ml). The resulting mixture was stirred at ice temperature for 1 h, then filtered. The filtrate was kept at -20" overnight, and the resultant oily solid was collected by filtration and recrystallized from etherlmethanol (from room temperature to -20') to give the hyponitrite (6) as a white crystalline solid (1.61 g, 23%), m.p. 69-70" (with nitrogen evolution). A small portion was recrystallized from pentane (from room temperature to -20') to give an analytical sample, m.p. 69-71' (lit.18 66-70) (Found: C, 72.3; H, 7.5; N, 9.4. Calc. for ClaHz2N20,: C, 72.5; H, 7.4; N, 9.6%). 1 .,,, (cyclohexane) (loge) 213, 225sh nm (4.18, 3.91). v,,, 1607, 1500, 1450, 1388, 1379, 1268, 1262, 1150, 1105, 1078, 990, 845, 765, 7 0 0 ~ m - ~ . N.m.r. 6 7.37, m, Whiz 2 Hz, 5H, ArH; 1.73, S, 6H, Me.

Kinetic Detevminntions

The rates of decomposition of dicumyl hyponitrite (6) were determined by following the decrease in u.v. absorption at 226 nm of a cyclohexane solution.

The reactions were carried out in a Perkin-Elmer u.v. cell which was heated by circulating water (peristaltic pump) from a constant-temperature bath. The solution temperature was monitored by means of an immersed thermocouple connected to a Technoterm digital instant-action thermometer. In a typical run, the preheated cell was filled with a c. 5 x M solution of the hyponitrite (6) in cyclohexane, the temperature probe was inserted, and the absorbance decrease was recorded until a constant value was reached. Appropriate absorbance values were used to construct plots of In[(A, - A,)/(Ao - A,)] against time, where A, is the absorbance at time t, A, is the final absorbance, and A. is the absorbance at 'zero' time, which was chosen as a convenient time soon after the solution temperature had reached a constant value. The first-order rate constants (slopes) were calculated by linear regression analysis of the In[(A,- A,)/(A,- A,)] and time data points, and had correlation coefficients r 0.999. Activation parameters were calculated from the Eyring equation

where k is the rate constant at temperature T, and kB, h and R are the Boltzmann, Planck and gas constants respectively. Linear regression analysis of Ink against l / T was used to calculate the slope, -AH :/R, and the intercept, In(kBT/h)+ (AStlR).

Decomposition of Dicumyl Hyponitrite (6) in Methyl Methac~ylnte

Dicumyl hyponitrite (6) (298 mg, 1.00 mmol) and isoindolinyloxyl (1) (422 mg, 2.22 mmol) were dissolved in freshly purified methyl methacrylate (50 ml), making the solution 0.020 M in initiator (6) and 0.0444 M in trapping agent (1). The solution was degassed by several freeze-evacua- tion-thaw cycles (liquid N2 being used), then sealed under vacuum (c. 0.01 Torr) and placed in a constant-temperature bath at 60' for 2 h.* Quantitative determination, at 270 nm, of the alkoxv

* Measurement of the u.v. absorbance of the solution at 430 nm before and after heating indicated consumption of 95 % of the theoretical amount of isoindolinyloxyl. The 5 %deficiency can be accoun- ted for by the formation of the cage recombination product dicurnyl peroxide (16). Heating for longer times produced no further change.

41 Malinowskii, M. S., and Jaworowskii, A. A., Zh. Obshch. Khim., 1955, 25, 2209.


amines (10)-(15) and dicumyl peroxide (16) was carried out by h.p.l.c., ethanollwater mixtures being used for elution. The mixture was further analysed by g.1.c. (105") to determine yields of cumyl alcohol (5), acetophenone, and methoxyisoindoline (11). That alkoxy amines (10)-(14) possess identical molar extinction coefficients at 270 nm has been s h o ~ n ; ~ . ~ h.p.1.c. response factors for alkoxy amine (15) and cumyl peroxide (16) relative to alkoxy amines (10)-(14) were therefore determined from mixtures of known composition of (15) and (16) with methoxyisoindoline (11) (see below). Similarly, g.1.c. detector response factors for cumyl alcohol (5) and acetophenone relative to (11) were determined from mixtures of known composition. The yields of cumyl alcohol and acetophenone could thereby be related to the yields of the non-volatile products (10) and (12)-(16).

The identities of all the products except the cumyloxyl adduct (15) were determined in the first instance by co-injection on h.p.1.c. and/or g.1.c. with authentic samples, and subsequently by isolation and comparison of spectral properties with those of authentic samples (see Acknowledgments). Alkoxy amine (15) was conclusively identified after isolation and complete spectral characterization.

The products of the reaction were isolated as follows. The reaction mixture was concentrated under reduced pressure to c. 3-5 ml and subjected to preparative h.p.1.c. separation of its components. Each fraction was concentrated to the point of cloudiness, then extracted with distilled ethyl acetate. The extracts were dried (Na2S04) and evaporated leaving the various products as oils or solids, which were characterized by their mass, and, where possible, n.m.r. and infrared spectra. In this way was isolated 443 mg [52% based on (16)J of the alkoxy amine (15) as a viscous oil. The crude product was dissolved in methanol, cooled to -20°, and filtered, whereupon crystallization occurred in the filtrate. The fine white needles of (15) (260 mg), m.p. 67-74", were crystallized twice from methanol to give methyl 2-merhyl-3-(I-methyl-I-phenylethoxy)-2-(l,I,3,3-tetramethylisoindolin-2-yl- oxy)propionate (15) (194 mg, 26%), m.p. 80-86" (Found: C, 73.1 ; H, 8.0; N, 3.1. C26H35N04 requires C, 73.4; H, 8.3; N, 3.3 %). I,,, (EtOH) (loge) 215, 258, 264, 270 nm (3.70, 2.88, 3.04, 3.05). v,,, 1730, 1450, 1358, 1268, 1165, 1085, 988,860, 815, 753,697cm-'. N.m.r. 87.2-7.5, m, 7H, ArH; 7.05-7.15, m, 2H, ArH; 3.76, s, C02Me; 3.49, d, J 7.7 Hz, lH, OCH,C; 3.33, d, J 7.7Hz, IH, 0CH2C; 1.63, s, Me; 1.54, s, 2xMe; 1.45, s, Me; 1.36, s, Me; 1.34, s, 2xMe.

Decomposition of Dicumyl Hyponitrite (6) in Styrene

A solution of dicumyl hyponitrite (6) (300 mg, 1 .Ol mmol; 0.0202 M) and isoindolinyloxyl (1) (419 mg, 2.21 mmol; 0.0442 M) in freshly purified styrene (50 ml) was degassed and heated at 60" for 2 h (see footnote on p. 2022). Quantitative determination, at 270 nm, of alkoxy amines (1 1) and (23)-(25), acetophenone and dicumyl peroxide (16) was carried out by h.p.l.c., methanollwater mixtures being used for elution. Further analysis by g.1.c. (105") verified the absence of cumyl alcohol (5). H.p.1.c. response factors for cumyloxyl adduct (24), acetophenone and dicumyl peroxide (16) were determined from mixtures of known composition with methoxyisoindoIine (11); the relative molar extinction coefficients of (23) and (1 1) had been determined pre~iously.~ Insufficient quantities of adducts (25) were available for accurate determinations. Identification of (1 I), (16), (23) and acetophenone was made initially by co-injection with authentic samples, and subsequently by isolation and spectral comparison (see Acknowledgments). Cumyloxyl adducts (24) and (25) were identified after isolation and spectral characterilation.

The products of the reaction were isolated as follows. The reaction mixtures from two separate but identical runs were combined, styrene was removed at 0.01 Torr, and the resulting cloudy oil was clarified with dichloromethane, diluted with ethanol and cooled to -20". Fine white needles of (24) (930 mg, 5473, m.p. 84-85', were collected, and the mother liquors were concentrated and crystallized from ethanol to give a second crop (280 mg, 16%). Preparative h.p.1.c. and isolation were carried out as described above on the mother liquors. This gave a further 136 mg of crystalline (24) (total 1.35 g, 79%). A small sample was recrystallized from carbon tetrachloride/ethanol, giving 2-[2-(l-methyl-l-phenylethoxy)-l-phenylethoxy]-l,l,3,3-tetra~1ethylisoindoline (24) as fine white needles, m.p. 84-85" (Found: C, 81.0; H, 8.1 ; N, 3.3. C29H35N02 requires C, 81.1 ; H, 8.2; N, 3.3%). A,,, (EtOH) (loge) 216, 258, 264, 271 nm (3.85, 3.02, 3.11, 3.08). v,,, 1497, 1455, 1362, 1265, 1166, 1082, 1062, 1000, 895, 756, 704cm-'. N.m.r. 6 7.15-7.45, m, 11H, ArH; 7.05-7.15, m, lH, ArH; 6.95-7.05, m, lH, ArH; 4.93, m, Whiz 8 HZ, CH2CHON; 3.68, dd, J 10, 8 Hz, lH, OCH,CH; 3.23, dd, J 10, 4.5Hz, lH, 0CH2CH; 1.72, s, Me; 1.56, s, Me;

E. Rizzardo, A. K. Serelis and D. H. Solonlon

1.55, s, Me; 1.52, s, Me; 1.25, s, Me; 0.76, s, Me. m/e430(8%, M+1), 312(7), 194(15), 192 (lo), 191 (24), 190 (17), 181 (4), 176 (G), 174 (6), 160 (4), 147 (13), 120 (ll), 119 (LOO), 108 (10). In addition there were isolated two minor components in quantities insufficient for complete characterization. The structures of the diastereomeric 2-[4-(1-methyl-1-phenylethoxy)-l,3-diphenylbutoxy]- 1,1,3,3-tetramethylisoindolines (25) were assigned on the basis of n.m.r. and mass spectral properties as follows. N.m.r. 6 6.9-7.5, m, 19H, ArH; 4.38, dd, J 12, 4 Hz, CH2CHON; 3.1-3.35, m, 0CH2CH; 2.4-2.65, m, CHCH,CH; 2.25-2.4, m, CH,CHCH2; 1.46, s, Me; 1.42, s, Me; 1.39,s,Me; 1937,s,Me; 1.26,s,Me; 1.16,s,Me. rn/e534(29%,M+1),518(3),456(4),417(8), 416 (26), 299 (4), 298 (17), 207 (6), 183 (3), 182 (23), 181 (13), 180 (13), 176 (12), 174 (6), 147 (13), 120 (lo), 119 (loo), 107 (4), 105 (3), 91 (3). N.m.r. 6 6.9-7.4, m, 19H, ArH; 4.81, dd, J 7 Hz, CH2CHON; 3.29, m, 0CH2CH; 2.92, quintet, J 7 Hz, 0CH2CHCH2; 2.74, dt, J 14, 7 Hz, lH, CHCH,CHON; 2.11, dt, J 14, 7Hz, lH, CHCH2CHON; 1.48, s, 4 x M e ; 1.25, s, Me; 1.22, s, Me, m/e 534 (38%, M + I), 518 (3), 458 (3), 457 (7), 444 (6), 418 (3), 417 (15), 416 (55), 415 (14), 398 ( 9 , 338 (8), 299 (17), 298 (77), 207 (4), 193 (3), 192 (25), 191 (16), 190 (15), 176 (lo), 174 (4), 147 (lo), 120 (lo), 119 (100).

A stirred solution of ferrous sulfate (heptahydrate; 1.25 g, 4.5 mmol) and isoindolinyloxyl (1) (570 mg, 3.0 mmol) in dimethyl sulfoxide (2.5 ml), under an argon atmosphere and at ambient temperature, was treated dropwise with a mixture of 30% hydrogen peroxide (0.77 ml, 9 .0 mmol) and dimethyl sulfoxide (10 ml) over 0.25 h. The reaction mixture was stirred for a further 0.5 h, then diluted with water (350 ml) and extracted with light petroleum (b.p. 3040") (100 ml). The organic layer was washed with water (3 x 100 ml), dried (Na2S04), and evaporated to give a colourless oil (570 mg). Dissolution in methanol and addition of water caused immediate crystallization. The mixture was kept at -20' overnight and filtered to give (11) (470mg, 7773, m.p. 38.5-39.5". Recrystallization from aqueous methanol (from room temperature to -20") afforded 2-methoxy- 1,1,3,3-tetramethylisoindoline (11) as long white needles (355 mg, 58%), m.p. 39-40" (Found: C, 76.4; H, 9.3; N, 6.8. Calc. for CI3Hl4NO: C, 76.1; H, 9.3; N, 6.8%). A,,, (EtOH) (log&) 215, 257, 263, 270nm (3.43, 2.75, 2.95, 3.00). v,,, 1490, 1465, 1377, 1364, 1325, 1272, 1167, 1052, 1025, 752cm-'. N.m.r. 67.2-7.3, m, 2H, ArH; 7.05-7.15, m, 2H, ArH; 3.80, s, Me; 1.44, s, 4 x Me.

Acknowledgments

We thank R. Eibl for technical assistance, R. I. Willing and Dr S. R. Johns for determining the 250-MHz 'H n.m.r. spectra and for helpful discussions, and I. Vit for determining the mass spectra. We also thank Dr P. G. Griffiths for providing samples and spectra of alkoxy amines (10)-(14), and Dr G. Moad for providing samples and spectra of the methyl adduct (23).

Manuscript received 1 March 1982

* We thank Dr P. G. Griffiths for informing us of his successful application of isoindolinyloxyl (1) to the trapping of methyl radicals generated by the method of Torsell et al?'

42 Bertillsson, B. M., Gustafsson, B., Kuhn, I., and Torsell, K., Acta Chem. Scand., 1970, 24. 3590; Rudqvist, U., and Torsell, K., Acta Chem. Scand., 1971, 25, 2183.

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