Download Linear Functionalized Polyethylene Prepared with Highly Active Neutral Ni(II) Complexes

Linear Functionalized Polyethylene Prepared with Highly
Active Neutral Ni(II) Complexes
ERIC F. CONNOR,1 TODD R. YOUNKIN,1 JASON I. HENDERSON,1 SONJONG HWANG,1 ROBERT H. GRUBBS,1
WILLIAM P. ROBERTS,2 JOHNATHAN J. LITZAU2
1
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, Division of Chemistry and Chemical Engineering,
California Institute of Technology, Pasadena, California 91125
2
Cryovac, Incorporated, Sealed Air Corporation, 100 Rogers Bridge Road, Building A, Duncan, South Carolina 29334
Received 13 April 2002; accepted 22 May 2002
Published online 00 Month 2002 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/pola.10370
Neutral Ni(II) salicylaldimine catalysts (pendant ligand ⫽ NCMe or PPh3)
were used to copolymerize ethylene with monomers containing esters, alcohols, anhydrides, and amides and yielded linear functionalized polyethylene in a single step.
␣-Olefins and polycyclic olefin comonomers carrying functionality were directly incorporated into the polyethylene backbone by the catalysts without any cocatalyst, catalyst
initiator, or other disturber compounds. The degree of comonomer incorporation was
related to the monomer structure: tricyclononenes ⬎ norbornenes ⬎ ␣-olefins. A wide
range of comonomer incorporation, up to 30 mol %, was achieved while a linear
polyethylene structure was maintained under mild conditions (40 °C, 100 psi ethylene).
Results from the characterization of the copolymers by solution and solid-state NMR
techniques, thermal analysis, and molecular weight demonstrated that the materials
contained a relatively pure microstructure for a functionalized polyethylene that was
prepared in one step with no catalyst additive. © 2002 Wiley Periodicals, Inc. J Polym Sci
ABSTRACT:
Part A: Polym Chem 40: 2842–2854, 2002
Keywords: ␣-olefins; functionality; heteroatom-containing polymers; linear polyethylene; neutral nickel catalyst; norbornene; salicylaldamine; tricyclononene
INTRODUCTION
The incorporation of functional groups into the
hydrocarbon backbones of polyolefins has been a
highly desired modification for many years.1,2 It is
expected to expand the scope of polyolefin applications by improving adhesion, polymer miscibility, gas diffusion characteristics, and toughness.
In addition, the possibility of introducing reactive
functional groups along a polyolefin backbone
would allow for mild chemical modification and,
Correspondence to: R. H. Grubbs (E-mail: rhg@its.
caltech.edu)
The supplementary material referred to in this article can
be found at http://www.interscience.wiley.com/jpages/0887624X/Suppmat/2002/40/v40.2842.html.
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 40, 2842–2854 (2002)
© 2002 Wiley Periodicals, Inc.
2842
therefore, further alteration by the chemist.3 For
polyethylene to function as a high-performance
thermoplastic material, a high molecular weight
and a crystalline melting point (Tm) are usually
among the most desired properties.4 However, the
incorporation of functionality directly into polyethylene often leads to a material that fails to
meet one or both of these characteristics.5 Typical
examples include commercial samples of poly(ethylene-co-alkylacrylate) that are prepared with extreme pressures (2000 atm) and high temperatures (⬎200°C) via the radical reaction of ethylene and acrylates.6 Materials formed in this
fashion are characteristically branched copolymers that exhibit low Tm’s (⬃100 °C).7
The coordination polymerization of olefins offers an approach to more linear polymers (Scheme
LINEAR FUNCTIONALIZED POLYETHYLENE
Scheme 1
1). Polyolefin catalysts are typically electrophilic
or oxophilic and, therefore, are poisoned by the
presence of functionalized olefins.1 Heteroatoms
have been incorporated into polyethylene using
early metal and metallocene catalysts but often
require the following reaction modifications:
masking the functionality as an innocuous species (such as a borate), pre-complexation of functional groups by stoichiometric amounts of Lewis
acidic species, and block copolymerization via two
reactions.1,8 –15 More promise, however, has been
observed with late metal catalysts because they
are more tolerant of polar functionalized compounds.16 The neutral nickel catalysts based on a
PO chelate used on an industrial scale in the
Shell higher olefin process (SHOP) to generate
ethylene oligomers have been reported to incorporate functional groups into polyethylene.16 Klabunde and Ittel17 showed that functionalized linear low-density polyethylene (LLDPE) could be
prepared with POO chelate nickel complexes. For
respectable catalytic turnover numbers (TONs),
these catalysts often require high ethylene pressures and/or a cocatalyst to form higher molecular
weight products. Longer chain ␣-olefins carrying
functionality were reported to be incorporated
into the polyethylene backbone.17,18 Recently
Mecking demonstrated that neutral Ni complexes
containing a SHOP NO type chelate operate in
pure water to give high molecular weight polyethylene.19,20 Gibson recently reported the incorporation of acrylates into polyethylene with Ni catalysts with bulky PO ligands.21 Interestingly, acrylate incorporation was found at the chain ends
of the polyethylene.
Cationic nickel catalysts are believed to form
intramolecular chelates with acrylate-type monomers, and so copolymerizations with these types
of monomers have proven to be difficult.22 A significant breakthrough for these systems was re-
2843
ported recently by Johnson and McLain.23,24 They
described the direct incorporation of alkyl acrylates into polyethylene with cationic nickel catalysts by the addition of an excess of borate salt
that allowed the further insertion of ethylene.
The addition of another catalyst component was
required for the cationic nickel systems to operate
in the presence of heteroatom functionality. However, the cationic palladium systems were singlecomponent for polyethylene-functionalized materials. The insertion of methacrylate has been
shown to form four-, five, and six-membered chelates with the carbonyl coordinated to the palladium metal.25 The hydride elimination–reinsertion reaction cycle ring-expanded chelate structures and allowed the subsequent insertion of
ethylene. Up to 20 mol % methacrylate could be
incorporated into polyethylene. The polymers produced were highly branched (⬃100 branches per
1000 carbons), and the functionality was at the
end of the branches. This provided novel materials with glass-transition temperatures (Tg’s) of
⫺67 to ⫺77 °C.26 –28
Functionalized norbornenes containing esters
and free carboxylic acids have been homopolymerized via an insertion mechanism with cationic
palladium catalysts to give high molecular weight
polymers.29 –31 By reducing the steric bulk on the
diimine ligands of the cationic late metal systems,
Goodall et al. synthesized random copolymers of
methacrylate and norbornene, obtaining wide
control over the comonomer content.32 SHOP
nickel catalysts were used by the same group to
give ethylene-functionalized norbornene copolymers.33 Generally, the norbornene content was
high in Goodall et al.’s materials (⬎40%), and this
allowed the preparation of amorphous polymers
with Tg’s ranging from 30 to 300 °C and higher,
which depended on the bicyclic comonomer content.
We have reported the neutral nickel catalysts 1
[pendant ligand (L) ⫽ NCMe] and 2 (L ⫽ PPh3),
which provide high molecular weight, linear polyethylene in the presence of polar solvents such as
ethers, esters, alcohols, amides, amines, and even
water (Scheme 2).34,35 These catalysts contain
specifically tailored bulky salicylaldamine ligands, which greatly increase both the catalyst
activity and lifetime. The complexes operate at
low pressures of ethylene (⬃1–7 atm), allowing
reactions of less reactive olefins with respect to
ethylene. Catalysts 1 and 2 operate by themselves, with no cocatalyst additive, and allow the
incorporation of sensitive functionalities directly
into polyethylene.
2844
CONNOR ET AL.
Scheme 2
The resting state of the catalysts is believed to
be the ligated species, in which NCCH3 or PPh3
occupies the open coordination site. Although dissociation of the pendant ligand leads to the formation of the same active species for complexes 1
and 2, 1H NMR experiments have shown that a
higher percentage of initiation over the same
timeframe and the same conditions occurs with
the nitrile complex 1. Therefore, polymerizations
conducted at lower ethylene pressures are more
productive with catalyst 1. Accordingly, functionalities that do not strongly coordinate to the metal
center were found to be successful with this system. We report the preparation of a number of
functionalized polyolefins, ranging from semicrystalline materials (Tm ⬃ 80 –128 °C; 0.1– 6 mol %
comonomer) to amorphous materials (Tg ⬃ 72 °C;
30 mol % comonomer). We describe the limitations and advantages of polymerization reactions
that have been obtained with the single-component neutral nickel catalysts 1 and 2.
EXPERIMENTAL
available hex-1-ene epoxide and then hydrolyzed
via stirring in dilute H2SO4 overnight. Extraction
with diethyl ether was followed by stirring in the
presence of Na2SO4. The diethyl ether was removed with reduced pressure, and this yielded
the diol. Subsequent protection with 2-methoxypropene in the presence of amberlyst resin gave
the dioxolane 5. The characterization matched
published data.37,38
Preparation of Tricyclo[4.2.1.00,0]non-7-ene-3carboxylic Acid t-Butyl Ester (10)
Quadricyclane (12 mL) and t-butyl acrylate (92
mL) were placed in a thick-walled Schlenk tube.
The components were degassed and then sealed
under an atmosphere of nitrogen. The reaction
mixture was heated to 96 °C for 8 h. The product
was obtained by distillation. The isolated yield
with respect to quadricyclane was 80% (bp ⫽ 120
°C at 1.00 mmHg).
NMR spectroscopy revealed that endo and exo
isomers were present (3/1 ratio). The assignments
of the endo isomer follow.
1
H NMR (CDCl3, 300 MHz, 25 °C): 5.90, 5.91
(2H, bs), 2.68 (1H, bs), 2.60 (1H, bs), 2.28 –2.27
(2H, m), 2.20 (1H, m), 1.98 (1H, d, JHH ⫽ 9.3 Hz),
1.61 (1H, d, JHH ⫽ 10.0 Hz), 1.41 (10H, s), 1.26
(1H, d, JHH ⫽ 10.0 Hz). 13C NMR (CDCl3, 75.47
MHz, 25 °C): 175.32, 135.78, 134.57, 79.76, 44.04,
43.83, 40.41, 40.27, 38.37, 34.11, 28.03, 23.66. IR
(KBr): 1718.3 cm⫺1 {␯[C(O)]}. ELEM. ANAL. Calcd.
for C14H20O2: C, 76.33%; H, 9.15%. Found: C,
76.41%; H, 9.12%.
Materials
All manipulations and polymerizations were carried out in an N2-filled drybox or with standard
air-sensitive or vacuum-line techniques under argon. Argon was purified by passage through columns of BASF RS-11 (Chemalog) and Linde 4-Å
molecular sieves. Ethylene was purified by passage through an O2 scrubber (Matheson model
6410 oxygen-absorbing purifier), and water was
removed by molecular sieves.
Solvents were rigorously degassed in 18-L reservoirs and passed through two sequential purification columns. Protic contaminants were removed with activated alumina, and a supported
copper catalyst was used to remove trace oxygen
from hydrocarbons.36 Compound 2 was synthesized according to a published procedure.34 The
characterization and synthesis of 1 will be reported in a forthcoming article on the catalysts.
Compound 5 was prepared from commercially
Synthesis of Tricyclo[4.2.1.00,0]non-7-ene-3,4dicarboxylic Acid t-Butyl Ester (11)
A previously reported adduct of quadricyclane
and maleic anhydride39 was hydrolyzed by 16.2 g
of the white solid being placed in a flask with 100
mL of H2O. NaOH (0.4 g) was added to the solution, which was then set to reflux for 8 h. The
water was removed to give a white solid that was
dried under reduced pressure. t-BuOH (50 mL),
the white solid, and 1.5 mL of concentrated
H2SO4 were placed into a glass bomb with a pressure gauge. Isobutylene gas (70 mL) was condensed in the glass bomb (⫺70 °C). The flask was
allowed to warm to room temperature. After the
reaction was stirred for 8 h, the flask was heated
to 84 °C (70 psig in the flask) for 3 h. The flask
was cooled to ⫺70 °C, and the isobutylene gas was
allowed to evaporate. The oil that remained was
dissolved in pentane and washed with a saturated
LINEAR FUNCTIONALIZED POLYETHYLENE
water solution of K2CO3. After the pentane layer
was dried with MgSO4, 22.0 g of the crude product
was obtained, and it crystallized upon standing.
The product was purified by column chromatography with a 1/9 mixture of ethyl acetate and
hexanes.
NMR spectroscopy revealed that exo and endo
isomers were present (3/1 ratio).
Spectral Data for the Endo Isomer
1
H NMR (CDCl3, 300 MHz, 25 °C): 5.97 (2H, bs),
2.74 (2H, bs), 2.58 (2H, m, JHH ⫽ 3.5 Hz), 2.23
(2H, m, JHH ⫽ 2.1 Hz), 1.5–1.3 (20H, maxima at
1.48, 1.43, and 1.36). 13C NMR (CDCl3, 75.47
MHz, 25 °C): 172.32, 135.33, 80.42, 43.45, 40.89,
40.81, 37.90, 28.07.
Spectral data for the Exo Isomer
1
H NMR (CDCl3, 300 MHz, 25 °C): 5.98 (2H, bs),
3.34 (2H, m, JHH ⫽ 9.6 Hz), 3.14 (2H, bs), 2.12
(2H, m, JHH ⫽ 9.3 Hz), 2.02 (1H), 1.43 (18H, bs),
1.26 (1H). 13C NMR (CDCl3, 75.47 MHz, 25 °C):
169.98, 136.49, 80.00, 42.98, 41.83, 37.88 (two
overlapping peaks), 28.07. IR (KBr): 1721.5 cm⫺1
{␯[C(O)]}. ELEM. ANAL. Calcd. for C14H20O2: C,
71.22%; H, 8.81%. Found: C, 71.22%; H, 8.76%.
Synthesis of N-Butyl-tricyclo[4.2.1.00,0]non-7-ene3,4-dicarboxyimide (13)
A previously reported adduct of quadricyclane
and maleic anhydride39 was placed in a flask with
toluene and 1.1 equiv of n-butyl amine. The mixture was refluxed for 12 h with a Dean–Stark
apparatus. The product was isolated by distillation to give a light yellow solid. The isolated yield
with respect to the anhydride starting material
was 85% (bp ⫽ 142 °C at 0.25 mmHg).
1
H NMR (CDCl3, 300 MHz, 25 °C): 6.01 (2H,
bs), 3.51 (2H, t, J ⫽ 7.2 Hz), 2.94 (2H, bs), 2.65
(2H, bs), 2.10 (2H, bs), 1.6 –1.4 (6H, m), 1.3–1.2
(6H, m), 0.89 (3H, t, 7.5 Hz). 13C NMR (CDCl3,
75.47 MHz, 25 °C): 179.10, 135.81, 44.54, 41.71,
41.05, 40.95, 38.92, 30.10, 20.37, 14.05. IR (KBr):
1769.3, 1697.2 cm⫺1 {␯[C(O)]}. ELEM. ANAL. Calcd.
for C15H19NO2: C, 73.44%; H, 7.81%; N, 5.71%.
Found: C, 73.34%; H, 7.94%; N, 5.76%.
General Procedure for the Polymerization of
Ethylene with Comonomers via Ni Complexes
A 6- or 12-oz Fisher–Porter glass pressure bottle
was charged with the appropriate amount of the
Ni complex under an atmosphere of nitrogen (for
2845
the polymerization at 14 atm, a 200-mL autoclave
was used). Toluene (80 mL) was then cannulatransferred into the reactor and was followed by a
solution of comonomer in toluene (10 mL). The
ethylene pressure was raised to a particular value
and maintained for the specified times. MeOH/
HCl workup afforded polyethylene, which was filtered and dried in vacuo.
General Procedure for the High-Resolution Liquid
C NMR Characterization of Polyethylene
Copolymers
13
It was shown by Randall40 that the nuclear Overhauser effect (NOE) factor for many polyethylene
systems is nearly full and constant for all carbons
of the copolymer. Therefore, WALTZ (wideband
alternating phase low power technique for zero
residual splitting) decoupling sequences were
used for an improved signal-to-noise ratio, which
allowed the number of scans to be reduced. This is
important when the polymer of interest is sparingly soluble in the solvent blend. When the integrations did not agree with each other via
WALTZ, an inverse-gated experiment was run to
check if some of the signals were being selectively
enhanced. Little difference in the integration was
observed in most polyethylenes; with calculations
based on the averaging of several signals, any
error was minimized. Other things that were considered were the potentially long relaxation times
(T1’s) of end groups (e.g., methyls and methylenes
at or near the end of the polymer chain); these
signals were not used for quantitation because
they would not have fully relaxed when the next
pulse fired and, as a result, signal was lost in
these carbons. In a typical copolymer, the branch,
␣, and ␤ carbons were used to integrate. The sum
of all of these signals was made and then divided
by the number of carbons that they were attributed to get an averaged integration unit per carbon for that constituent. A 90° pulse (based on the
main methylene resonance at 30.0 ppm) and at
least 32,000 data points were collected on a
Bruker DMX-400 MHz NMR instrument with
high-temperature capabilities. Three thousand
scans, with a 10-s recycle delay, which amounted
to an 8-h run, constituted a typical experiment.
This was done to satisfy the requirements for the
signal-to-noise ratio (ASTM Method D 5017-96).
The NMR solvent mixture [⬃150-mg sample,
added in small aliquots, with 2.0 mL of 1,2,4trichlorobenzene (protonated), 0.75 mL of benzene-d6 (lock), and 0.25 mL of hexamethyldisiloxane (internal reference); 10-mm tubes] worked
2846
CONNOR ET AL.
Scheme 3
well for most polyolefins. The dissolution heating
block and NMR experiment were maintained at
130 °C because good temperature control was important (benzene-d6 begins to boil at ⬃130 °C,
and going above this could prove disastrous).
Small slits in the top of the NMR tube caps were
cut to avoid pressure buildup in the tube.
to be approximately 80 and 0.1 s at 4.7 T for the
crystalline and amorphous phases, respectively.
There was no noticeable NMR signal growth observed when the repetition time was increased to
1000 s.
RESULTS AND DISCUSSION
General Procedure for Gel Permeation
Chromatography (GPC) Analysis
GPC analysis was determined with a Waters 150
GPCV liquid chromatograph. It was equipped
with three detectors: a Waters differential refractometer, a Waters single-capillary viscometer,
and a Wyatt DAWN DSP light scattering detector. Several samples were prepared and yielded
approximately 0.1 wt % solutions in 1,2,4-trichlorobenzene. Four Waters high-temperature
␮Styragel columns (106, 105, 104, and 103 Å) at
140 °C were used to determine the molecular
weight distributions (MWDs) of the polymers.
The instrument was calibrated with TSK narrowpolydispersity polystyrene standards. A universal
calibration curve was created to determine the
MWDs. The data were collected and calculated
with Water’s Millennium software.
General Procedure for Solid-State NMR
The solid-state 13C NMR experimental procedure
was as follows. All magic-angle spinning (MAS)
13
C NMR experiments were carried out with a
Bruker DSX 200 (50 MHz for 13C) and a 7-mm
cross-polarity/magic-angle spinning (CP-MAS)
probe, except for a sample of 10 mol % comonomer
incorporation for which a Bruker DSX 500 (125
MHz for 13C) and a 4-mm BL CP-MAS probe were
used because of the limited amount of the sample.
Polymer powder (160 mg) was packed in a 7-mm
MAS rotor and was spun at 4.0 kHz and 4.7 T.
The 13C block decay signal was acquired with a
4-␮s, 90° pulse with high-power H-decoupling,
and the typical repetition time used was 500 s.
The spin–lattice relaxation times were estimated
Copolymerization of Ethylene and ␣-Olefins
Initially, we tested the commercially available
ethyl undecylenoate 3 to determine whether catalysts 1 and 2 could incorporate functionalized
␣-olefins and produce high molecular weight ethylene copolymers (Scheme 3). Trials 1a– d (Table
1) outline our discoveries with catalysts 1 and 2.
In analogy to our observations for ethylene homopolymerization, the acetonitrile catalyst 1
demonstrated a slightly higher activity for ethylene/ethyl undecylenoate copolymerizations than
the phosphine-containing catalyst 2 (cf. trials 1a
and b to trials 1c and d). The observed turnover
numbers (TONs) for both catalysts are ⬃2 ⫻ 105
grams of polymer ⫻ mol of catlayst⫺1 ⫻ h⫺1,
which is approximately 1 order of magnitude
slower than for ethylene homopolymerizations
performed in a noncoordinating solvent (e.g., toluene or benzene, ⬃6 ⫻ 106) but compares well
with the rates observed for ethylene homopolymerization in a more coordinating medium (e.g.,
ether, ⬃3.1 ⫻ 106, or ethyl acetate, ⬃6.8 ⫻ 105).
This rate suppression could be due to the coordination of the functional moiety or merely a reflection of a decreased insertion rate for ␣-olefins.
Klabunde and Ittel reported a similar effect with
their POO chelate nickel complexes.41 However,
a threefold increase in the concentration of 3 led
to only a minor decrease in the activity of both
catalysts 1 and 2 (cf. trials 1a and b and 1c– d).
Higher incorporation of 3 was obtained from both
catalysts in more concentrated solutions.
When a large excess of ethylene was employed,
the conversion of comonomer 3 into the polymer
was low (Table 2, trial 2c). However, high molec-
2847
LINEAR FUNCTIONALIZED POLYETHYLENE
Table 1. Comparison of Catalysts 1 and 2 for the Copolymerization of Ethylene and Ethyl Undecyleneoate 3a
Trial
Catalyst
Comonomer
3 (M)
TON
(⫻105)b
Branch
Contentc
Tm
(°C)
Comonomer
Incorporatedd
1a
1b
1c
1d
1
1
2
2
0.62
2.07
0.62
2.07
1.88
1.80
1.76
1.32
5
6
9
10
118
108
125
110
1.0
3.8
0.2
1.8
a
Conditions: 25 ␮mol of catalyst (2.5 mM); volume of toluene ⫹ comonomer 3 ⫽ 10 mL; 120 psig ethylene; reaction time ⫽ 15
min 40 °C bath.
b
TON ⫽ (g of polymer) ⫻ (mol of catalyst)⫺1 ⫻ (h)⫺1.
c
Total Me ⫹ Et ⫹ Pr ⫹ Bu branches per 1000 carbons as determined by 13C NMR.
d
Incorporation ⫽ molar percentage.
cause of protonation of the polymer chain on the
ligand from the catalyst. Protecting the diol functionality as the dioxolane monomer 5 led to
higher ethylene consumption by catalyst 1, and
this was reflected in both the molecular weight
and TON (cf. trials 2d and e).
The copolymerization of 1-octene 6 under similar conditions demonstrated a few noticeable differences (Table 3). First, it indicated that the
presence of functionality on the monomer affected
comonomer incorporation into polymer. Higher
contents of ␣-olefins (12.5 mol %, trial 3f) could be
obtained with 1-octene at lower concentrations
than for the functionalized ␣-olefins previously
discussed (Tables 1 and 2). As the molarity of the
␣-olefin in the polymerization mixture increased,
a corresponding decrease in the polymer molecular weight resulted (trials 3a, b, d, and e). This
contrasts with the functionalized ␣-olefins, for
which the molecular weight was less affected by
the comonomer concentration (trials 2a– c).
ular weight, linear copolymers were accessible,
and these materials fell into the category of highdensity polyethylene.4 When the polymerization
was run in almost neat functionalized ␣-olefins,
comonomer incorporation into polyethylene could
reach levels of nearly 10 mol %, and so the material moved toward an LLDPE.4 With high levels
of comonomer incorporation, the resulting polymer exhibited a decrease in Tm as predicted (in
trial 1b, 3.8 mol % incorporation reduced Tm to
108 °C).42 13C NMR spectra of poly(ethylene-coethyl undecylenoate)s of different compositions
are displayed in Figure 1. One can observe more
functional groups than branches (cf. the intensity
of the C1 peak with that of the methyl peak in
Fig. 1). When one considers the branch content in
functionalized polyethylenes that are produced as
a result of side reactions in a free radical polymerization process, we find that both spectra illustrate that these catalysts produced a material
with a relatively pure microstructure.5,42
␣-Olefins with free alcohols (5) could be incorporated (trial 2e). Again, the catalyst lifetime was
affected, as for the homopolymerization of ethylene in the presence of methanol,35 possibly be-
Branch Analysis
When the 13C NMR spectra of copolymers from
1-octene and ethyl undecylenoate were examined
Table 2. Copolymerization of Ethylene and Functionalized-␣-Olefin Comonomers with Catalyst 1a
Trial
2a
2b
2c
2d
2e
3
3
3
4
5
Comonomer
(M)
Mw
(⫻103)
PDI
(Mw/Mn)
Branch
Contentb
Tm
(°C)
Ethylene
TONc
Comonomer
TONc
Comonomer
Incorporatedd
3.61
2.16
0.72
0.72
0.72
176
204
165
172
54
2.4
2.1
2.5
2.3
2.4
8
7
8
5
8
125
128
124
124
125
5,396
6,011
12,400
6,080
1,362
17
10
25
19
5
0.4
0.2
0.2
0.2
0.3
Conditions: 65 ␮mol of catalyst (0.72 mM); 90 mL of toluene; 100 psig ethylene; 40 °C bath; reaction time ⫽ 8 h.
Total Me ⫹ Et ⫹ Pr ⫹ Bu branches per 1000 carbons as determined by 13C NMR.
TON ⫽ mol of substrate converted/mol of catalyst.
d
Incorporation ⫽ molar percentage.
a
b
c
2848
CONNOR ET AL.
Figure 1. 13C NMR spectra (75% C6H3Cl3, 25% C6D6, 400 MHz, 130 °C) of a copolymer of ethylene and monomer 3 produced by catalyst 1. The polymer contains (a) 10
mol % 3 or (b) 0.4 mol % 3.
at similar incorporation levels, differences in the
comonomer insertion mechanism became apparent. Therefore, when we compared the branch
content of polymers produced in trials 1b and 3a,
it was apparent that the presence of functionality
in the reaction solution reduced the branch content from 58 branches per 1000 carbons (trial 3a)
to 6 branches per 1000 carbons. An examination
of Table 2 demonstrates that the presence of heteroatoms, whether in the solvent or in the monomer, resulted in polymers with fewer branches in
comparison with the comonomers produced with
octene (Table 3).
With neutral nickel salicylaldamine catalysts,
copolymerizations with vinyl-functionalized or al-
lyl-functionalized monomers such as vinyl acetate
or methyl acrylate were unsuccessful under the
conditions used in this study. With methyl acrylate, no polymer was obtained. A solution of
methyl acrylate with catalyst 2, examined by
NMR, showed no observable interaction of the
monomer and the catalyst. Polymerizations with
catalysts 1 and 2 in the presence of methyl acrylate gave no isolable material; this suggested that
intramolecular chelation occurred during polymerization and prevented polymer chain growth.
With vinyl monomers such as vinyl ether, polyethylene was produced, but the incorporation of
the comonomer into the polyethylene backbone
was not observed.
LINEAR FUNCTIONALIZED POLYETHYLENE
2849
Table 3. Copolymerization of Ethylene and 1-Octene 6 with Catalysts 1 and 2a
1-Octene
Trial
Catalyst
mL
mM
Activityb
Mw
(⫻103)
PDI
(Mw/Mn)
Branch
Contentc
Comonomer
Incorporatedd
Tm
(°C)
3a
3b
3c
3d
3e
3f
1
1
2
1
1
1
0.1
0.5
1.0
1.0
2.5
10.0
2.2
10.5
22.5
22.5
55.0
235.0
5.27
2.75
3.27
2.48
1.22
0.95
12.8
8.8
—e
9.3
7.4
3.3
3.30
2.50
—e
2.10
3.40
2.10
58
67
76
66
82
106
1.7
2.4
1.7
4.0
6.5
12.5
111.5
104.8
120.0
89.4
Broad
Broad
a
Standard conditions (unless stated otherwise): 28 ␮mol of catalyst (2.8 mM); volume of toluene ⫹ comonomer 6 ⫽ 10 mL; 120
psig ethylene; 40 °C bath; reaction time ⫽ 1 h.
b
Activity ⫽ 105 g of polymer ⫻ (mol of catalyst)⫺1 ⫻ (h)⫺1.
c
Total Me ⫹ Et ⫹ Pr ⫹ Bu branches per 1000 carbons as determined by 13C NMR.
d
Incorporation ⫽ molar percentage.
e
Not determined.
Copolymerization of Functionalized Norbornenes
Although ␣-olefins were successfully copolymerized with ethylene, the results suggest that we
could employ a slightly more reactive olefin for
the reaction process (Scheme 4). Norbornene
monomers contain 19.2 kcal/mol of ring strain,43
and the relief of strain energy via insertion into
the saturated bicyclic structure (14.4 kcal/mol)
has been considered to be an important factor for
their increased activity in comparison with that of
an ␣-olefin. In addition, it has been observed that
the ␤-hydrogen elimination reaction occurs rarely
with norbornene monomers, presumably because
of the unfavorable geometry of the ␤ hydrogen on
the bicyclic monomer.44 There are reports that
under standard conditions, diimine Pd and Ni
polyethylene catalysts give no isolable ethylene–
norbornene copolymers, and so a study of these
types of monomers was conducted and reported.45
Reaction data for these monomer types with
catalysts 1 and 2 (Table 4) show that they were
readily incorporated into the polyethylene backbone with an improved molar percentage with
respect to that of ␣-olefins. Although the norbornene monomers were incorporated in a high
molar percentage, the overall TON was noted to
be lower than with the functionalized ␣-olefins. It
is likely that this reflects the reduced rate of
Scheme 4
insertion of ethylene into the metal norbornene
bond due to steric factors. The polycyclic ethylene
copolymers gave lower molecular weight materials, but the comonomer yield into the polymer
was higher.
Functionalized tricyclononene monomers (10–
13) contain an exocyclobutane (Scheme 5 and 6).
It was predicted that these monomers would be
incorporated more readily than their norbornene
analogues as a result of the functionality being
held further away from the inserting olefin (cf.
trials 5c and g). These monomers were also easy
to prepare in one step according to the method
described by Tabushi.39
The copolymerization data obtained for ethylene and 10 with the neutral nickel catalyst are
displayed in Table 4. The molecular weight decreased moderately when the ethylene pressure
was decreased from 7 to 1 bar (cf. trials 5d– g). As
observed, very high levels of comonomer incorporation (30 mol %) could be obtained with these
monomers by a lower pressure of ethylene being
applied (⬃5 psig) to initiate the complex in the
presence of comonomer (trial 5d). Because of the
reactivity difference between the smaller ethylene monomer and the less reactive cyclic olefin, it
was thought that with the concentration of ethylene decreasing in the polymerization solution, the
reaction of the less reactive monomer would occur
more frequently. A decrease in the TON was observed for both ethylene and the polycyclic
comonomer at lower ethylene pressures. It is believed that this resulted because of a combination
of incomplete catalyst initiation and catalyst decomposition.
2850
CONNOR ET AL.
Table 4. Ethylene Polymerization with Catalysts 1 and 2 in the Presence of Monomers 7–13a
Trial
R
Comonomer/
Ni
Ethylene
(psig)
Ethylene
TONb
Comonomer
TONb
Branch
Contentc
Mw
(⫻103)d
PDI
(Mw/Mn)
Tm
(°C)
Comonomer
Incorporatede
5a
5b
5c
5d
5e
5f
5g
5h
5ih
5j
7
8
9
10
10
10
10
11
12
13
0.16
0.14
0.36
0.14
0.15
0.15
0.36
0.18
0.02
0.13
100
100
100
⬍5
10
30
100
100
20
100
458
1540
2474
102
2014
2600
8585
1360
3240
7600
34
39
60
51
129
123
200
5
20
8
9
9
9
—f
11
14
14
5
43
7
17
70
74
30
55
41
76
59
—i
—i
1.6
2.1
1.6
1.2
1.6
1.6
2.3
2.9
—i
—i
105
110
109
—g
80
90
112
124
112
124
5
4
2
31
6
4
2
0.3
0.5
0.1
a
Conditions: 65 ␮mol (0.72 mM) of catalyst; 90 mL of toluene; 40 °C bath; reaction time 8 h. Trials 5a– b were conducted with
catalyst 2, and trials 5c–j were obtained with catalyst 1.
b
TON ⫽ mol of substrate converted/mol of catalyst.
c
Total Me ⫹ Et ⫹ Pr ⫹ Bu branches per 1000 carbons.
d
In g ⫻ mol⫺1.
e
Incorporation ⫽ molar percentage, as determined by 13C NMR.
f
No branches observed.
g
No Tm observed.
h
2,6-Di-tert-butylpyridine was added to the monomer solution before the reaction.
i
Data not recorded.
It was observed that the polymers with increasing comonomer incorporation reflected a corresponding decrease in Tm. With 6 mol % of the
comonomer 10 in polyethylene, Tm, as determined
by differential scanning calorimetry (DSC), appeared broad, and as the comonomer content in
the polymer increased, the melting endotherm
disappeared (Fig. 2). With 30 mol % comonomer
incorporation (trial 5d), DSC thermal analysis
showed an endothermic thermal transition between 68 and 75 °C (maximum rate, 71 °C) that
was assigned as Tg; no melting endotherm was
observed. These results agree with data from ethylene–norbornene copolymers made with early
metal metallocenes.46 The 13C NMR spectroscopic
analysis of these ethylene copolymers indicated
that the branch content within these copolymers
was generally less than 15 branches per 1000
carbons, with methyl branches being the most
prevalent (trials 5e– g, which contained 11, 14,
and 14 methyl branches per 1000 carbons, respectively). The 13C NMR spectra for the polycyclic
Scheme 5
copolymers described in this article indicated that
there were no comonomer dyads present, even
when the comonomer incorporation was high (see
Fig. 3). Resonances with a shift downfield from ␦
⫽ 46.0 ppm, typical of norbornene dyads in polyethylene, were not observed in our spectra.47 No
homopolymer was obtained from any norbornene
monomer with the single-component Ni catalysts.
Therefore, the theoretical limit of polycyclic olefin
incorporation for catalysts 1 and 2 is likely to be
50 mol %. It is probable that norbornene insertion
occurs preferentially at alternating ethylene
units because of steric encounters that occur with
the norbornene monomer and the ligand on catalyst 1. This characteristic is frequently exhibited
with metallocene systems.48 For this reason,
Goodall et al. modified the cationic nickel and
palladium catalysts containing diimine ligands so
that ethylene–norbornene copolymers could be
prepared.32 Although we have not yet found the
appropriate conditions or monomer, it should be
possible to form a strictly alternating ethylene–
tricyclononene or ethylene–norbornene copolymer with these catalysts. Moreover, monomer 10
contained the acid-sensitive tert-butyl ester
group, and it can be seen from the 13C NMR
spectrum (Fig. 2) that this functionality remained
fully intact. We think that this is illustrative of
how mild these nickel catalysts are and that this
demonstrates the advantage of using a neutral-
LINEAR FUNCTIONALIZED POLYETHYLENE
2851
Scheme 6
based catalyst rather than a cationic complex
more prone to generating highly acidic byproducts during the polymerization.
Copolymerizations of monomer 12 with ethylene demonstrated the incorporation of anhydride
functionality into an ethylene copolymer (trial 5i).
The catalyst TON was increased by treatment of
the anhydride monomer solution with a proton
sponge for the removal of trace amounts of acid
because carboxylic acids have been shown to protonate the ligand from the metal catalyst. For
further improvements of the TONs, the di-tertbutyl ester analogue 11 was prepared and incorporated into polyethylene (trial 5h). The complete
conversion of the di-tert-butyl ester to the anhydride was accomplished by the heating of the
functionalized polyethylene to 200 °C under re-
duced pressure (Scheme 7). This conversion could
be effected in a polymer melt during processing.
The structures of the anhydride– ethylene copolymers prepared from both 11 and 12 were found,
with 13C NMR spectroscopic analysis, to be similar. Tricyclononene n-butyl imide 13, prepared
from the anhydride 11, was also incorporated into
ethylene polymers. Polymerizations carried out
with 13 proceeded in a noticeably controlled fashion. This observation is reflected in the high ethylene TONs observed (trial 5j) and demonstrates
that certain functionalities actually stabilized the
nickel complexes toward side reactions over the
course of the polymerization.
The concentration of the methanol fraction
used to precipitate the polymer and analysis by
1
H NMR spectroscopy revealed the following
Figure 2. DSC heating curves for a copolymer of ethylene and monomer 10 (in air at
5 °C/min after cooling from 140 °C at 10 °C/min) showing both the melt transition and
the transition for thermal cleavage of the t-butyl ester: (a) polyethylene with 6 mol % 10
incorporated, (b) polyethylene with 4 mol % 10 incorporated, and (c) polyethylene with
2 mol % 10 incorporated.
2852
CONNOR ET AL.
Figure 3. 13C NMR spectrum (75% C6D3Cl3, 25% C6D6, 400 MHz, 130 °C) of a
copolymer of ethylene and monomer 10 produced by catalyst 1. The polymer contains 31
mol % 10.
when strained cyclic monomers were used. No low
molecular weight polyethylene fractions were ever
observed; only unreacted monomer and ligand
were identified. An analysis of the unreacted
methyl ester norbornene monomer 9 revealed
that there did not appear to be a significant
change in the initial–final isomer distribution.
This indicates that norbornene monomers did not
appear to exhibit an obvious reactivity difference
between the endo and exo isomers for the neutral
nickel catalysts 1 and 2.49 This was confirmed
from an NMR analysis of the polymers that contained an endo/exo ratio of 60/40, which was similar to the ratio found in the monomer feed. Similarly, 10 and 11 were found to contain anti/syn
isomer ratios of 40/60 and 45/55, respectively. The
tricyclononenes differed from the norbornene
monomers in that a reactivity difference between
stereoisomers was observed. The monomer isomer ratio found in the methanol from polymer
precipitation indicated that the anti isomer was
consumed faster than the syn isomer. The preference for one isomer is also reflected by the isomer
Scheme 7
ratio (determined by 13C NMR spectroscopic analysis) present in the isolated polymers.
A study was carried out with solid-state 13C
NMR techniques to characterize the crystallinity
within linear polyethylenes containing comonomer 3 (further details are given in the supplementary material). The polyethylene samples examined ranged from 3.8 to 0.1 mol % in comonomer
content. Spectra from these samples exhibited
solid-state 13C NMR signals between 26 and 35
ppm, and these were fitted with three Lorentzian
lines, as shown in the Figure 4. The peak at 31.3
ppm with a full width at half-maximum of 126 Hz
represents the amorphous-phase methylene carbons, whereas the two relatively narrow peaks at
32.8 and 34.1 ppm with a full width at half-maximum of 30 – 45 Hz were assigned as methylene
carbons in the crystalline phase.50 –52 The two
crystalline peaks indicate the presence of two
crystallographically distinct phases, which are
called the orthorhombic phase (32.8 ppm) and the
monoclinic phase (34.1 ppm) in previous reports.50,51 The quantification of the three different phases was carried out from the spectral peak
area of fitted lines and is summarized in Table 5.
The content of the crystalline phase for the samples examined was over 60%, and over 20% of this
phase was found to be the monoclinic crystalline
phase. Within the comonomer incorporation
range examined (which is expressed as a molar
percentage), the crystallinity increased as the incorporation decreased, as expected. No strong cor-
LINEAR FUNCTIONALIZED POLYETHYLENE
2853
Figure 4. Solid-state 13C NMR spectra of functionalized polyethylene containing
monomer 3. (a) Experimentally obtained spectra are shown on the top, and the bottom
curve shows the Lorentzian fit with the resulting simulated curve in the middle. (b)
Solid-state spectra are shown that were obtained experimentally for polyethylene with
different monomer loadings.
relation between the incorporation and the fraction of the monoclinic phase was observed.
CONCLUSIONS
Neutral, late transition-metal catalysts similar to
the SHOP catalyst were shown to be useful in
preparing a number of functional-group-containing, high molecular weight, lightly branched,
semicrystalline ethylene copolymers under mild
laboratory conditions. Significantly, these were
obtained directly from monomers containing heteroatoms with a single catalyst system. The potential that catalysts of the neutral nickel type
may have for making a variety of useful functionalized polyethylene materials was demonstrated.
High ethylene TONs were observed for catalysts 1
and 2, demonstrating a tolerance of these catalysts for both olefin functionality and a variety of
different oxygen and nitrogen functionalities. The
tricyclononene and norbornene monomers exhibited higher incorporation into the polyethylene
Table 5. Relative Ratio of the Crystalline Phases
Trial
Incorporation
(mol %)
Crystalline
Part (%)
Fraction of
Monoclinic Phase
(%)
2b
2c
2a
1b
0.2
0.2
0.4
3.8
63.3
65.5
60.5
66.7
26.7
30.9
28.4
10.6
backbone in comparison with ␣-olefins such as
ethyl undecylenoate. Functional groups that were
successfully incorporated directly into ethylene
copolymers in this manner included ester, imide,
and anhydride groups. The indirect incorporation
of anhydride groups was also demonstrated with
the thermolysis of vicinal di-tert-butyl ester
groups.
Support was provided by the National Institute for
Standards and Technology (through the Advanced
Technology Program) and Cryovac, a division of Sealed
Air Corp. GPC and polymer NMR analyses were carried out by Cryovac.
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