Decreasing the alkyl branch frequency in precision polyethylene: Pushing the limits toward longer run lengths

Article abstract of DOI:10.1021/ja2040046

A symmetrical α,ω-diene monomer with a 36 methylene run length was synthesized and polymerized, and the unsaturated polymer was hydrogenated to generate precision polyethylene possessing a butyl branch on every 75th carbon (74 methylenes between branch points). The precision polymer sharply melts at 104 °C and exhibits the typical orthorhombic unit cell structure with two characteristic wide-angle X-ray diffraction (WAXD) crystalline peaks observed at 21.5° and 24.0°, corresponding to reflection planes (110) and (200), respectively.

Full text of DOI:10.1021/ja2040046

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Decreasing the Alkyl Branch Frequency in Precision Polyethylene:
Pushing the Limits toward Longer Run Lengths
Bora Inci and Kenneth B. Wagener*
Center for Macromolecular Science and Engineering, The George and Josephine Butler Polymer Research Laboratory, Department of
Chemistry, University of Florida, Gainesville, Florida 32611-7200, United States
S Supporting Information
b
Key to success in this research is the preparation of a
ABSTRACT: A symmetrical R,ω-diene monomer with a 36
methylene run length was synthesized and polymerized, and
the unsaturated polymer was hydrogenated to generate
precision polyethylene possessing a butyl branch on every
75th carbon (74 methylenes between branch points). The
precision polymer sharply melts at 104 °C and exhibits the
typical orthorhombic unit cell structure with two character-
istic wide-angle X-ray diffraction (WAXD) crystalline peaks
observed at 21.5° and 24.0°, corresponding to reflection
planes (110) and (200), respectively.
symmetrical diene monomer, such that the built-in branch run
length strictly dictates the spacing between the two consecutive
branch points. Previous publications from our laboratory have
reported the successful preparation of polyethylene with precise
38 methylene carbon run lengths (i.e., a branch being placed
on every 39th carbon along the polyethylene backbone).7
Herein, we report the synthesis and characterization of preci-
sion polyethylene possessing a butyl branch on every 75th carbon
(74 methylenes between branch points). Primary structure char-
acterization and morphological examination by wide-angle X-ray
diffraction (WAXD) analysis are also reported in this communication.
A multistep synthetic route is necessary for the preparation
of a symmetrical diene monomer (11). Figure 1 illustrates the
elaborate synthetic effort, which consists of a nine-step procedure
including alkenyl bromide (8) preparation10 from 10-undecenol
(1), which is a renewable feedstock derived from castor oil,11
dialkylation of hexanenitrile (9),12 and decyanation of alkylcyano
R,ω-diene (10).13 The spacing between the two consecutive
branch points along the polyethylene backbone (see structure of
polymer 13 in Figure 1) is set in the dialkylation step of
hexanenitrile, where the alkenyl bromide run length (in this case
36 methylene units) plays a crucial role. The systematic increase
of run length from 9 to 36 requires challenging purification steps.
For example, in the dehydrohalogenation of alkyl dibromide (4)
with t-BuOK, the crude mixture contains the desired alkenyl
bromide (20-bromoicos-1-ene 5), the dielimination product, and
unreacted starting material, all with close retardation factors by
thin layer chromatography. The situation is even more compli-
cated for dehydrohalogenation of alkyl dibromide (7), where the
polarity difference within the crude mixture becomes almost
unnoticeable as the number of methylene units is doubled. Silica
gel with a small particle size distribution (5À20 μm) was selected
for the column separation. Consecutive column chromatography
passes, and the relatively low room temperature solubility of
compound 8 in hexane (used as an eluent for the separation)
made the purification of the desired compound, 38-bromoocta-
triacont-1-ene (8), tedious.
olyethylene prepared via step growth polymerization chem-
P
istry (the ADMET reaction) has recently proved to generate
unique macromolecular structures for morphological examina-
tion of polyolefins.1À4 These polymers, while structurally akin to
copolymers made via chain copolymerization of ethylene and vinyl
comonomers, have unique properties, because use of symme-
trical R,ω-diene monomers ensures the precise spacing of chain
branches.5À8 Systematic alterations in branch identity and posi-
tion on the polymer backbone are made during monomer
synthesis to yield predictable changes in packing behavior and
lamella thickness.
ADMET polyethylene prepared by polycondensation of 1,9-
decadiene compares well with chain-polymerized commercial
high density polyethylene. (Both preparations melt at 134 °C
and form orthorhombic unit cells.) Introduction of pendant alkyl
moieties onto the polyethylene backbone disrupts the crystal-
linity and results in a decrease in melting point.1 For example,
insertion of butyl branches on every 39th carbon generates a
semicrystalline polymer that melts sharply at 75 °C.7 The
thermal and morphological behavior of this polymer was recently
compared to that of a copolymer of ethylene and 1-hexene having
the same net concentration of butyl branches along the polymer
backbone (although not regularly spaced) prepared via chain
polymerization with a metallocene catalyst.9 Both polymers have
orthorhombic unit cells, but the ADMET polymer has a sharper
melting transition and narrower lamella thickness distribution.
On the other hand, more frequent branch placement in ADMET
polyethylene generates polymers with lower melting points and
in some cases amorphous materials (with as few as 4 methylene
groups between branches). Because this behavior diminishes the
possibility of systematic comparisons of ADMET polyethylene
with frequent branches with chain-polymerized commercial
polyethylene, we are now focusing on the preparation of poly-
mers with less frequent branch placement.
Dialkylation of hexanenitrile (9) is the other challenging step
in this synthesis route. Because of the low solubility of 8 in THF
at low temperatures, the traditional procedure (i.e., addition of
base at À78 °C and alkylation at 0 °C) for nitrile alkylation was
not followed, and 50 °C was selected as the reaction temperature.
The elevated temperature increased the likelihood of undesired
Received: May 1, 2011
Published: July 19, 2011
r
2011 American Chemical Society
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1
Figure 2. (a) H NMR spectra of monomer (11), unsaturated (12),
and saturated (13) polymers. (b) Olefinic region shown in higher
magnification.
Table 1. Molecular Weight Data for Polymers 12 and 13
Figure 1. Synthetic scheme for the synthesis of polymer 13.
side reactions, such as alkyl bromide elimination. After the
purification of 2-butyl-2-(octatriacont-37-en-1-yl)tetracont-39-
enenitrile (10), the decyanation step gave 39-butylheptahepta-
conta-1,76-diene (11) in a reasonably high yield.
Because of the relatively high melting point of monomer 11
(Tm = 77 °C), the polymerization temperature was 100 °C where
Ru-based catalysts are prone to have low turnover numbers and
to generate RuÀH species,14À16 which would cause isomeriza-
tion problems and disrupt the symmetrical nature of the mono-
mer.17,18 Therefore, monomer 11 was condensed to form an
unsaturated ADMET polymer using Schrock's [Mo] catalyst for
clean metathesis chemistry. The disappearance of terminal olefin
signals in 1H NMR spectra (see Figure 2) proves complete con-
version, which is necessary for any step-growth polymerization.
The desired unsaturated linear polymer (12) was isolated with
no detectable side reactions and hydrogenated utilizing Wilk-
inson's catalyst to yield the precision polyethylene (polymer 13)
possessing a butyl branch on every 75th carbon. Figure 2
illustrates the saturation of internal olefins in an unsaturated
polymer (12) with complete loss of 1H NMR olefin signals. Total
hydrogenation is also evidenced by infrared spectroscopy, as
confirmed by the complete loss of the alkene CÀH out-of-plane
bending peak at 967 cmÀ1. It is important to note that the
solubility characteristics of polymer 13 (soluble in toluene,
dichlorobenzene, trichlorobenzene, etc. at high temperatures)
are similar to those of polyethylene prepared by chain polymer-
ization. Molecular weight data were obtained using high tem-
perature GPC in 1,2,4-trichlorobenzene (Table 1).
a Molecular weight data were collected by GPC in 1,2,4-trichloroben-
zene at 135 °C relative to polystyrene standards. b PDI, polydispersity
index (Mw/Mn).
transitions at 95.5 and 104 °C, and as expected, saturation
increases both the melting points and the heat of fusion values.
The melting transitions of both polymers are sharper than those
for chain-polymerized poly(ethylene-co-hexene) and other ethy-
lene/R-olefin copolymers, illustrating the importance of preci-
sion placement of the branches.19À21
Thermal data for polymer 13 and other previously reported
precision polyethylenes with more frequent butyl branches6
(Table 2) clearly demonstrate the effect of precision run length
in these materials. As the run length increases between two
consecutive branch points, the melting point gradually increases,
approaching that of ADMET PE (linear PE without branches,
Tm = 134 °C). Experimental data in butyl branched precision
polymers were compared with theoretical predictions for infi-
nitely long polyethylene interpreted by Flory.22 A plot of melting
Figure 3 shows the DSC thermograms of the unsaturated and
saturated polymers in both the second heating and cooling
cycles. The polymers are semicrystalline, having the melting
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Figure 3. DSC exotherms (down) and endotherms (up) for unsatu-
rated (12) and saturated (13) polymers.
Figure 5. WAXD diffractograms of precision polymer (13), ADMET
PE, and commercially available High Density Polyethylene (HDPE)
prepared with ZieglerÀNatta catalyst. Data were collected at 30 °C
for all polymer samples.
Table 2. DSC Data for Precision Polyethylenes Possessing
Butyl Branch
all three polymers exhibit the typical orthorhombic unit cell
structure with two characteristic crystalline peaks observed at
21.5° and 24.0°, corresponding to reflection planes (110) and
(200), respectively. Orthorhombic packing suggests the exclu-
sion of the butyl branch from the unit cell, consistent with the
previously documented measurements on the chain-polymerized
poly(ethylene-co-hexene) copolymer.9 Transmission Electron
Microscope (TEM) measurements are underway to better
understand the morphology of polymer 13.
butyl branch
on every nth
carbon, n
butyl branches
per 1000
Tm
ΔHm
carbon atoms
(°C)
(J/g)
5
200
67
48
26
13
0
amorphous
À33
14
--
15
13
21
47
39
75
66
75
104
152
204
’ ASSOCIATED CONTENT
ADMET PE
134
S
Supporting Information. Experimental details. This ma-
b
terial is available free of charge via the Internet at http://pubs.acs.
org.
’ AUTHOR INFORMATION
Corresponding Author
wagener@chem.ufl.edu
’ ACKNOWLEDGMENT
The authors thank the National Science Foundation (DMR-
0703261) and the International Max Planck Research School for
Polymer Materials for financial support. The Army Research
Office contributed to the catalyst work necessary to make
proper selections. We also thank Dr. Markus Mezger, Dr. Werner
Steffen, and Michael Bach at Max Planck Institute for Polymer
Research for their help with X-ray measurements and Sandra
Seywald at Max Planck Institute for Polymer Research for her
help with GPC measurements.
Figure 4. Plot of melting temperature vs butyl branch frequency in
precision polyethylenes.
’ REFERENCES
(1) Sworen, J. C.; Smith, J. A.; Wagener, K. B. J. Am. Chem. Soc. 2003,
125, 2228–2240.
(2) Sworen, J. C.; Smith, J. A.; Berg, J. M.; Wagener, K. B. J. Am.
Chem. Soc. 2004, 126, 11238–11246.
(3) Sworen, J. C.; Wagener, K. B. Macromolecules 2007, 40,
4414–4423.
(4) Mutlu, H.; de Espinosa, L. M.; Meier, M. A. R. Chem. Soc. Rev.
2011, 40, 1404–1445.
point vs number of butyl branches per 1000 carbons in butyl
branched precision polymers (Figure 4) shows good linearity, an
indication of the similarity in thermal behavior between ADMET
PE and infinitely long PE.
Morphological examination by WAXD is illustrated in Figure5
for polymer 13, ADMET PE, and commercial High Density
Polyethylene (HDPE) made with ZieglerÀNatta catalyst. The
similarity of polymer 13 to ADMET PE and HDPE is obvious:
11874
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COMMUNICATION
(5) Rojas, G.; Berda, E. B.; Wagener, K. B. Polymer 2008,
49, 2985–2995.
(6) Rojas, G.; Wagener, K. B. Macromolecules 2009, 42, 1934–1947.
(7) Zuluaga, F.; Inci, B; Nozue, Y.; Hosoda, S.; Wagener, K. B.
Macromolecules 2009, 42, 4953–4955.
(8) Rojas, G.; Inci, B.; Wei, Y.; Wagener, K. B. J. Am. Chem. Soc.
2009, 131, 17376–17386.
(9) Hosoda, S.; Nozue, Y.; Kawashima, Y.; Suita, K.; Seno, S.;
Nagamatsu, T.; Wagener, K. B.; Inci, B.; Zuluaga, F.; Rojas, G.; Leonard,
J. K. Macromolecules 2011, 44, 313–319.
(10) Baughman, T. W.; Sworen, J. C.; Wagener, K. B. Tetra hedron
2004, 60, 10943–10948.
(11) (a) Naughton, F. C. Journal of the American Oil Chemists’ Society
1974, 51, 65–71.(b) Flinn, R. A. In Encyclopedia of Chemical Processing
and Design, Vol. 6; Calcination Equipment to Catalysis; McKetta, J. J.,
Ed.; New York, 1978; pp 401À420. (c) Marijke Van der Steen,
M. V. D.; Stevens, C. V. ChemSusChem 2009, 2, 692–713.
(12) Rojas, G.; Baughman, T. W.; Wagener, K. B. Synth. Commun.
2007, 37, 3923–3931.
(13) Rojas, G.; Wagener, K. B. J. Org. Chem. 2008, 73, 4962–4970.
(14) Ulman, M.; Grubbs, R. H. J. Org. Chem. 1999, 64, 7202–7207.
(15) Hong, S. H.; Day, M. W.; Grubbs, R. H. J. Am. Chem. Soc. 2004,
126, 7414–7415.
(16) Hong, S. H.; Wenzel, A. G.; Salguero, T. T. J. Am. Chem. Soc.
2007, 129, 7961–7968.
(17) Courchay, F. C.; Sworen, J. C.; Ghiviriga, I.; Abboud, K. A.;
Wagener, K. B. Organometallics 2006, 25, 6074–6086.
(18) Fokou, P. A.; Meier, M. A. R. J. Am. Chem. Soc. 2009, 131,
1664–1665.
(19) Alamo, R. G.; Mandelkern, L. Macromolecules 1989, 22,
1273–1277.
(20) Mirabella, F. M. J. Polym. Sci., Part B: Polym. Phys. 2006,
44, 2369–2388.
(21) Mirabella, F. M.; Crist, B. J. Polym. Sci., Part B: Polym. Phys.
2006, 42, 3416–3427.
(22) Flory, P. J. Trans. Faraday Soc. 1955, 51, 848–857.
11875
dx.doi.org/10.1021/ja2040046 |J. Am. Chem. Soc. 2011, 133, 11872–11875