Ethylene/hindered phenol substituted norbornene copolymers: Synthesis and NMR structural determination

Article abstract of DOI:10.1002/pola.26270

A new family of ethylene-based copolymers with controlled amounts of a norbornene comonomer (N_ArOH) bearing a stabilizing antioxidant functionality (2,6-di-tert-butyl phenol) was prepared. Due to unavoidable exo/endo equilibrium operative in N_ArOH comonomer, a complete and detailed NMR assignment of the structure of the prepared ethylene/N _ArOH copolymers was carried out for the determination of the exo/endo ratio inside the polymer. These novel functionalized comonomers can be considered suitable starting material for preparing ethylene-based copolymers, with tunable comonomer content, as non-releasing macromolecular antioxidant additives for specific application in safe food and/or drug packaging 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012 Copolymers of ethylene with an antioxidant phenolic functionalized norbornene comonomer have been prepared and their structure fully determined by NMR. These novel materials can be used as non-releasing macromolecular antioxidant additives for specific application in safe food and/or drug packaging. Copyright

Full text of DOI:10.1002/pola.26270

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Ethylene/Hindered Phenol Substituted Norbornene Copolymers:
Synthesis and NMR Structural Determination
1
1
2
2
Caterina Viglianisi, Stefano Menichetti, Giulio Assanelli, Maria Carmela Sacchi,
2
2
Incoronata Tritto, Simona Losio
1
Dipartimento di Chimica ‘‘Ugo Schiff,’’ Universit aꢀ di Firenze, Via della Lastruccia 13, 50019 Sesto Fiorentino (FI), Italy
2
ISMAC – CNR, Via Bassini 15, 20133 Milano, Italy
Correspondence to: C. Viglianisi (E-mail: caterina.viglianisi@unifi.it) or S. Losio (E-mail: simona.losio@ismac.cnr.it)
Received 13 May 2012; accepted 12 July 2012; published online
DOI: 10.1002/pola.26270
ABSTRACT: A new family of ethylene-based copolymers with
controlled amounts of a norbornene comonomer (NArOH) bear-
ing a stabilizing antioxidant functionality (2,6-di-tert-butyl phe-
nol) was prepared. Due to unavoidable exo/endo equilibrium
operative in NArOH comonomer, a complete and detailed NMR
assignment of the structure of the prepared ethylene/NArOH
copolymers was carried out for the determination of the exo/
endo ratio inside the polymer. These novel functionalized
comonomers can be considered suitable starting material for
preparing ethylene-based copolymers, with tunable comono-
mer content, as non-releasing macromolecular antioxidant
additives for specific application in safe food and/or drug pack-
aging VC 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym
Chem 000: 000–000, 2012
KEYWORDS: antioxidants; ethylene/norbornene copolymeriza-
tion; macromolecular additives; metallocene catalysts; NMR;
packaging
INTRODUCTION Any polymer used for food packaging appli-
devise feasible synthetic routes for the preparation of comono-
mers bearing the antioxidant functionality and an easily poly-
merizable double bond. Norbornene (N) is proved to be an
extremely effective monomer in polymerization with metallo-
cation is neither durable nor processable without antioxidant
1
,2
additives that inhibit or retard degradation. Unfortunately,
most of the commonly used additives are low molecular
weight compounds, more polar than the polyolefin matrices,
and consequently they are extractable to some extent into
the food due to diffusion and volatility. In the last few years,
we carried out a research aimed to propose innovative solu-
tions to the unavoidable physical migration of low molecular
antioxidant additives from plastic films used in food or drug
6
,7
cene-based catalysts.
Moreover, literature reports several
examples of copolymerizations of ethylene with functionalized
8–13
norbornene derivatives.
These amorphous copolymers are
strong, stiff, highly transparent materials with very high mois-
ture barrier as well as interesting electrical proprieties that
allows their use primarily in applications requiring glass-like
clarity. Thus we focused our efforts on the synthesis of a norbor-
nene comonomer (NArOH) bearing, in exo and/or endo-position,
a hindered phenol (2,6-di-tert-butyl-phenol) as a pendant stabi-
lizing substituent. The copolymerizations were then designed to
achieve the controlled insertion of the comonomer up to about
2% mol: indeed with higher comonomer concentrations, we
have experienced a poor miscibility of the functionalized copoly-
3
–5
packaging.
Within the frame of such research, the most
satisfactory results arose studying new macromolecular anti-
oxidant additives bearing tunable amounts of a selected anti-
oxidant functionality. Specifically, a synthetic route to pro-
duce a series of novel comonomers A–D bearing a highly
efficient phenolic antioxidant unit and different methylene
spacers between the aromatic ring and the polymerizable
olefinic double bond was devised (Fig. 1).
5
mers with the polyolefinic matrix. In general, exo-isomers of
substituted norbornene are reported to be more reactive than
Comonomers A–D were copolymerized with ethylene (E) to
obtain copolymers, with a controlled incorporation of the anti-
oxidant group along the polyethylene chain (Fig. 1), which
represent a new family of non-releasing macromolecular antiox-
idant additives to be used as ‘‘masterbatches’’ in blends with
commercial polyolefins, for specific application in safe food
and/or drug packaging. The crucial point of this approach is to
14–16
the endo-isomers in the polymerization reaction.
Therefore,
this article discusses stereodefined synthesis of NArOH comono-
mer as well as the role of such stereochemistry on the formation
of the corresponding ethylene copolymers.
For this purpose, a careful NMR study was conducted on the
comonomer and on the prepared copolymers for the
Additional Supporting Information may be found in the online version of this article.
2012 Wiley Periodicals, Inc.
V
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0.22 mmol) was added followed by ammonium formate
(
9
1.15 g, 18.32 mmol) and the reaction mixture was stirred at
0 C overnight. The mixture was cooled to room tempera-
ꢁ
ture, diluted with dichloromethane (DCM) (200 mL), washed
with 0.1M hydrochloric acid (150 mL) and water (3ꢂ 150
mL), the organic layer was dried over Na SO filtered and
2
4
concentrated in vacuum. The crude product was purified by
flash chromatography on silica gel using 15:1 petroleum
ether:DCM as eluent to give the title compound as a color-
1
less oil in 68% yield. H NMR (400 MHz, CDCl , d): 1.40–
3
FIGURE 1 Comonomers (A–D) and copolymers previously pre-
1
.44 (m, 1H, H ), 1.46 (s, 18H, H ), 1.57–1.63 (m, 2H, H ,
7 f 6
pared that inspired the present study.
H ), 1.71–1.76 (m, 1H, H ), 2.64 (dd, J ¼ 8.2, 5.0 Hz, 1H, H ),
7
6
5
2.84 (bs, 1H, H ), 2.96 (bs, 1H, H ), 5.04 (s, 1H, OH), 6.15
4 1
determination of the exo/endo ratio. Mono- and multidimen-
sional NMR data were either found in the literature for ethyl-
ene/norbornene (E/N) or ethylene/functionalized norbor-
(dd, J ¼ 5.6, 2.8 Hz, 1H, H ), 6.26 (dd, J ¼ 5.6, 2.8 Hz, 1H,
2
13
H ), 7.09 (s, 2H, H ); C NMR (100 MHz CDCl , d): 30.4
3
b
3
(6C ), 33.6 (C ), 34.4 (2C ), 42.2 (C ), 43.7 (C ), 45.8 (C ),
f
6
e
1
4
7
6
–13,17–24
nene copolymers
or registered on E/N copolymers
48.7 (C ), 124.1 (2C ), 135.5 (2C ), 136.4 (C ), 137.1 (C ),
5 b c a 2
appositely prepared. The next sections illustrate how NMR
data were utilized to achieve the complete assignment of the
structure of these new antioxidant pending comonomers and
copolymers.
137.5 (C ), 151.6 (C ); IR (KBr): m ¼ 3639, 3156, 2957,
3
d
ꢀ1 þ
1659, 1460.35 cm ; MS (m/z (%)): 298 (2) [M ], 232
(100), 217 (78), 57 (41). Anal. calcd for C H O: C 84.51, H
2
1 30
10.13; found: C 84.44, H 10.50.
(endo)-2,6-Di-tert-butyl-4-(bicyclo[2.2.1]hept-5-en-2-
EXPERIMENTAL
yl)phenol (endo-1)
Synthesis
All the reactions were monitored by thin layer chromato-
graphy (TLC) on commercially available precoated plates
(
silica gel 60 F 254) and the products were visualized with
acid vanillin solution. Silica gel 60, 230–400 mesh, was used
1
for column chromatography. Unless otherwise noted,
H
1
3
NMR and C NMR spectra were recorded with a Varian
Gemini 200, Varian Gemini 300, and with a Varian Mercury
Plus 400 in CDCl solutions. Residual CHCl was used as ref-
3
3
Isolated by flash chromatography after partial RDA/DA equil-
erence at 7.26 and 77.00 ppm. Infrared spectra (IR) were
recorded on a FTIR Infrared Spectrometer 1600 Perkin
Elmer in KBr pellets or in CDCl solutions using a CaF cell.
Adsorptions are reported in cm . Mass spectra were meas-
ured with a Shimadzu QP5050, by FAB (m-nitrobenzyl alco-
hol as matrix) or by electron spray ionization (ESI) using
JEOL MS station JMS700. All commercially compounds were
used as received, unless otherwise stated and without fur-
ther purifications.
1
ibration. H NMR (200 MHz, CDCl , d): 1.17–1.26 (m, 1H,
3
H
6
), 1.33–1.57 (m, 2H, 2H
7 f
), 1.42 (s, 18H, H ), 2.14–2.26 (m,
3
ꢀ
2
1
1H, H ), 2.92 (bs, 1H, H ), 3.02 (bs, 1H, H ), 3.24–3.33 (m,
6
1
4
1
H, H ), 4.99 (s, 1H, OH), 5.89 (dd, J ¼ 5.8, 2.8 Hz, 1H, H ),
5
3
6
.26 (dd, J ¼ 5.8, 2.8 Hz, 1H, H ), 6.95 (s, 2H, H ); IR (KBr):
2
b
ꢀ
1
m ¼ 3639, 3156, 2957, 1659, 1460.35 cm ; MS (m/z (%)):
þ
298 (2) [M ], 232 (100), 217 (78), 57, (41). Anal. Calcd for
C H O: C 84.51, H 10.13; found: C 84.44, H 10.50.
2
1 30
(
exo)-4-(Bicyclo[2.2.1]hept-5-en-2-yl)-2,6-di-tert-butyl-
(
exo)-2,6-Di-tert-butyl-4-(bicyclo[2.2.1]hept-5-en-2-yl)
phenyl acetate (exo-2)
phenol (exo-1)
A Schlenck tube was charged with 4-bromo-2,6-di-tert-butyl-
phenol (1.27 g, 4.47 mmol) and triphenylphosphine (118
mg, 0.45 mmol) then was evacuated and refilled with nitro-
gen three times, dry dimethylformamide (DMF) (7 mL) and
bicycle[2.2.1]hepta-2,5-diene (norbornadiene) (2.5 mL, 23
mmol) were added through the rubber stopper via a syringe
and stirred to dissolve the solid. Palladium acetate (49 mg,
A Schlenck tube was charged with 4-bromo-2,6-di-tert-butyl-
phenyl acetate (1.50 g, 4.58 mmol) and triphenylphosphine
(118 mg, 0.45 mmol) then was evacuated and refilled with
nitrogen three times, DMF dry (7 mL) and bicycle[2.2.1]-
hepta-2,5-diene (norbornadiene) (2.5 mL, 23 mmol) were
added through the rubber stopper via a syringe and stirred
to dissolve the solid. Palladium acetate (49 mg, 0.22 mmol)
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and ammonium formate (1.13 g, 17.88 mmol) were added in
sequence and the reaction mixture stirred at 90 C overnight.
Then, it was cooled to room temperature, diluted with DCM
The X-ray CIF file for this structure has been deposited at
the Cambridge Crystallographic Data Center with the deposi-
tion number CCDC 857071. Copies of the data can be
obtained, free of charge, from CCDC, 12 Union Road,
Cambridge, CB2 1EZ UK (e-mail: deposit@ccdc.cam.ac.uk;
internet://www.ccdc.cam.ac.uk).
ꢁ
(
200 mL), washed with 0.1M hydrochloric acid (150 mL)
and water (3ꢂ 150 mL), the organic layer was dried over
Na SO filtered and concentrate in vacuo. The crude product
2
4
was purified by flash chromatography on silica gel using 2:1
Copolymerization
petroleum ether:DCM as eluent to give the title compound as
All experiments and manipulations involving air-sensitive com-
pounds were carried out under a dry nitrogen atmosphere in a
glovebox apparatus or using standard Schlenk line techniques.
Methylaluminoxane (MAO) (10% wt as toluene solution,
Crompton) was used after removing all volatiles and drying the
resulting powder at 50 C for 3 h in vacuum (0.1 mmHg). Triiso-
butylaluminoxane (TIBA) (Witco, in solution of toluene) was
used as received. rac-Et(Ind) ZrCl2 was kindly provided by
1
colorless oil in 88% yield. H NMR (400 MHz, CDCl
3
, d):
1
1
3
2
.350 (s, 9H, H ), 1.352 (s, 9H, H ), 1.40–1.44 (m, 1H, H ),
f f 7
.57–1.63 (m, 2H, H , H ), 1.71–1.76 (m, 1H, H ), 2.34 (s,
6
7
6
H), 2.67 (dd, J ¼ 8.6, 4.6 Hz, 1H, H ), 2.88 (bs, 1H, H ),
5
1
.97 (bs, 1H, H ), 6.15 (dd, J ¼ 5.6 2.8 Hz, 1H, H ), 6.26 (dd,
4
2
ꢁ
J ¼ 5.6 2.8 Hz, 1H, H ), 7.209 (s, 1H, H ), 7.210 (s, 1H, H );
3
b
b
1
3
C NMR (100 MHz, CDCl
3
, d) 22.7 (1C), 31.5 (6C
f
), 33.6
2
(
1C ), 35.4 (2C ), 42.2 (1C ), 43.8 (1C ), 45.8 (1C ), 48.3
6 e 1 4 7
Basell Poliolefine Italia S.r.l. Toluene was dried and distilled
from sodium under nitrogen atmosphere. Nitrogen and ethyl-
ene gases were dried and deoxygenated by passage over col-
(
1C ), 125.5 (1C ), 125.6 (1C ), 137.3 (1C ), 137.4 (1C ),
5 b b 2 3
1
41.66 (1C ), 141.68 (1C ), 142.4 (1C ), 145.6 (1C ), 171.2
c c a d
(1C).
2
umns of CaCl , molecular sieves, and BTS catalysts.
Typical Copolymerization Procedure
(
exo)-2,6-Di-tert-butyl-4-(bicyclo[2.2.1]hept-5-en-2-yl)
The copolymerizations were performed in a round-bottom
phenol (exo-1)
ꢁ
flask at 25 C and 0.5 atm of ethylene gas pressure. The total
Lithium aluminum hydride (0.557 g, 14.66 mmol) was sus-
pended in tetrahydrofurane (THF) (50 mL) under a nitrogen
atmosphere at 0 C. A solution of the exo-2 (1.634 g, 4.79
mmol) in THF (50 ml) was added to the suspension and the
mixture was refluxed for 16 h. A solution 0.1M of hydrochlo-
ric acid (200 mL) was added to the reaction mixture under
ice cooling and extracted with DCM (3ꢂ 150 mL), the com-
bined organic layers were dried over Na SO filtered and
volume of toluene (100 mL) was introduced into the evacuated
ꢁ
and N purged flask. The synthesized comonomer was added
2
with TIBA (ratio between TIBA and comonomer ¼ 1.2) and the
solution was stirred for 2 h under N pressure. Afterward the
2
flask was filled with 6 mmol of MAO solution, 2 mmol of the cat-
alyst (ratio between Al and Zr ¼ 3000), and ethylene at 0.5 atm
pressure (ratio between ethylene and the comonomer ¼ 6.67),
2
4
maintained with the correct proportion of N and ethylene gas
2
concentrated in vacuum to afford exo-1 as a colorless oil in
8% of yield.
in the flask. The copolymerization was stopped after 30 min,
through the addition of ethanol and 37% hydrochloric acid. The
reaction product was stirred for several hours then filtered,
washed with ethanol, and dried in vacuum.
9
X-Ray Analysis
exo)-4-(bicyclo[2.2.1]Hept-5-en-2-yl)-2,6-di-tert-butylphenyl
acetate (exo-2): C H O , M ¼ 340.49, Monoclinic, space
(
Reduction of Copolymer E/NArOAc
In a flat-bottom PE syringe, equipped with sintered Teflon
filters, copolymer E/NArOAc (41 mg, loading 1.36% mol) was
2
3 32 2
group P21/c, a ¼ 9.821(1), b ¼ 17.719(1), c ¼ 12.742(1) Å,
ꢁ
3
b ¼ 111.41(1) , V ¼ 2064.3(3) Å , Z ¼ 4, D ¼ 1.096 mg/
c
3
ꢀ1
m , m ¼ 0.68 mm , F(000) ¼ 744, experiment T ¼ 293 K.
pre-swelled in dry THF (1 mL) for 6 h, then LiAlH (38 mg)
4
1
3,203 reflections were collected with a 4.31< y < 29.34
was added and the suspension was swelled at room temper-
ature overnight. The reaction mixture was purred into ice
and 0.1M aqueous solution of hydrochloric acid and filtered
through the syringe which was equipped with Teflon tubing
and valves to wash under suction with THF, EtOAc, DCM (3ꢂ
20 mL each). The resulting copolymer was dried under vac-
uum over KOH pellets. We verify the formation of the
required copolymer E/NArOH by comparison of the corre-
sponding FTIR spectra (see Supporting Information).
range; 4850 were independent; the parameters were 226 and
the final R index was 0.0935 for reflections having I>2rI. The
non-hydrogen atoms were refined anisotropically. Aromatic,
methylic, methylenic, and methynic hydrogens were assigned
in calculated positions, all of them were refined as isotropic.
X-ray analysis was carried out with a Goniometer Oxford Dif-
fraction KM4 Xcalibur2. Graphite-monochromated Mo/Ka
radiation (40 mA/ꢀ40 kV) and a KM4 CCD/SAPPHIRE detec-
tor were used for cell parameter determination and data col-
lection. The integrated intensities, measured using the x
scan mode, were corrected for Lorentz and polarization
effects. The substantial redundancy in data allows empiri-
cal absorption corrections (SADABS ) to be applied using
multiple measurements of symmetry-equivalent reflections.
NMR Analysis
1
3
C NMR spectra of comonomer was recorded in CDCl CDCl
2
2
ꢁ
ꢁ
at 28 C and 103 C on a Bruker Avance 400 spectrometer
operating at 100.58 MHz, internal chemical shift reference:
1% hexamethyldisiloxane. Conditions: acquisition time 1.80
s; relaxation delay 16 s. All the copolymers were recorded in
2
5
2
6
2
7
ꢁ
ꢁ
The structure was solved by direct methods of SIR2004
and refined using the full-matrix least squares on F pro-
vided by SHELXL97.
CDCl CDCl2 at 103 C. Conditions: 10 mm probe; 90 pulse
2
2
angle; 64 K data points; acquisition time 5.56 s; relaxation
delay 20 s; 3–4 K transients. Proton broad-band decoupling
2
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was achieved with a 1D sequence using bi_waltz16_32
1
3
power-gated decoupling. C DEPT NMR spectra were meas-
ured with composite pulse decoupling using the sequence
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
s -90 -s -180 , 90 -s -135 , 180 -s -CPD-acquire, with
1
2
2
2
ꢁ
delays s of 5 s, and s of 3.8 ms and 90 pulse widths of
4.3 and 28.1 ls and for C and H, respectively.
1 2
1
3
1
1
ꢁ
1
Two dimensional NMR parameters: 90 pulse widths for H
1
3
and C were 6.95 and 13.90 ls respectively; relaxation delay,
.2 s; gradient heteronuclear single quantum coherence
gHSQC) experiments were carried out with a delay of 1.67 ms
1
(
1
corresponding to a J ¼ 150 Hz, for the creation of antiphase
CH
magnetization. Data were zero filled and weighted with a
shifted sinebell function before Fourier transformation.
SCHEME 1 Alternative available access to comonomer
gHMBC experiments, with a two-fold low pass J-filter, to sup-
press one-bond correlations, were carried out with a delay
NArOH 1.
of 100 ms for the evolution of long-range coupling and
equipped with a differential refractometer. The column set
was composed of three mixed TSK-Gel GMHxl-XT columns
from Tosohaas (mobile phase o-dichlorobenzene; tempera-
ture 145.8 C; flow rate 0.8 mL/min; injection volume 300
mL). Universal calibration with polystyrene standards was
used.
1
delays corresponding to
JCH values of 130 and 160 Hz, for
the creation of the antiphase magnetization.
ꢁ
On the basis of the proposed assignments, the NArOH content
in the produced copolymers was calculated by the following
equations:
½
NArOHꢃ
RESULTS AND DISCUSSION
NArOH ðmol%Þ ¼
ꢂ 100
½
NArOHꢃ þ ½Eꢃ
Synthesis of 2,6-Di-t-butyl-phenol-norbornene
comonomers
We focused the synthetic efforts to the preparation of (exo)-
2,6-di-tert-butyl-4-(bicyclo[2.2.1]hept-5-en-2-yl)phenol, indi-
cated as exo-1 thereafter, since it bears a 2,6-di-tert-butyl
substituted phenol that ensures an high antioxidant protec-
tion and it possesses the correct exo geometry for an effi-
cient participation to polymerization.
inspired to BHT (butylated hydroxy toluene, i.e., 2,6-di-tert-
butyl-4-methylphenol) an antioxidant additive world-widely
used, for the protection of, inter alia, plastic materials to-
ward oxidative damage.
INArOH
NArOH ðmol%Þ ¼
ꢂ 100
#
(1)
(2)
INArOH þ IE
"
2
1
X
1
INArOH ¼
ðInx þ InnÞ
21
n¼1
where Inx indicates the area of the signal of the carbon n of
the exo isomer and Inn the area of the signal of carbon n of
the endo isomer.
14–16
Indeed, exo-1 was
In particular:
INArOH ¼ ðI7x þ I7nÞ
(3)
The synthesis of exo-1 can be envisaged either via a Diels–
Alder (DA) reaction of cyclopentadiene with the proper
styrene (Scheme 1, path a) or via a reductive Heck cross-
coupling of norbornadiene with 2,6-di-tert-butyl-4-bromo-
phenol (Scheme 1, path b). While a mixture of exo-1 and
For the above equation, the signals of the exo–endo pair of
C7 methylene carbon are chosen as being the only ones not
overlapped with other signals.
2
9,30
endo-1 is expected using a DA procedure, literature data
1
IE ¼ IE
(4)
support the use of a reductive Heck reaction for achieving
the complete control of the exo stereochemistry of norborna-
diene arylation (Scheme 1).
2
where I is the total area of methylene peaks, from 28.12 to
E
2
7.40 ppm. Due to the overlap of S , S , and cB , the inten-
ad cd 1
Commercially available 2,6-di-tert-butyl-4-bromophenol was
reacted in DMF at 90 C with an excess of norbornadiene in
sities of such peaks can be separated using the following
expression that involves the correlation between integrated
peak areas of signals of sequences linked by stoichiometric
relationships (bB ¼ cB ):
ꢁ
the presence Pd(AcO) and PPh (5% and 10% mol, respec-
2
3
tively), using 4 equiv. of ammonium formate as reducing
agent (Scheme 2). After 24 h, we observed the complete
reaction of the starting bromide and the formation of the
adduct exo-1 (vide infra) as the sole isomer, regrettably, con-
taminated by a significant amount (ꢄ 20%) of 2,6-di-tert-
butylphenol, formed as the result of a competitive reductive
de-bromination. This process, not reported in the case of
1
1
S þ S ¼ area of the peak at 28.12 ppm—area correspond-
ad
cd
ing to bB peak at 25.18 ppm.
1
Molecular Analysis
Molecular mass and molecular mass distributions were
determined by using a GPCV2000 system from Waters,
2
9,30
unsubstituted 4-halophenols,
is particularly critical since
4
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SCHEME 3 RDA/DA mediated stereolability of exo-1.
SCHEME 2 Reductive-Heck preparation of exo-1 and exo-2.
derivative exo-2 as described in Scheme 2, undergoes an
unavoidable isomerization that, depending on the storage pe-
riod and conditions, causes the formation of variable
amounts of the endo-1 stereoisomer. As a matter of fact, the
copolymerizations described in the next section were carried
out using two different samples of comonomer 1, namely a
the two products are very difficult to separate. Considering
the high nucleophilicity of the aromatic ring as one of the
possible reasons of the observed de-bromination, the proce-
dure was repeated under the same reaction condition but
with the O-acetylated 2,6-di-tert-butyl-4-bromophenol as
starting material. Positively, the corresponding O-acetylated
norbornene adduct 2 (NArOAc) was obtained in good yields,
with only trace amounts (ꢄ 4%) of the corresponding de-
brominated derivative, yet again as single exo diaster-
eoisomer (Scheme 2).
1
.1/1 and a 6/1 mixture of exo/endo isomers respectively as
1
verified registering the H NMR just before to run the
copolymerization. Once the double bond of exo-1 (or endo-1)
has participated to the polymerization, the RDA process is
no more allowed, hence, in the final copolymers the [2.2.1]
bicyclic system results stereochemically and structurally very
stable.
Lithium aluminum hydride reduction of the acetyl group of
exo-2 and silica gel column chromatography allowed the isola-
tion of exo-1 as pure compound (vide infra). The exo geome-
try of NArOH and NArOAc was demonstrated by comparison of
their spectroscopic data with those of similar derivatives
Ethylene/2,6-Di-t-butyl-phenol-norbornene
copolymerizations
The copolymerization of ethylene with 2,6-di-t-butyl-phenol-
norbornene (NArOH) with the homogeneous MAO-activated
3
1
available in the literature. Moreover, it was possible to
obtain suitable crystals of O-acetylated exo-2 to run a X-ray
analysis which definitely demonstrated the exo geometry of
compounds deriving from the reductive Heck reaction (Fig. 2).
rac-Et(Ind) ZrCl (EI) catalyst was investigated. EI was cho-
2
2
sen because it is able to incorporate bulky comonomers, spe-
cifically norbornene (N), in ethylene-based copolymers. Trii-
sobutylaluminum (TIBA) was used to protect the hydroxyl of
Interestingly, we were able to point out how compound exo-
was not stereostabile on standing, since variable amount
NArOH in order to prevent the catalyst deactivation. The
1
copolymerization reactions were performed at 0.5 atm of
ethylene gas pressure: the reduced pressure was chosen to
promote the insertion of the bulky polar comonomer with
respect to ethylene. Polymerization times were chosen since
to avoid the complete comonomer conversion to prevent the
formation of polyethylene chains. The protected hindered
phenol does not act as a poison for metallocene complex
of the endo-1 isomer, along with other minor by-products,
are formed keeping exo-1 either in solution or as pure com-
pound. Such behavior was not observed in exo-2 that was
configurationally stable for months. Most likely, this phenom-
enon is caused by a retro Diels–Alder (RDA)/DA process that
occurs from both free phenolic isomeric derivatives exo-1
and endo-1 and foresees the formation of cyclopendadiene
and 2,6-di-tert-butyl-4-vinylphenol as reactive intermediates
and, under the proper polymerization conditions, could posi-
35–38
tively affect the polymerization activity.
ization procedure is sketched in Scheme 4.
The copolymer-
(
Schemes 3 and 1a).
Literature data on the peculiar electronic characteristics, and
consequential attitude to RDA processes, of norbornene-
The two NArOH isomeric mixtures (1.1/1 and 6/1) were pre-
treated in toluene solution for 2 h with 1.2 equiv of TIBA.
An ethylene/norbornene (E/N) copolymer was also pre-
pared, under the same conditions, as a model. In order to
obtain a copolymer containing exclusively an exo comonomer,
the copolymerization of ethylene was carried out with exo-2
3
2
based cycloadducts, as well as the well-known anionic RDA
3
3,34
acceleration,
which in this case can be effective only on
phenolic compounds 1 but not on ester 2, corroborate our
hypothesis. Indeed, comonomer exo-1, obtained as single
stereoisomer by reduction of the corresponding acetylated
(
NArOAc). In this case NArOAc was pre-treated with 2.2 equiv
of TIBA to ‘‘protect’’ both oxygens of the carboxylic group. In
spite of the protection, a noticeable catalyst deactivation was
FIGURE 2 X-ray structure of exo-2 (NArOAc).
SCHEME 4 Ethylene/NArOH copolymerization procedure.
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a
TABLE 1 Ethylene/NArOH and Ethylene/NArOAc Copolymerizations With rac-[Et(Ind)
2
2
ZrCl ]/MAO: Analytical Results
Incorporated
b
Activity
Comonomer
[mol%—(exo/
endo)]
c
Comonomer
PE
[E]/
Yield (gpol/
mmolcatꢅhꢅP
Conversion
(wt %)
M
w
3
c
[
mmol-exo/endo)] (Atm) [com] (mg)
E
)
(ꢂ10 )
w n
M /M
d
(
(
(
(
(
Run 1) N
1
1
13.34
6.67
547
470
450
390
230
1,642
1.98
36.9
7.0
322
418
n.d.
n.d.
314
2.2
3.0
Run 2) NArOH
Run 3) NArOH
Run 4) NArOH
Run 5) NArOAc
1–(1.1/1)
2–(1.1/1)
0.5–(6/1)
1 (>99/1)
0.5
0.5
0.5
0.5
1,880
1,800
1,560
184
0.43–(3/2)
1.20–(3/2)
0.78
3.35
8.6
n.d.
n.d.
9.8
13.34
6.67
20.1
9.7
e
f
1.36–(>99/1)
a
d
Polymerization conditions: solvent ¼ toluene, total volume ¼ 100 mL,
t
pol ¼ 10 min.
e
[
com]/[TIBA] ¼ 1:1.2, tpol ¼ 15 min, [catalyst] ¼ 2 mmol, [MAO]/[Zr] ¼
[com]/[TIBA] ¼ 1:2.2, [catalyst] ¼ 5 mmol, [MAO]/[Zr] ¼ 2000 (mol/mol),
ꢁ
3
b
,000 (mol/mol), T ¼ 30 C.
t
pol ¼ 30 min.
13
Determined by C NMR.
f
13
Determined by C NMR after reduction.
c
n w n
M and M /M determined by SEC.
ꢁ
ꢁ
observed when the polymerization conditions adopted for
ArOH were applied to NArOAc and negligible amounts of co-
polymer were produced. Different polymerization conditions
specifically, higher amount of catalyst and increased poly-
monly conducted at 28 C were repeated at 103 C to facilitate
the comparison with the spectra of the ethylene base copoly-
mers that are soluble only at high temperature. The complete
peak assignment is available as Supporting Information.
N
(
merization time) were required to achieve acceptable
yield of copolymer. Eventually, the copolymer obtained with
The general structure and carbon indexing of an E/NArOH co-
polymer chain is sketched in Figure 4. For the backbone
methylene carbons (S) the two Greek subscripts indicate the
distances from the neighboring methines. The carbons
related to the exo and endo isomers are designated by sub-
scripts x and n respectively.
E/NArOAc was reduced with LiAlH , as described in the
4
Experimental Section, since to obtain the free ArOH group
and hence a pure exo-E/NArOH copolymer. The copolymeriza-
tion results are summarized in Table 1.
1
3
NMR Structural Determination of Comonomer
and E/NArOH Copolymers
Figure 3 shows the C indexing and the NMR spectrum of the
For the C NMR assignment of E/NArOH structure the copol-
ymer containing 1.20% mol of comonomer (Table 1, Run 3)
1
3
was used. The complete C NMR spectrum is presented in
Figure 5. In the expanded aromatic region the signals of the
carbons of the 2,6-di-t-butyl-phenolic substituent (C –C ) of
both the exo and endo isomers were assigned by comparison
with the spectrum of the comonomer (see previous section).
No traces of further olefinic species are detectable in this
region.
6/1 exo/endo sample of comonomer 1 (Fig. 3).
a
d
The aromatic region of the spectrum was assigned by com-
parison with literature data regarding a different comonomer
3
–5
bearing the same phenolic substituent.
A DEPT 135
experiment was acquired to discriminate methyl and methine
groups. The use of HSQC and HMBC NMR techniques helped
for the distinction between C2 and C3 olefinic carbons, as
In Figure 5(a, b), the expanded aliphatic region from 24 to
48 ppm of the spectrum of Figure 4 is shown along with the
corresponding region of the spectrum of the E/N copolymer
6 5
being three bond correlated to the H methylene and H
methine protons, respectively. C , C , and C are thus assigned
4
1
7
(
see Supporting Information). The NMR experiments com-
1
3
FIGURE 4 Carbon indexing in E/NArOH copolymer and C NMR
spectrum of the E/NArOH copolymer (1.20 mol % of comono-
mer) and assignments of the aromatic region.
1
3
FIGURE 3 Carbon indexing and C NMR spectrum of a sample
of a 6/1 exo/endo-1 mixture.
6
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FIGURE 6 Expanded aliphatic regions of the spectrum of Fig-
ure 4 (a) and of the copolymer containing the pure exo-1
comonomer (b).
low field cross-peaks arise only from H (C ) long-terms con-
b
b
nectivities, these resonances were considered as a starting
point for the other resonances assignments, as it follows. Hb
proton (7.5 ppm) correlates with the aromatic carbons C
149.45 ppm), Ca (136.18 ppm), Cc (133.95 ppm), and Ce
d
(
1
3
FIGURE 5 C NMR spectrum of a model ethylene/N copoly-
mer (a) and expanded aliphatic regions of the spectrum of Fig-
ure 4 (b).
(32.46 ppm). Noticeably, it correlates with C5x that conse-
quently was safely assigned at 46.27 ppm. To discriminate
between C1x and C4x, we took advantage of the specific cor-
relation of H (C ) with the C carbon.
4
4
a
(
Table 1, Run 1). By comparing the two spectra, the well
The bidimensional technique also permitted to check most
of the above described assignments. Signals of comonomer
detected signals between 28.12 and 27.93 ppm could be
safely assigned to the ad, cd, and bd methylene carbons of
6
,17
the chain.
The presence of these signals is diagnostic of
the occurred insertion of the comonomer into the chain, an
evidence boosted by the absence of any trace of unreacted
comonomer in the olefinic region of the spectra.
In Figure 6 (a, b) the expanded aliphatic region from 30 to
48 ppm of the spectrum of Figure 4 is shown along with the
corresponding region of the spectrum of the pure exo-copoly-
mer prepared reducing the E/NArOAc copolymer. The compar-
ison of the two patterns allowed for the complete distinction
between the chemical shifts of the endo and exo inserted
comonomer forms.
By comparison with the assignment of C and C methine in
2
3
1
8
ethylene/hydroxyl norbornene copolymers,
we assigned
the signals at 45.75 and 44.37 ppm to C2x and C3x, respec-
tively, and the signals at 45.66 and 43.83 ppm, of equal in-
tensity, to the corresponding C2n and C3n.
In the DEPT spectrum, two CH signals are detected at 37.73
2
and 32.76 ppm, respectively, while the former can be
3
9
assigned to C7x on the basis of data reported in literature,
the latter is not as easily attributed. According to literature,
we hypothesized that the 6x and 6n methylenes are likely
overlapped by the intense signals of C carbon atom belong-
f
1
8
ing to the tert-butyl group at 28.80 ppm.
The three
methine carbons C C5x, and C4x were assigned with the
1
x,
1
13
help of the H– C bidimensional heteronuclear NMR techni-
ques (Fig. 7). Considering that in the HMBC spectrum the
FIGURE 7 HSQC (a) and HMBC (b) spectra of Run 3 in Table 2.
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1
3
TABLE 2 C NMR Chemical Shift Assignments for E/NArOH and
accrued out using MAO-activated rac-Et(Ind) ZrCl2 as cata-
2
for E/N Copolymers
lyst and protecting the polar function of comonomers with
TIBA. In comparison to N, the TIBA protected NArOH showed
an obviously lower incorporation rate into the polymer
chain. This result was expected due to high steric demand of
the TIBA protected comonomer. A sensible difference in
incorporation was also observed among the 6/1 and 1.1/1
comonomer samples. The results reported in Table 1 showed
that exo-enriched comonomer (6/1) is markedly more reac-
tive than the 1.1/1 sample. As a consequence, the former
allows a higher comonomer conversion (ꢄ 20%). Both
copolymers resulted enriched in the more reactive exo form
with respect to the starting comonomer, confirming the gen-
eral preferential insertion of the exo isomer in the growing
chain. However, despite such selectivity, because of the ster-
eochemical and structural instability of the comonomer, the
presence of a variable amount of the endo form in the
copolymers appears unavoidable. Actually, the whole antioxi-
dant effect of macromolecular additive will be directly
related to the amount of the inserted comonomer independ-
ently on its exo/endo ratio. In this regard, the present study
will be particularly useful for future applications of these
additives since it establishes a relationship between the
amount of comonomer insertion as function of the exo/endo
ratio of the comonomer used. The copolymerization of the
pure exo-2 comonomer, followed by reduction of the ester
group, allowed the preparation of a copolymer containing
the pure exo form and a comonomer content about three
times higher than that obtained in Run 2 with the same
comonomer molar fraction. However, the low polymerization
activity deriving from the partial poisoning of the catalyst
discourages the use of such procedure. In this context, our
aim was mainly to obtain copolymers suitable for micro-
structural investigation as a mandatory requisite for future
studies on the effectiveness of this new family of polymeric
additives in protecting the polyolefin matrix.
a
E/NArOH Chemical
Shift (ppm)
E/N Chemical
Shift (ppm)
Carbon
Isomer
C
C
C
C
C
C
C
C
C
C
d
a
c
exo
149.45
149.49
136.18
131.92
133.95
133.75
121.46
122.44
44.37
43.83
46.27
45.39
45.75
45.66
46.67
40.64
40.07
36.44
37.73
32.88
35.28
32.46
32.33
28.80
28.80
28.73
28.73
28.12
27.93
27.73
25.18
17.93
endo
exo
endo
exo
endo
exo
b
3
5
2
4
1
7
endo
exo
45.03
28.33
45.03
39.50
39.50
30.90
endo
exo
endo
exo
endo
exo
endo
exo
endo
exo
endo
aB
1
C
C
C
e
6
f
exo
endo
exo
28.33
endo
exo
endo
S
S
S
adþScdþcB
bd
1
28.07–28.12
27.95
CONCLUSIONS
dd
27.73
bB
1
1
A norbornene comonomer bearing an antioxidant hindered
phenol was prepared and, using a metallocene catalyst,
reacted with ethylene to give new copolymers with variable
amounts of the stabilizing unit. A carefully and exhaustive
NMR structural determination was carried out to quantify
the exo/endo ratio on comonomer and copolymers and to
study its effect in the rate of polymerization. The results
obtained allowed to establish the suitable condition for the
preparation and NMR characterization of E/NArOH copoly-
mers to be used as ‘‘masterbatches’’ in blends with commer-
cial polyolefins, for specific application in safe food and/or
drug packaging. A forthcoming paper will illustrate more in
detail the production of polymeric films, using these macro-
molecular additives and the properties that make them suita-
ble for food packaging. Specifically, the follow-up paper will
examine the studied films’ effectiveness in terms of thermal,
thermo-oxidative, photo-stability, and no releasing character
compared with those of low density polyethylene (LDPE)
films containing traditional antioxidant additives.
1B
a
17
Ref.
homosequences were not detectable, suggesting that the
obtained products are random copolymers containing only
isolated comonomer units. A list of complete peak assign-
ment is shown in Table 2.
DISCUSSION
Using a reductive Heck coupling of norbornadiene and
an acetylated 4-bromophenol it is possible to obtain a new
antioxidant bearing comonomer NArOH that, in its free pheno-
lic form, is not stereostabile. Two exo/endo mixtures of
derivative 1 were tested as potential new comonomer for
the preparation of macromolecular antioxidant additives by
copolymerization with ethylene. Polymerizations were
8
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ACKNOWLEDGMENTS
16 Bergstrom, C. H.; Ruotoistenmaki, J.; Aitola, E. T.; Seppala,
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This study was supported by Cariplo Bank Fundation: Project
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partners of the Project: Paola Stagnaro and Sara Limbo. Thanks
are due to Antonella Caterina Boccia for expertly performing
NMR analysis and to Daniele Piovani for SEC measurements.
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