Initiation mechanisms for radical polymerization of methyl methacrylate with tert-butyl peroxypivalate

Article abstract of DOI:10.1021/ja962341b

The reaction of tert-butyl peroxypivalate (2) with methyl methacrylate (3) has been studied by the radical trapping technique employing 1,1,3,3-tetramethyl-2,3-dihydro-1H-isoindol-2-yloxyl (1) as a scavenger. Thermolysis of 2 generated tert-butoxyl, tert-butyl, and methyl radicals in the ratios of 48:50:2 at 60 °C in 3. Both alkyl radicals underwent selective tail addition to 3. tert-Butyl radicals reacted about twice as fast as methyl radicals with 3. The absolute rate constant for addition of tert-butyl radicals to 3 was estimated to be 2.3 x 10⁶ M^-1 s^-1 at 60 °C. The overall ratio of addition to H abstraction in the reaction of 2 with 3 was 5:1.

Full text of DOI:10.1021/ja962341b

10824
J. Am. Chem. Soc. 1996, 118, 10824-10828
Initiation Mechanisms for Radical Polymerization of Methyl
Methacrylate with tert-Butyl Peroxypivalate
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; CSIRO, DiVision of Chemicals and Polymers, PriVate Bag 10,
Rosebank MDC, Clayton, Victoria 3169, Australia; and Fine Chemicals and Polymers Research
Laboratory, NOF Corporation, Taketoyo-cho, Chita-gun, Aichi 470-23, Japan
ReceiVed July 8, 1996X
Abstract: The reaction of tert-butyl peroxypivalate (2) with methyl methacrylate (3) has been studied by the radical
trapping technique employing 1,1,3,3-tetramethyl-2,3-dihydro-1H-isoindol-2-yloxyl (1) as a scavenger. Thermolysis
of 2 generated tert-butoxyl, tert-butyl, and methyl radicals in the ratios of 48:50:2 at 60 °C in 3. Both alkyl radicals
underwent selective tail addition to 3. tert-Butyl radicals reacted about twice as fast as methyl radicals with 3. The
absolute rate constant for addition of tert-butyl radicals to 3 was estimated to be 2.3 × 106 M-1 s-1 at 60 °C. The
overall ratio of addition to H abstraction in the reaction of 2 with 3 was 5:1.
Introduction
technique is based on the fact that 1 reacts with carbon-center-
ed radicals at almost diffusion-controlled rates10 but not with
oxygen-centered radicals.
The initiation process in free radical polymerization plays
an important role in determining polymer properties such as
stability, molecular weight distribution, and composition.1 In
particular, initiator-derived end groups can have profound effects
on the stability of the polymer toward thermal and/or photo-
chemical degradation. The radical trapping technique developed
by the CSIRO,2 employing the nitroxide 1 as radical scavenger,
However, there have been some observations of nitroxide
trapped products arising from reactions of carbon-centered
radicals (e.g., methyl radicals generated by â-scission of tert-
butoxyl radicals, phenyl radicals11 from decarboxylation of
benzoyloxyl radicals, and solvent-derived carbon-centered radi-
cals12), with monomer. More recently, we have reported the
reaction of “second generation” carbon-centered radicals,13
phosphorus-centered radicals,14 and sulfur-centered radicals15
with monomer followed by trapping by 1. Thus we have shown
that the radical trapping technique can also be used to study
reactions of radicals other than oxygen-centered ones with
monomer.
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 tert-
alkyl peroxypivalates) with monomers. This paper reports the
results of a study of the reaction of tert-butyl peroxypivalate
(2) with methyl methacrylate (3) in the presence of the nitroxide
1. In the radical polymerization of acrylates and methacrylates,
2 is a widely used commercial initiator.16 However, no reports
has been used extensively to elucidate initiation mechanisms
in free radical polymerization. Most early work involved the
reaction of oxygen-centered radicals, that is, tert-butoxyl,2,3
benzoyloxyl,4,5 isopropoxycarbonyloxyl,5 cumyloxyl,6 hydroxyl,7
isopropoxyl,8 and ethoxyl radicals9 with monomer, since this
† Griffith University.
‡ CSIRO.
§ NOF Corp.
(9) Busfield, W. K.; Jenkins, I. D.; Thang, S. H.; Rizzardo, E.; Solomon,
D. H. Eur. Polym. J. 1993, 29, 397-400.
X Abstract published in AdVance ACS Abstracts, October 15, 1996.
(1) (a) Moad, G.; Solomon, D. H. Aust. J. Chem. 1990, 43, 215-239.
(b) Krstina, J.; Moad, G.; Solomon, D. H. Eur. Polym. J., 1989, 25, 767-
777.
(10) Beckwith, A. L. J.; Bowry, V. W.; Moad, G. J. Org. Chem. 1988,
53, 1632-1641.
(11) Moad, G.; Rizzardo, E.; Solomon, D. H. Aust. J. Chem. 1983, 36,
1573-1588.
(2) Rizzardo, E.; Solomon, D. H. Polym. Bull. 1979, 1, 529-534.
(3) (a) Griffiths, P. G.; Rizzardo, E.; Solomon, D. H. J. Macromol. Sci.,
Chem. 1982, 17, 45-50. (b) Busfield, W. K.; Jenkins, I. D.; Thang, S. H.;
Rizzardo, E.; Solomon, D. H. J. Chem. Soc., Perkin Trans. 1 1988, 485-
490.
(12) (a) Grant, R. D.; Griffiths, P. G.; Moad, G.; Rizzardo, E.; Solomon,
D. H. Aust. J. Chem. 1983, 36, 2447-2454. (b) Bednarek, D.; Moad, G.;
Rizzardo, E.; Solomon, D. H. Macromolecules 1988, 21, 1522-1528.
(13) Bottle, S. E.; Busfield, W. K.; Heiland, K.; Jenkins, I. D.;
Meutermans, W.; Monteiro, M. Progress in Pacific Polymer Science 3;
Ghiggino, K. P., Ed.; Springer-Verlag: Berlin: Heidelberg, 1994; pp 85-
97.
(4) (a) Moad, G.; Rizzardo, E.; Solomon, D. H. J. Macromol. Sci., Chem.
1982, 17, 51-59. (b) Moad, G.; Rizzardo, E.; Solomon, D. H. Macro-
molecules 1982, 15, 909-914.
(5) Cuthbertson, M. J.; Moad, G.; Rizzardo, E.; Solomon, D. H. Polym.
Bull. 1982, 6, 647-651.
(14) Busfield, W. K.; Grice, I. D.; Jenkins, I. D. Aust. J. Chem. 1995,
48, 625-634.
(6) Rizzardo, E.; Serelis, A. K.; Solomon, D. H. Aust. J. Chem. 1982,
35, 2013-2024.
(15) (a) Busfield, W. K.; Heiland, K.; Jenkins, I. D. Tetrahedron Lett.
1994, 35, 6541-6542. (b) Busfield, W. K.; Heiland, K.; Jenkins, I. D.
Tetrahedron Lett. 1995, 36, 1109-1112.
(7) Grant, R. D.; Rizzardo, E.; Solomon, D. H. J. Chem. Soc., Perkin
Trans. 2 1985, 379-384.
(16) Sheppard, C. S. 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.
(8) Busfield, W. K.; Jenkins, I. D.; Rizzardo, E.; Solomon, D. H.; Thang,
S. H. J. Chem. Soc., Perkin Trans. 1 1991, 1351-1354.
S0002-7863(96)02341-4 CCC: $12.00 © 1996 American Chemical Society

Radical Polymerization of Methyl Methacrylate
J. Am. Chem. Soc., Vol. 118, No. 44, 1996 10825
regarding the mechanism of initiation by 2 have been published.
Chart 1
Results and Discussion
Initially the thermolysis of 2, both in the presence and in the
absence of 1, was carried out in the absence of monomer in
order to test for the possible induced decomposition of 2 by 1.
It has been reported that 1 catalyzed the decomposition of certain
organic peroxides (e.g., diacylperoxides5,17 and peroxydicar-
bonates5). The decomposition of 2 in cumene at 60 °C,
measured by monitoring its disappearance by HPLC, satisfied
first-order kinetics. The rate constant in the presence of 1, 2.66
× 10-5 s-1, was consistent with that in the absence of 1 (k )
2.67 × 10-5 s-1) and also with the literature value, (k ) 2.70
× 10-5 s-1).18 This clearly indicates that 1 does not influence
the thermal decomposition of 2. The initiator 2 is known to
generate equimolar amounts of the two radicals via concerted
two-bond scission in a solvent, such as cumene (Scheme 1).18,19
Table 1. Ratios of Abstraction Products to the Tail Addition
Product of tert-Butoxyl Radicals and Methyl Methacrylate (4) for
Two Initiators at 60 °C
relative product yields
initiator
4
5
6
7
8
9
DTBPOa
100
100
47.5
47.1
5.3
5.4
0.8
0.7
6.6
6.7
0.8
0.8
2b
Scheme 1
a DTBPO ) di-tert-butyl peroxyoxalate, [DTBPO]0 ) 0.020 M, [1]0
) 0.088 M, reaction time ) 1.25h. b [1]0 ) 0.080 M, [2]0 ) 0.040 M,
reaction time ) 0.5h.
Scheme 2
Following the reaction of initiator 2 (0.040 M) with methyl
methacrylate (3) as solvent in the presence of 1 (0.080 M) at
60 °C in vacuo, the reaction mixture was analyzed by HPLC
and HPLC-MS. Alkoxyamines 4-11 (Chart 1) were formed
in the relative percentage yields as shown. As expected,
alkoxyamine 4 was the major product derived from tert-butoxyl
radical addition to methyl methacrylate, 8 and 9 were minor
products derived from methyl radical direct trapping and
addition, respectively, and 10 and 11 were the products derived
from competitive trapping and addition of tert-butyl radicals.
Products 5, 6 and 7 are products formed by hydrogen
abstraction from 3, followed by trapping (5 and 6) or further
monomer tail addition and trapping (7). In a separate experi-
ment, diacetyl peroxide was used as initiator to provide an
exclusive source of methyl radicals. The only products formed
were alkoxyamines 8 (78%) and 9 (22%). Thus 3 undergoes
negligible H-abstraction by methyl radicals. In a further separate
experiment, di-tert-butyl peroxyoxalate was used as initiator to
provide a major source of tert-butoxyl radicals. Products 4-9
were all observed; the ratios of the abstraction products to the
main tail addition product 4 are compared with the correspond-
ing products from the reaction with pivalate 2 in Table 1. The
close similarity of the ratios suggests that all the abstraction
products from the pivalate reaction arise from tert-butoxyl
radicals and that tert-butyl radicals result in negligible abstrac-
tion from 3. The reaction selectivity (preference for addition
over abstraction) of both methyl and tert-butyl radicals is
therefore similar to that of cyclohexyl radicals reported by Giese
et al.20
The range and yields of products can be adequately explained
by Scheme 2. The yield of products derived from tert-butyl
radicals (49.3%) was slightly less than that derived from tert-
butoxyl radicals (50.7%) presumably due to partial product
decomposition. A series of experiments was carried out for
various times in order to test the stability of products in the
reaction system. Product 11 was a prime suspect whereas the
tert-butyl adduct 10 is expected to be stable under the reaction
conditions.10 A lower trap concentration was used to enhance
the yield of 11, which, according to Scheme 2, is formed from
addition of tert-butyl radicals to methyl methacrylate in
(17) Moad, G.; Rizzardo, E.; Solomon, D. H. Tetrahedron Lett. 1981,
22, 1165-1168.
(18) The rate constant was calculated from the literature values at three
temperatures (45, 55, and 65 °C). Fort, Jr., R. C.; Franklin, R. E. J. Am.
Chem. Soc. 1968, 90, 5267-5268.
(19) (a) Bartlett, P. D.; Simons, D. M. J. Am. Chem. Soc. 1960, 82, 1753-
1756. (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.
(20) Giese, B.; Meixner, J. Chem. Ber. 1981, 114, 2138-2145.

10826 J. Am. Chem. Soc., Vol. 118, No. 44, 1996
Nakamura et al.
Table 2. Yields of All Products Relative to the Tail Addition Product of tert-Butoxyl Radicals and Methyl Methacrylate (4) at 60 °Ca
relative product yields
entry
[T]0/M
reaction time/h
4
5
6
7
8
9
10
11
4-9
10 + 11
1
2
3
4
5
6
7
8
9
0.040
0.040
0.040
0.040
0.040
0.060
0.080
0.120
0.160
0.5
1.0
2.0
3.0
5.0
0.5
0.5
0.5
0.5
100
100
100
100
100
100
100
100
100
47.1
46.9
47.2
47.7
48.5
47.0
47.1
47.1
47.2
5.0
5.0
5.0
5.0
4.7
5.3
5.4
5.6
5.8
1.3
1.4
1.5
1.6
1.8
0.9
0.7
0.5
0.3
5.9
6.0
6.0
6.0
5.9
6.5
6.7
6.9
7.1
1.5
1.4
1.6
1.6
2.0
1.1
0.8
0.6
0.4
104.2
101.1
98.1
90.6
84.9
118.0
128.3
136.7
142.6
46.6
40.4
28.9
22.3
11.9
35.7
28.0
20.3
15.3
160.8
160.7
161.3
161.9
162.9
160.8
160.7
160.8
160.8
150.8
141.5
127.0
112.9
96.8
153.7
156.3
157.0
157.9
a [2]0 ) 0.040 M.
Scheme 3
competition with reaction with trap. Relative product yields
are shown in Table 2. The results clearly indicate that product
11 is decomposing under the reaction conditions. A blank
experiment was carried out in order to test the stability of 11.
Thermolysis of 11 in methyl methacrylate in the presence of 1
and in the absence of 2 indicated the rapid decomposition of
11 (75% of 11 was decomposed after 2 h at 60 °C) to form
hydroxyamine 15. This type of decomposition has been reported
previously.21 The fact that compound 15 was not detected in
the reaction products of 2 with methyl methacrylate may be
due to oxidation of 15 by 2.21b
Figure 1. Thermolysis of 2 in cumene in the presence (b) and in the
absence of 1 (0) at 60 °C.
The total yield of alkyl radicals, (R′•), is given by eq 2. As the
conversion of pivalate is very low (half-life of 2 at 60 °C is ca.
7 h in cumene), the amounts of MMA and T consumed are
very small, and these concentrations can be assumed to be
constant and equal to the initial values ([MMA]0 ) 9.1 M at
60 °C6 and [T]0 as shown in Table 2). Therefore, according to
eq 3 (derived from eqs 1 and 2), the plot of 1/(R′-T) vs 1/[T]0
should be linear with 1/(R′•) as the intercept.
Methyl, tert-butyl, and acyloxymethyl radicals 14 all under-
went competitive addition/trapping reactions (Scheme 3). A
series of experiments was carried out for various initial
concentrations of 1 in order to investigate the reactions of these
alkyl radicals. The product yields, relative to the yield of the
major addition product 4 (taken as 100), are summarized in
Table 2. As expected, the relative amounts of tert-butoxyl
radical-derived products was basically independent of the initial
concentration of T ([T]0), that is, 4:5:6 + 7:8 + 9 ) 100:47.1:
6.1:7.6. On the other hand, tert-butyl radical-derived products
were obtained in higher yield at higher [T]0 since the yield of
11 (and therefore the amount of the decomposition of 11) was
reduced relative to the formation of 10. The ratios of products
derived from competitive reactions of methyl, tert-butyl, and
acyloxymethyl radicals 14, should be proportional to the ratio
of the concentrations of T and MMA [see eq 1, where kadd and
(R′•) ) (R′-T) + (R′-MMA-T)
(2)
(3)
k
add[MMA]0
kT(R′•)
1
1
[T]0
1
(R′•)
)
+
(R′-T)
The linear relationships, shown in Figure 2, provide strong
support for the validity of the experimental data and for Scheme
2. The intercepts give the total yield of radicals based on the
yields of the directly trapped radical products, R′-T, and is
independent of the yields of the products R′-MMA-T. This
eliminates the problem of the decomposition of 11. The total
yield of tert-butyl radicals is 162.9 (relative to 4 as 100), in
very close agreement with the total yield of products from tert-
butoxyl radicals (see Table 2). This shows that 2 generates
equimolar amounts of the two radicals in monomer as well as
in cumene.18,19 The yields of other alkyl radicals relative to
alkoxyamine 4 were also confirmed [methyl (6.1) and acyl-
oxymethyl radical 14 (7.6) respectively]. Assuming that tert-
butyl radicals (the yield of 11) ) (the total yield of tert-butoxyl
kadd
kT
(R′-MMA-T) [T]
)
(1)
[MMA]
(R′-T)
kT are the general rate constants for the reaction of radical R′
with MMA and T respectively and (R′-T) and (R′-MMA-
T) are the yields of these compounds relative to the yield of 4].
radical-derived products) - (the yield of 10), the ratios of kadd/
kT for methyl, tert-butyl, and acrylate radicals are obtained from
the linear (R′-MMA-T)/(R′-T) vs 1/[T]0 plot according to
eq 1 (shown in Figure 2).
(21) (a) Howard, J. A.; Tait, J. C. J. Org. Chem. 1978, 34, 4279-4283.
(b) Grattan, D. W.; Carlsson, D. J.; Howard, J. A.; Wiles, D. M. Can. J.
Chem. 1979, 57, 2834-2842.
The resulting values were 1.1 × 10-3, 2.4 × 10-3, and 1.2
× 10-3, respectively, and if kT is assumed to have the same

Radical Polymerization of Methyl Methacrylate
J. Am. Chem. Soc., Vol. 118, No. 44, 1996 10827
Thus tert-butyl radicals add to methyl methacrylate about twice
as fast as do methyl radicals.23 On the other hand, 14 shows
about the same reactivity as methyl radicals. Probably the
increased steric bulk of 14 (relative to methyl radicals) is
compensated by an increased nucleophilicity resulting from the
R-oxygen atom. Caronna et al.24 have reported that the relative
reactivity of alkyl radicals in addition reactions to diethyl vinyl
phosphonate is in the general order methyl < n-alkyl < sec-
alkyl < tert-alkyl radicals. Also Baban et al.25 reported that
isopropyl radicals react with acrylonitrile and methyl acrylate
about 7 times faster than n-heptyl radicals at 0 °C. From these
results, Giese26 concluded that for relatively small alkyl sub-
stituents on the carbon-centered radical, polar effects, which
increase the nucleophilicity of the radical with an increasing
number of alkyl groups at the radical center, are apparently more
important than substituent effects on the stability of the radicals.
The absolute rate constant for addition of tert-butyl radicals to
methyl methacrylate, kadd(But•), was also estimated to be ca.
2.2 × 106 M-1 s-1 using the value10 of kT ) 9.1 × 108 M-1
s-1. This is consistent with a literature report27 that the addition
of tert-butyl radicals to methyl methacrylate occurs >7.6 times
faster than the corresponding addition to styrene (k ) 2.3 ×
105 M-1 s-1 at 60 °C).28 Furthermore, the resulting value, kadd
-
•
(CH3 ) ) 1.0 × 106 M-1 s-1 at 60 °C, agrees with the value
•
recently reported by Zytowski et al.,29 kadd(CH3 ) ) 4.9 × 105
M-1 s-1 at 24 °C and Ea ) 16.0 kJ mol-1
.
In conclusion, in the reaction of 2 with 3, tert-butyl radicals
and methyl radicals generated from the thermolysis of 2
underwent selective tail addition to 3, while tert-butoxyl radicals
were responsible for all of the H-abstraction products. The rate
constant for addition of tert-butyl radicals to 3 was about twice
that for addition of methyl radicals and it was estimated to be
2.2 × 106 M-1 s-1 at 60 °C. About 83% of initiator-derived
free radicals, that is, tert-butoxyl (31%), tert-butyl (50%), and
methyl radicals (2%), underwent addition to 3.
Figure 2. 1/(R′-T) vs 1/[T]0 plots for alkyl radicals. (a) tert-Butyl
radicals (0). (b) Methyl radicals (4) and acyloxymethyl radicals 14
(O).
Experimental Section
HPLC was performed using Shimadzu LC-9A liquid chromatograph
fitted with a Waters Nova-Pak C18, ODS column, connected to a
Shimadzu UV spectrophotometric detector set at 270 nm and a CR-
6A computing integrator. Peak areas from HPLC chromatograms were
converted directly into percent molar yields of products. Alkoxyamine
compounds containing 1 mol equiv of the radical trapping moiety as
UV chromophore have been shown to have almost identical molar
extinction coefficients at 270 nm.6
NMR spectra were recorded on a Varian Gemini-200 (200MHz)
spectrometer, using deuterated chloroform or methanol as solvent.
Chemical shifts for 1H NMR spectra are relative to residual CHCl3 (δ
7.24 ppm) and for 13C NMR spectra are relative to the central peak of
the triplet resonance due to CDCl3 (δ 77.0 ppm) or the central peak of
seven-line multiplet resonance due to CD3OD (δ 49.0 ppm).
HPLC-electrospray mass spectra were obtained with a Single
Quadrupole VG Platform II mass spectrometer, coupled to a MassLynx
data system.
Figure 3. (R′-MMA-T)/(R′-T) vs 1/[T]0 plots for alkyl radicals.
tert-Butyl radicals (0), methyl radicals (4), and acyloxymethyl radicals
14 (O).
value for any carbon-centered radical,22 the relative reactivities
of the three radicals toward tail addition can be estimated as
follows:
(23) If the trapping rate of methyl radicals is faster than that of tert-
butyl radicals (9.1 × 108 M-1 s-1 at 18 °C),10 this ratio becomes smaller
correspondingly.
(24) Caronna, T.; Citterio, A.; Ghirardini, M.; Minisci, F. Tetrahedron
1977, 33, 793-796.
(25) Bahan, J. A.; Roberts, B. P. J. Chem. Soc., Perkin Trans. 2 1981,
161-166.
•
kadd(CH3 ):kadd(But•):kadd(14) ) 1.0:2.2:1.1
(26) Giese, B. Angew. Chem., Int. Ed. Engl. 1983, 22, 753-764.
(27) Moad, G.; Solomon, D. H. ComprehensiVe Polymer Science;
Eastmond, G. C., Ledwith, A., Russo, S., Sigwalt, P., Eds.; Pergamon
Press: Oxford, 1989; Vol. 3, p 108.
(22) Though the rate constants for nitroxide trapping of alkyl radicals
by 1 were reported to be almost diffusion controlled (ca. 1 × 109 M-1
s-1),10 the values depend on the structure of the radicals trapped (e.g., steric
protection of the radical center and resonance stability of the radical).
Therefore, rate constants for radicals, 12 and 13 are likely to be lower than
109 M-1 s-1 (Bowry, V. W.; Ingold, K. U. J. Am. Chem. Soc., 1992, 114,
4992-4996).
(28) The value was calculated from the Arrhenius equation (log A )
7.6 ( 0.2 M-1 s-1, Ea ) 14.3 ( 1.0 kJ mol-1). Mu¨nger, K.; Fischer, H.
Int. J. Chem. Kinet. 1985, 17, 809-829.
(29) Zytowski, T.; Fischer, H. J. Am. Chem. Soc. 1996, 118, 437-439.

10828 J. Am. Chem. Soc., Vol. 118, No. 44, 1996
Nakamura et al.
Materials. Methyl methacrylate 3 was washed with 5% NaOH,
dried over anhydrous Na2SO4, and distilled at atmospheric pressure.
Cumene was washed with concentrated H2SO4, dried over anhydrous
Na2SO4, and distilled at reduced pressure. Both solvents were stored
in a refrigerator (-20 °C). tert-Butyl peroxypivalate (2) was prepared
by the reaction of pivaloyl chloride with tert-butyl hydroperoxide in
alkaline solution. 2 was 99.7% pure (iodometric titration30); νmax(neat)/
cm-1 1769vs (CdO); m/z 197 (M + Na)+, 175 (M + H)+. Acetyl
peroxide was prepared by the reaction of acetic anhydride with hydrogen
peroxide in the presence of sodium carbonate.31 Di-tert-butyl peroxy-
oxalate,32 nitroxide 1,33 and hydroxyamine 158 were prepared by
literature procedures.
Kinetic Experiment. Thermolysis of 2 in the Presence of 1 in
Cumene. Cumene solutions (5 mL) containing 2 (0.05 M) and 1 (0.11
M) were placed in glass ampules, which were purged with nitrogen,
sealed, and immersed in a constant temperature bath regulated at 60
°C. At successive time intervals, ampules were removed from the bath.
The concentration of peroxide in each solution was measured by HPLC
(at 250 nm). The reaction was followed up to 50% decomposition
and exhibited first-order kinetics.
Trapping Experiments. Typical Procedure for Reaction of 2 with
3 in the presence of 1. A solution of 2 and 1 in freshly distilled 3 was
degassed by repeated freezing and thawing (three cycles) on a vacuum
line (<10-4 mmHg). The reaction vessel was then sealed under vacuum
and heated at 60 ( 0.1 °C for 0.5 h. Excess of monomer was then
removed by distillation under reduced pressure prior to analysis by
reverse phase HPLC with methanol/water mixtures as the eluent. The
HPLC separated products were identified by electrospray mass
spectrometry. The new compound (alkoxyamine 11) and some known
compounds (4, 9, and 10) were isolated by preparative HPLC and
characterized by the NMR data listed below (NMR data not previously
reported).
Methyl 3-tert-butoxy-2-methyl-2-(1,1,3,3-tetramethyl-2,3-dihydro-1H-
isoindol-2-yloxy)propanoate3a (4). 1H NMR (CDCl3) δ 7.26-7.19 (2H),
7.12-7.06 (2H), 3.77 (s, 3H), 3.67 (d, J ) 8.2 Hz, 1H), 3.45 (d, J )
8.2 Hz, 1H), 1.58 (s, 3H), 1.49 (s, 3H), 1.40 (s, 3H), 1.36 (s, 6H), 1.19
(s, 9H); 13C NMR (CDCl3) δ 174.0, 145.2, 145.0, 127.2, 121.6, 84.2,
73.1, 68.1, 67.9, 67.0, 51.7, 29.5, 27.5, 25.6, 25.3, 18.8; m/z 386 (M +
Na)+, 364 (M + H)+.
(1,1,3,3-Tetramethyl-2,3-dihydro-1H-isoindol-2-yloxy)methyl 2-
Methylpropenoate7 (6). m/z 312 (M + Na)+, 290 (M + H)+.
3-Methoxycarbonyl-3-(1,1,3,3-tetramethyl-2,3-dihydro-1H-isoindol-
2-yloxy)butyl 2-methylpropenoate3a (7). m/z 412 (M + Na)+, 390 (M
+ H)+.
2-Methoxy-1,1,3,3-tetramethyl-2,3-dihydro-1H-isoindole6,34 (8). m/z
206 (M + H)+.
Methyl 2-Methyl-2-(1,1,3,3-tetramethyl-2,3-dihydro-1H-isoindol-2-
yloxy)butanoate3a (9). 1H NMR (CDCl3) δ 7.26-7.19 (2H), 7.13-
7.06 (2H), 3.77 (s, 3H), 2.20-1.72 (m, 2H), 1.53 (s, 3H), 1.47 (s, 3H),
1.44 (s, 3H), 1.36 (s, 3H), 1.34 (s, 3H), 0.95 (t, J ) 7.5 Hz, 3H); 13C
NMR (CDCl3) δ 175.3, 145.5, 144.8, 127.3, 127.2, 121.6, 121.5, 84.7,
67.9, 67.8, 51.7, 33.3, 29.7, 29.5, 25.7, 25.0, 20.0, 8.9; m/z 328 (M +
Na)+, 306 (M + H)+.
2-tert-Butoxy-1,1,3,3-tetramethyl-2,3-dihydro-1H-isoindole7 (10). 1H
NMR (CDCl3) δ 7.27-7.20 (2H), 7.15-7.09 (2H), 1.51 (s, 6H), 1.34
(s, 6H), 1.32 (s, 9H); 13C-NMR (CD3OD) δ 146.7, 128.2, 122.7, 77.5,
69.2, 31.3, 29.8, 26.2; m/z 248 (M+H)+.
Methyl 2,4,4-Trimethyl-2-(1,1,3,3-tetramethyl-2,3-dihydro-1H-isoin-
dol-2-yloxy)pentanoate (11). 1H NMR (CDCl3) δ 7.26-7.19 (2H),
7.12-7.06 (2H), 3.75 (s, 3H), 2.08 (d, J ) 14Hz, 1H), 1.76 (d, J ) 14
Hz, 1H), 1.62 (s, 3H), 1.46 (s, 3H), 1.45 (s, 3H), 1.35 (s, 3H), 1.32 (s,
3H), 0.98 (s, 9H); 13C NMR (CDCl3) δ 175.6, 144.6, 127.3, 127.1,
121.7, 121.5, 83.8, 67.7, 53.9, 51.5, 30.8, 30.5, 29.8, 29.3, 29.2, 26.1,
25.0, 22.5; m/z 370 (M + Na)+, 348 (M + H)+. Note that compound
11 was unstable and partially decomposed during the running of the
13C NMR spectrum.
Reaction of Methyl Radicals with 3 in the Presence of 1.
A
solution of diacetyl peroxide (0.20 M) with 3 in the presence of 1 (0.050
M) was reacted at 60 °C for 3 h in the same manner as above.
Reaction of tert-Butoxyl Radicals with 3 in the Presence of 1.
The reaction of di-tert-butyl peroxyoxalate (0.020 M) and 3 in the
presence of 1 (0.088 M) was carried out at 60 °C for 1.25 h in the
same manner as above.
Thermolysis of 11 in the Presence of 1. A MMA solution of 11
(0.030 M) and 1 (0.040 M) was heated at 60 °C for 2 h in the same
manner as above. The resulting solution was concentrated and followed
by HPLC analysis (alkoxyamine 10 was used as an internal standard).
Methyl 2-[(1,1,3,3-tetramethyl-2,3-dihydro-1H-isoindol-2-yloxy)-
methyl]propenoate7 (5). m/z 312 (M + Na)+, 290 (M + H)+, 258 (M
- MeO)+.
Acknowledgment. We thank Dr. P. Van Le for useful
discussions. Financial assistance from NOF Corp., Griffith
University, and the Australian Research Council is gratefully
acknowledged.
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