Macromolecules
Article
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(250 mg, 1.45 mmol) and triethylamine (182 mg, 0.251 mL, 1.79
mmol) in toluene (3.45 mL) at room temperature and left for 18 h
before heating to 70 °C for 2 h. Filtration from a precipitate yielded
orange liquid, which was reduced under vacuum to yield orange
crystals that were then recrystallized from cyclohexane. Yield: 49%; mp
79 °C. 1H NMR (400 MHz, CDCl3, δ, ppm): 6.15 (1 H, s), 5.64 (1 H,
s). IR (NaCl, thin film, cm−1): 1715 (CO), 1620 (CC). HRMS
(m/z): calcd for C13H22NO3 240.1600; found 240.1597.
removed under vacuum to yield a viscous, tan liquid. H NMR (500
MHz, CDCl3, δ, ppm): 6.38 (1 H, dd, J1 = 17.3 Hz, J2 = 1.26 Hz), 6.10
(1 H, dd, J1 = 17.5 Hz, J2 = 10.4 Hz), 5.81 (1 H, dd, J1 = 10.4 Hz, J2 =
1.10 Hz), 5.10 (1 H, tt, J1 = 11.3 Hz, J2 = 4.41 Hz), 3.63 (3 H, s), 1.88
(2 H, dd, J1 = 11.2 Hz, J2 = 2.68 Hz), 1.60 (2 H, t, J1 = 11.8 Hz), 1.20−
1.25 (12 H, m). HRMS (m/z): calcd for C13H23NO3 241.1678; found
241.1673.
4-Cinnamoyloxy-2,2,6,6-tetramethylpiperidine-N-oxyl (1c).
Cinnamoyl chloride (300 mg, 1.80 mmol) in toluene (1.00 mL)
added dropwise to a solution of TEMPOH (250 mg, 1.50 mmol) and
triethylamine (177 mg, 1.80 mmol, 0.244 mL) in toluene (3.45 mL)
and stirred for 3 h at 70 °C. The solids were allowed to settle, and the
liquid decanted. The reduced liquid produced orange crystals under
vacuum. Yield: 38%; mp 79 °C. 1H NMR (500 MHz, CDCl3, δ, ppm):
7.77 (1 H, m), 7.58 (2 H, m), 7.44 (2 H, m), 7.47 (1 H, m), 6.50 (1 H,
m), 3.14 (1 H, s), 1.00−3.00 (16 H). IR (NaCl, thin film, cm−1): 1709
(CO), 1638 (CC). HRMS (m/z): calcd for C18H24NO3
302.1756; found m/z 302.1765.
4-Crotonoyloxy-2,2,6,6-tetramethylpiperidine-N-oxyl (1d). A
solution of crotonoyl chloride (188 mg, 1.80 mmol, 0.127 mL) in
toluene (1.00 mL) was added dropwise to a solution of TEMPOH
(250 mg, 1.50 mmol) and triethylamine (177 mg, 1.80 mmol, 0.244
mL) in toluene (3.45 mL), and the mixture was heated to 70 °C for 3
h. An orange liquid was decanted from white solids and reduced under
vacuum to yield orange oil. Yield: 60%. IR (NaCl, thin film, cm−1):
1735 (CO), 1649 (CC). HRMS (m/z): calcd for C13H22NO3
240.1600; found 240.1611.
RESULTS AND DISCUSSION
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Model Compound Studies. If a reagent such as
AOTEMPO is to delay the onset of cross-linking without
compromising cross-link densities, then it must trap alkyl
radicals by combination with nitroxyl, as opposed to addition to
acrylate functionality. Ideally, selectivity for AOTEMPO
conversion to polymer-bound alkoxamine would be absolute,
with acrylate functionality only being activated once all nitroxyl
is consumed. To test the trapping selectivity of AOTEMPO, a
model compound approach was adopted, wherein cyclohexane
was used in place of LLDPE. This strategy is widely used to
generate unambiguous information regarding the structure of
reaction products when the low concentration of polymer-
bound functionality and the insolubility of polymer thermosets
make it impossible to accomplish for macromolecule systems.22
Our studies involved the thermolysis of known amounts of
DCP in a standard solution of AOTEMPO + cyclohexane. A
cyclic hydrocarbon was used to eliminate regioisomers from the
reaction products. The cyclohexyl and methyl alkoxyamines (2
and 3, Scheme 3), as well as the initiator byproducts
acetophenone and cumyl alcohol, were quantified by GC
analysis using authentic standards.
1-Cyclohexyloxy-2,2,6,6-tetramethylpiperidin-4-ol, 1-Me-
thoxy-2,2,6,6-tetramethylpiperidin-4-ol. DCP (5 wt %, 212 mg,
0.783 mmol) and TEMPOH (270 mg, 1.57 mmol) in cyclohexane
(4.23 g) were charged to a stainless steel vessel and pressurized with
N2 to 14 bar prior to heating to 160 °C for 1 h. The vessel was cooled
to room temperature and depressurized to give a crude reaction
product, which was subjected to flash chromatography using a silica
column (1:1 hexanes:ethyl acetate) to isolate the desired alkoxyamines
as white crystalline solids. 1-Cyclohexyloxy-2,2,6,6-tetramethylpiper-
idin-4-ol: mp 77 °C. 1H NMR (500 MHz, CDCl3, δ, ppm): 3.99 (1 H,
m, HC−OH), 3.63 (1 H, m, CHON) 2.07 (2 H, s, cyclohexyl CH),
1.84 (2 H, d, J1 = 9.54 Hz, piperidinyl CH), 1.77 (2 H, m, cyclohexyl
CH), 1.49 (2 H, t, J1 = 11.2 Hz, piperidinyl CH), 1.23 (6 H, s, CH3),
1.18 (6 H, s, CH3), 1.10−1.30 (6 H, cyclohexyl H), OH absent.
HRMS (m/z): calcd for C15H29NO2 [M + H]+ 256.2270; found
Scheme 3. Analyzed Products of AOTEMPO Model
Compound Reactions
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256.2277. 1-Methoxy-2,2,6,6-tetramethylpiperidin-4-ol: mp 83 °C. H
NMR (500 MHz, CDCl3, δ, ppm): 3.98 (1 H, m), 3.64 (3 H, s), 1.83
(2 H, d, J1 = 9.40 Hz), 1.49 (2 H, t, J1 = 11.9 Hz), 1.25 (6 H, s), 1.17
(6 H, s), OH absent. HRMS (m/z): calcd for C10H21NO2 187.1572;
found 186.1568.
1-Cyclohexyloxy-2,2,6,6-tetramethylpiperidin-4-yl Acrylate
(2). Acryloyl chloride (110 mg, 0.098 mL, 1.22 mmol) in toluene
(0.349 mL) was added dropwise to a solution of 1-cyclohexyloxy-
2,2,6,6-tetramethylpiperidin-4-ol (260 mg, 1.02 mmol) and triethyl-
amine (123 mg, 0.169 mL, 1.22 mmol) in toluene (2.53 mL), and the
mixture was stirred for 16 h at room temperature. A precipitate was
compacted by centrifugation and a clear liquid decanted prior to
evaporating residual solvent under high vacuum, yielding a pale yellow,
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crystalline solid; mp 50 °C. H NMR (500 MHz, CDCl3, δ, ppm):
6.40 (1 H, dd, J1 = 17.3 Hz, J2 = 1.10 Hz), 6.11 (1 H, dd, J1 = 17.0 Hz,
J2 = 10.3 Hz), 5.82 (1 H, dd, J1 = 10.5 Hz, J2 = 0.92 Hz), 5.13 (1 H, tt,
J1 = 11.5 Hz, J2 = 4.22 Hz, HC−O−C), 3.63 (1 H, m), 2.06 (2 H, m),
1.89 (2 H, dd, J1 = 12.3 Hz, J2 = 2.20 Hz), 1.78 (2 H, m), 1.67 (1 H,
s), 1.63 (2 H, t, J1 = 11.5 Hz), 1.56 (1 H, d, J1 = 12.1 Hz), 1.24 (12 H,
s), 1.0−1.4 (4 H). HRMS (m/z): calcd for C18H31NO3 309.2304;
found 309.2296.
1-Methoxy-2,2,6,6-tetramethylpiperidin-4-yl Acrylate (3).
Acryloyl chloride (86 mg, 0.077 mL, 0.950 mmol) in toluene (0.274
mL) was added to a solution of 1-methoxy-2,2,6,6-tetramethylpiper-
idin-4-ol (148 mg, 0.792 mmol) and triethylamine (80 mg, 0.11 mL,
0.950 mmol) in toluene (1.64 mL), and the mixture was stirred at
room temperature for 24 h. The liquid was decanted and solvent
At the temperatures used in polyolefin modifications, β-
scission of cumyloxyl to yield methyl radicals + aceteophenone
is competitive with hydrogen atom abstraction from a
hydrocarbon.23 The extent of radical fragmentation increases
with temperature and the C−H bond dissociation energy of
potential hydrogen atom donors.24,25 Independent studies of
hydrogen abstraction by cumyloxyl from cyclohexane have
recorded abstraction efficiencies on the order of 53% at 160 °C,
with cumyl alcohol:aceteophenone ratios of about 1:1.14.26 In
the present context, alkoxamines 2 and 3 should be produced in
approximately these proportions. The data plotted in Figure 1
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dx.doi.org/10.1021/ma3016135 | Macromolecules 2012, 45, 8147−8154