Modeling branched polyethylene: Copolymers possessing precisely placed ethyl branches

Article abstract of DOI:10.1021/ja047850p

A structural investigation of precise ethylene/1-butene (EB) copolymers has been completed using step polymerization chemistry. The synthetic methodology needed to generate four model copolymers is described; their primary and higher level structure is ch

Full text of DOI:10.1021/ja047850p

Published on Web 08/19/2004
Modeling Branched Polyethylene: Copolymers Possessing
Precisely Placed Ethyl Branches
John C. Sworen, Jason A. Smith,† Jessica M. Berg, and Kenneth B. Wagener*
Contribution from The George and Josephine Butler Polymer Research Laboratory, Department
of Chemistry, UniVersity of Florida, GainesVille, Florida 32611-7200
Received April 14, 2004; E-mail: wagener@chem.ufl.edu
Abstract: A structural investigation of precise ethylene/1-butene (EB) copolymers has been completed
using step polymerization chemistry. The synthetic methodology needed to generate four model copolymers
is described; their primary and higher level structure is characterized. The copolymers possess an ethyl
branch on every 9th, 15th, and 21st carbon along the backbone of linear polyethylene. Melting points and
heats of fusion decrease with increased branch frequency. Differential scanning calorimetry and infrared
spectroscopy show highly disordered crystal structures favoring ethyl branch inclusion. On the other hand,
the EB copolymers contain high concentrations of kink and gauche defects independent of branch frequency.
These model copolymers are compared with random copolymers produced using traditional chain chemistry
and previously synthesized ADMET EP copolymers.
late transition metals.9,10 Similar to the results obtained for
ethylene/propylene (EP) copolymers,11 studies on randomly
Introduction
Macromolecules based on ethylene and centralized around
their copolymerization with R-olefins have been studied for more
than 60 years. These branched copolymers have garnered much
attention due to their enhanced mechanical properties, structural
simplicity, and industrial importance. However, the inability to
predict structure property functions for these simplest of
polymers has led them to be among the most thoroughly studied
macromolecules. Although such factors as mode of polymeri-
zation (radical, Ziegler-Natta, metallocene, etc.), catalyst
choice, reaction temperature/pressure, and molar mass bare
significant importance on ethylene/R-olefin copolymers, the
short-chain branching (SCB) content and its distribution are the
most prominent factors in linear low-density polyethylenes
(LLDPEs).1
branched ethylene/butylene (EB) copolymer systems have
shown that the density, enthalpy, degree of crystallinity, and
peak melting/crystallization points all decrease as the amount
of defect content (ethyl branch) is increased. In the past, the
interest of EB copolymers has been limited relative to ethylene/
propene versions of LLDPE. These butylenes-based copolymers
and homopolymers have garnered attention due to their unusual
combination of toughness and flexibility as well as their
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Linear low-density polyethylene is a statistical copolymer of
ethylene and an R-olefin (butene, hexene, and octene) where
the type, concentration, and distribution of these branches vary
and are highly dependent on the chosen polymerization mech-
anism. Typically, these random copolymers are produced using
Ziegler-Natta,2,3 metallocene catalysts,4-6 and anionically
synthesized hydrogenated butadienes,7,8 and when using other
† Current address: Milliken & Co., Spartanburg, SC.
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1972, 151, 97. (c) Bergstrom, C.; Avela, E. J. Appl. Polym. Sci. 1979, 23,
163. (d) Wild, L.; Ryle, T. R.; Knobeloch, D. C.; Peat, I. R. J. Polym. Sci.,
Polym. Phys. Ed. 1982, 20, 441. (e) Schouterden, P.; Groenickx, G.; Van
des Heijden, B.; Jansen, F. Polymer 1987, 28, 2099.
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Comotti, A.; Simonutti, R.; Camurati, I.; Sozzani, P. Macromolecules 2002,
35, 1677. (c) Zhang, F.; Song, M.; Lu¨, T.; Liu, J.; He, T. Polymer 2002,
43, 1453. (d) DesLauriers, P. J.; Rohlfing, D. C.; Hsieh, E. T. Polymer
2002, 43, 159. (e) Starck, P.; Malmberg, A.; Lo¨fgren, B. J. Appl. Polym.
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11238
J. AM. CHEM. SOC. 2004, 126, 11238-11246
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Modeling Branched Polyethylene
A R T I C L E S
Figure 1. Controlling ethyl branch content using ADMET polymerization.
resistance to creep and external stress.12 However, the synthetic
methodology used to study these copolymers may produce
unwanted side reactions and as a result unknown primary
structures. These defects, even in small quantities, can alter the
polymer’s macromolecular behavior and thermal response
depending on their frequency and identity.
Recently we presented a way to obviate the random nature
of branching in polyethylene along with unwanted side prod-
ucts.13,14 Doing so has been accomplished using the clean, step
polymerization chemistry offered by acyclic diene metathesis
(ADMET). This mild chemistry avoids chain transfer and other
catalyst “mistakes” encountered during chain propagation
processes, thereby producing a branched polymer with a
homogeneous composition distribution and known branch
identity (Figure 1).
A short time ago we reported the synthesis and thermal
behavior for a series of five model EP copolymers in which
the methyl branch was precisely placed on each 9th, 11th, 15th,
19th, and 21st carbon along the backbone, respectively.14 We
have also reported polyethylene-containing precise methyl
content, but with a statistical placement along the backbone
using ADMET copolymerization.15 The thermal behavior and
morphological analysis for this series of EP copolymers have
yielded unique results, in effect creating a new class of PE-
based materials using metathesis, based on the structural control
offered by precise branch identity coupled with their precise or
random placement.
In an effort to extend our LLDPE structural library, a synthetic
methodology was sought to lengthen the alkyl branch in these
model materials. This has proven to be a difficult task, for the
structural simplicity of symmetrically disposed, substituted
dienes is deceptive. We now report the successful synthesis of
R,ω-diene monomers in which the ethyl branch has been
symmetrically substituted on the hydrocarbon backbone. The
ADMET polymerization of these monomers and subsequent
hydrogenation has yielded the first ADMET model EB copoly-
mers wherein the ethyl branch is placed on each 9th, 15th, and
21st carbon along the backbone (Figure 1). Herein, we present
the monomer/polymer synthesis, characterization, and thermal
analysis for these new LLDPE model materials.
Results and Discussion
(A) Monomer Synthesis and Characterization. The mild
chemistry afforded by ADMET polymerization has proven a
useful mechanism for the modeling of perfectly branched
structures.13,14 The cornerstone of this perfectly branched PE
model study has been to produce a monomer (R,ω-diene) with
pure R-olefin functionality along with perfect branch identity
(Figure 1). Fulfilling both requirements has proven difficult
when expanding the branch length beyond the methyl group,14
leading to significant effort in formulating a synthetic pathway
that would successfully extend the branch identity without
sacrificing the integrity of the diene. Several methodologies were
investigated throughout the synthesis work to generate perfectly
branched LLDPE materials; multiple synthetic procedures were
used to complete this study starting either from ethyl aceto-
acetate (Figure 2), diethyl malonate (Figure 3), or ketodienes
(Figure 4). The first successful synthetic strategy to produce a
pure R,ω-diene monomer with a symmetrically substituted ethyl
branch is presented in Figure 2.
The conditions in step 1 (Figure 2) were modified from the
work of Krapcho et al.16 Ethyl acetoacetate is deprotonated with
base and readily effects the SN2 displacement of bromide upon
addition of 5-bromo-1-pentene. Subsequently, in the same pot,
the monosubstituted product is reacted with a second equivalent
of base and alkenyl halide, affording the disubstituted â-keto
ester (6). Compound 6 was decarboxylated using a dimethyl
sulfoxide (DMSO)/water/salt mixture16 to produce an R,ω-diene
with a pendant methyl ketone that is symmetrically substituted
along the monomer backbone (7). Reduction using lithium
aluminum hydride (LAH) yields the secondary alcohol (8),
which is further tosylated (9). The reducing agent Li(Et)3BH,
(12) (a) Al-Hussein, M.; Strobl, G. Macromolecules 2002, 35, 8515. (b)
Lindegren, C. R. Polym. Eng. Sci. 1970, 10, 163.
(13) O’Gara, J. E.; Wagener, K. B.; Hahn, S. F. Makromol. Chem., Rapid
Commun. 1993, 14, 657.
(14) Smith, J. A.; Brzezinska, K. R.; Valenti, D. J.; Wagener, K. B. Macro-
molecules 2000, 33, 3781.
(16) (a) Krapcho, A. P.; Weimaster, J. F.; Eldridge, J. M.; Jahngen, G. E., Jr.;
Lovey, A. J.; Stephens, W. P. J. Org. Chem. 1978, 43, 138. (b) Krapcho,
A. P. Synthesis 1982, 805. (c) Krapcho, A. P. Synthesis 1982, 893. (d)
Krapcho, A. P.; Gowrikumar, G. J. Org. Chem. 1987, 52, 1880.
(15) Sworen, J. C.; Smith, J. A.; Wagener, K. B.; Baugh, L. S.; Rucker, S. P.
J. Am. Chem. Soc. 2003, 125, 2228.
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A R T I C L E S
Sworen et al.
Figure 2. Ethyl-branch synthetic methodology for short methylene monomers.
Figure 3. Synthesis for longer methylene run length monomers shown for 3-(10-undecenyl)-13-tetradecene (2).
methyl monomers (Figure 3).14 Compound 11 is synthesized
using sodium hydride with 11-bromo-1-undecene, followed by
decarboxylation of the resulting diacid. The monoacid is reduced
into the primary alcohol and directly converted to the bromide
(13) using CBr4. A single carbon homologation was performed
by the addition of solid CO2 to the Grignard of compound 13.
Once again its reduction was followed by the formation of the
bromide 15. Monomer 2 was obtained by quenching the
Grignard of 15 with water. Noteworthy, the formation of the
Grignard must be achieved using sonication, for production of
the Grignard thermally causes compound 15 to dimerize. Again,
the key difference between either methods (Figures 2 and 3) is
the formation and reduction of the primary bromide (15) in
Figure 3 using Mg/H2O or if preferred the reduction of the
tosylated alcohol with Super Hydride. The displacement of a
primary tosylate using boron can produce the desired hydro-
carbon monomer with minimal amounts of the eliminated
compound (<5%).
The synthesis of symmetrical ethyl-branched dienes can be
accomplished using methodologies outlined in either Figure 2
or 3. However, in an attempt to simplify our monomer synthetic
procedure, we sought easier and more efficient ways to afford
any length branch through simple organic transformations. Our
initial endeavor focused on using Wittig couplings to produce
monomers containing a “masked” branch yielding the correct
ethyl branch upon exhaustive hydrogenation. In fact, the model
PEs derived from monomers made through Wittig coupling
Figure 4. Synthesis of Wittig monomers.
coined Super Hydride, first employed by H. C. Brown and co-
workers in the late 1970s, is used to reduce compound 9.17 This
final step yields a mixture of the symmetrical diene of interest
(1) and eliminated byproduct (10). These products can be
separated using HPLC or careful column chromatography with
hexane. The use of the hindered boron reducing agent is evident
since the reduction of the tosylated alcohol (9) with LAH
produces no traceable amount of compound 1, whereas a 68%
conversion was observed for the olefinic monomer 10.
Although monomer 1 (three methylenes) was successfully
produced by the method shown in Figure 2, difficulties were
encountered when trying to synthesize monomers containing
longer chain lengths. Complications arise during the reduction
of the secondary tosylated or mesylated alcohol derivatives. The
preferred synthesis for these longer run length monomers is
shown in Figure 3, outlined for 3-(10-undecenyl)-13-tetradecene
(2), to produce precisely placed ethyl branches. The synthesis
was modified according to the procedure for our precisely placed
(17) Krishnamurthy, S.; Brown, H. C. J. Org. Chem. 1976, 41, 3064.
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11240 J. AM. CHEM. SOC. VOL. 126, NO. 36, 2004

Modeling Branched Polyethylene
A R T I C L E S
Table 1. Molecular Weights for ADMET Model EB Materials
saturd copolymers
universalc
d
unsaturd copolymers (rel)b
rel (PS)b
M × 10-3
LALLS
model EB copolymer
n(ethyl on every nth backbone carbona)
M × 10-3
PDIe
PDIe
M × 10-3
PDIe
M × 10-3
PDIe
w
w
w
w
HPEB9
9
15
21
21
56.5
53.1
56.1
50.2
1.8
1.9
2.0
1.9
58.6
1.8
1.9
1.8
1.9
37.8
31.2
36.5
27.3
1.8
1.9
1.9
1.8
29.2
1.7
1.8
1.7
1.8
HPEB15W
HPEB21
HPEB21W
54.2
54.1
50.7
23.8
28.6
25.4
a Branch content based on the hydrogenated repeat unit. b Molecular weight data taken in tetrahydrofuran (40 °C) relative to polystyrene standards.
c Molecular weight data taken in tetrahydrofuran (40 °C) using viscosity law calibration relative to polystyrene standards. d Molecular weight data taken
using low-angle laser light scattering (LALLS) in tetrahydrofuran at 40 °C. e Polydispersity index (Mw/Mn).
(Figure 4) and ethyl acetoacetate addition (Figure 3) yield
polymers having the exact primary structure. Their comparison
will be discussed further.
that is comprised of only one type of repeat unit, plus the usual
amount of cyclics (<1-2%) found in bulk polycondensation
conversions. Exhaustive hydrogenation of the unsaturated pre-
polymer was accomplished using either palladium on carbon
(10 wt % Pd/C) or RuHCl(CO)(PCy3)2 (5 wt %); the homoge-
neous Ru catalyst was used with the trisubstituted olefinic
prepolymers due to its literature applications in this area.22 In
both cases the hydrogenations were carried out over 5 days using
500 psi for Pd; however, higher pressure was needed (2000 psi)
for the homogeneous catalyst to ensure complete hydrogenation.
The polymers were purified by filtration and simple precipitation
of the hydrogenation solution in acidic methanol (1 M). No side
reaction was detectable by thin-layer chromatography (TLC)
or nuclear magnetic resonance (NMR), and hydrogenation was
verified by both infrared (IR) spectroscopy and NMR analysis.
As previously observed, hydrogenation effectiveness is best
monitored by IR. The 967-969 cm-1 absorption in the
unsaturated polymer, which corresponds to the out-of-plane
C-H bend in the alkene, completely disappears after successful
hydrogenation. This is the first example of any model EB
copolymer being prepared containing precisely placed ethyl
branches on each and eVery 9th, 15th, and 21st carbon along
polyethylene’s linear backbone.
Wittig monomers (Figure 4) were synthesized using an in-
situ formation of the phosphorus ylide followed by direct attack
on the corresponding ketone. The starting ketones were syn-
thesized according to literature procedures.18 The branch identity
can be controlled by the starting bromoalkane used in the ylide
precursor; for example, ethyl branches would be obtained from
bromoethane. Their synthesis is done by refluxing the necessary
bromoalkane with triphenylphosphine in diethyl ether, where
the resulting salt is filtered, washed with excess ether, and dried
prior to use. Upon addition of base to the salt/ketone slurry the
solution turns yellow indicative of ylide formation. The reaction
is complete within 30 min of base addition, and the product
can be purified by flash chromatography in hexane. The clean
chemistry and easy synthesis afforded by Wittig chemistry will
allow formation of any branch length monomers readily from
available, inexpensive starting bromoalkanes.
(B) ADMET Polymerization and Hydrogenation Chem-
istry. The proper choice of the appropriate catalyst system
throughout this model study was crucial due to the differing
monomer structures employed; only Grubbs’ first generation19
or Shrock’s20 catalyst could be used for pure R,ω-diene
polymerizations. The Wittig monomers were polymerized only
with Grubbs’ catalyst, due to the presence of the “masked”
branch, the trisubsituted olefin. Since we are modeling polymers
containing exact primary structures, we have avoided all other
ruthenium-based catalyst systems due to their propensity to
isomerize external and internal olefins.21
In the following sections, all polymers are named using the
prefix HP (hydrogenated polymer) followed by the comonomer
type (EB, ethylene/butane, or EP, ethylene/propylene), and the
precise branch frequency (21); for example HPEB21 is desig-
nated as hydrogenated ethylene/butene copolymer containing
an ethyl branch on every 21st carbon. Due to the exact nature
of the polymers produced, the comonomer content can be easily
calculated using the branch frequency (n) following the relation-
ship
Monomer 1 was exposed to Schrock’s catalyst20 under mild
ADMET step polymerization conditions using typical catalyst
loadings (1000:1, monomer:catalyst). All other monomers were
polymerized using Grubbs’ first generation catalyst. The
chemistry proceeds cleanly to yield a linear, unsaturated polymer
2
n
mol % comonomer ) × 100
Table 1 confirms that the hydrogenation process does not
alter the molecular weight of the unsaturated polymers in this
study, which is consistent with our earlier experiments.13-15 The
saturated EB copolymers were analyzed by three molecular
weight determination methods consisting of the use of an internal
differential refractive index detector (DRI), differential viscosity
(18) Hopkins, T. E.; Wagener, K. B. Macromolecules 2003, 36, 2206.
(19) (a) Nguyen, S. T.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1993,
115, 9858. (b) Schwab, P.; France, M. B.; Ziller, J. W.; Grubbs R. H.
Angew. Chem., Int. Ed. Engl. 1995, 34, 2039. (c) Schwab, P.; Grubbs, R.
H.; Ziller, J. W. J. Am. Chem. Soc. 1996, 118, 100. (d) Grubbs, R. H.;
Marsella, M. J.; Maynard, H. D. Angew. Chem., Int. Ed. Engl. 1997, 36,
1101. (e) Dias, E. L.; Nguyen, S. T.; Grubbs, R. H. J. Am. Chem. Soc.
1997, 119, 3887. (f) Dias, E. L.; Grubbs, R. H. Organometallics 1998, 17,
2758. (g) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18, and
references therein.
(20) (a) Schrock, R. R.; Murdzek, J. S.; Bazan, G. C.; Robbins, J.; DiMare, M.;
O’Regan, M. J. Am. Chem. Soc. 1990, 112, 3875. (b) Bazan, G. C.;
Khosravi, E.; Schrock, R. R.; Feast, W. J.; Gibson, V. C.; O’Regan, M.
B.; Thomas, J. K.; Davis, W. M. J. Am. Chem. Soc. 1990, 112, 8378. (c)
Bazan, G. C.; Oskam, J. H.; Cho, H. N.; Park, L. Y.; Schrock, R. R. J.
Am. Chem. Soc. 1991, 113, 6899. (d) Fox, H. H.; Schrock, R. R.
Organometallics 1992, 11, 2763. (e) Feldman, J.; Murdzek, J. S.; Davis,
W. M.; Schrock, R. R. Organometallics 1989, 8, 2260. (f) Oskam, J. H.;
Schrock, R. R. J. Am. Chem. Soc. 1992, 114, 7588.
(21) (a) Lehman, S. E.; Wagener, K. B. Inorg. Chim. Acta 2003, 345, 190. (b)
Sworen, J. C.; Pawlow, J. H.; Case, W.; Lever, J.; Wagener, K. B. J. Mol.
Catal. A: Chem. 2003, 194, 69. (c) Bourgeois, D.; Pancrazi, A.; Nolan, S.
P.; Prunet, J. J. Organomet. Chem. 2002, 643, 247. (d) Arisawa, M.; Terada,
Y.; Nakagawa, M.; Nishida, A. Angew. Chem., Int. Ed. 2002, 41, 4732.
(22) (a) Martin, P.; McManus, N. T.; Rempel, G. L. J. Mol. Catal. A: Chem.
1997, 126, 115. (b) Otsuki, T.; Goto, K.; Komiya, Z. J. Polym. Sci., Part
A: Polym. Chem. 2000, 38, 4661. (c) McManus, N. T.; Rempel, G. L. J.
Macromol. Sci., ReV. Macromol. Chem. Phys. 1995, C35, 239.
9
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A R T I C L E S
Sworen et al.
Figure 6. (a) Infrared spectra for ADMET EB copolymers and (b) IR
comparison of precise methyl- and ethyl-branched copolymers (on every
21st carbon).
(ADMET). As a starting point we have used semiquantitative
13C NMR as the primary tool for investigation to verify primary
structure.
Figure 5 displays the 13C NMR spectra for the conversion of
monomer 1 to unsaturated polymer, UPEB9 (Figure 5b); the
13C spectrum for the fully saturated ADMET model EB
copolymer possessing an ethyl branch on every 9th carbon,
HPEB9 is presented in Figure 5c. These NMR data confirm
that the ADMET reaction has taken place. The absence of visible
end groups (114.5 and 139.2 ppm) implies that high polymer
has been obtained, a result consistent with the GPC results given
earlier. Further, the internal olefin resonance at 130.62 ppm
(Figure 5b) completely vanishes upon exhaustive hydrogenation
of the double bonds (confirmed by IR, Figure 6).
The clean and complete nature of the transformations depicted
in the spectra is typical for all ADMET model LLDPEs
synthesized thus far,14 data which illustrate the level of structural
control that is possible when choosing step condensation
chemistry as the method to model ethylene-co-R-olefin systems.
Moreover, the 13C NMR spectra of these ADMET copolymers
reveal the exact chemical shifts of a particular branch point and
its subsequent carbons. In effect, acyclic diene metathesis allows
for a direct correlation between branch identity and observed
NMR shift (ppm) due to the exact primary structure of the
polymers. In the case of precise ethyl-branched copolymers, the
observed shifts are 33.45 (R), 26.99 (â), 30.43 (γ), 30.31 (δ),
39.09 (methine), 26.12 (R′), and 11.12 (1Bn) ppm, which is in
very good agreement with experimental23,24 and predicted
values.25,26 These data suggest that ADMET model LLDPEs,
in conjunction with high-field NMR experiments, could be used
to derive new and improved mathematical parameters in the
Figure 5. 13C NMR of (a) monomer 1, (b) UPEB9, and (c) HPEB9.
detector (DP), and a precision light scattering detector (LS).
Using these three detectors in series, the molecular weights were
determined by universal calibration (a plot of log intrinsic
viscosity [η] × molecular weight vs retention time) calibrated
using polystyrene (PS) and low-angle laser light scattering
(LALLS). The results are shown in Table 1. The universal
calibration data were generated by calibrating the retention times
using 10 Polymer Laboratory polystyrene standards. As previ-
ously discussed for EP copolymer models,14 these ADMET
model EB copolymers exhibit molecular weights and polydis-
persities within a sufficient range to make it an excellent model
for commercial grades of LLDPE produced via metallocene
catalysis.4-6
(C) Structure Determination Using NMR and IR. Our goal
in modeling PE-based materials is to develop an understanding
of the relationship between the exact effect branch content and
identity and a given model copolymer’s micro- and macromo-
lecular properties. A direct transfer of monomer branch content
to the polymer is achieved using step metathesis chemistry
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Modeling Branched Polyethylene
A R T I C L E S
structural study of branched polyethylene. A wealth of informa-
tion can be collected concerning the amount, nature, and
partitioning of branched PE materials using 13C NMR studies.27
The precisely branched ADMET PEs synthesized thus far offer
a tremendous potential to study/model the direct impact a short-
chain branch and its distribution have on the final structure-
property relationships in ethylene-based materials. Nowhere is
this more evident than in the NMR results presented here.
Further, the majority of the methyl branches are known to in-
corporate into the repeating methylene sequences.31 Of course,
under equilibrium conditions the branches, even methyl, are
assumed to be rejected from the crystallites.32 Equilibrium is
seldom reached during crystallization, however, thereby favoring
an intermediate situation where the partial segregation of branch-
es exists between the amorphous and crystal regions.33 This
equilibrium, for branches longer than methyl, can be shifted by
the crystallization conditions to favor inclusion or exclusion.31a,34
On this basis, the bulk of the ethyl side group is at the boundary
between total exclusion and inclusion within the crystal lattice.
The intermediate situation of phase partitioning chain defects
seems most likely, and there have been numerous experi-
mental,27,31b,35 theoretical,36-39 and molecular modeling studies40
to help define a mechanism for branch inclusion. Typically,
X-ray diffraction coupled with high-field 13C NMR has been
used to determine branch inclusion. For ethyl-branched poly-
mers, the proportion of ethyl branch inclusion was determined
to be a function of SCB concentration and was estimated at
10:1 (17 SCB/1000C) and 5:1 (21 SCB/1000C) between the
amorphous and crystalline regions.27a,31b,34 More recently, studies
have assumed the existence of branch incorporation by consid-
ering possible structural perturbations and conformational
defects. These studies have proposed interstitial sites along the
polymer chains, known as kinks,36,37 arising from conforma-
tional gauche defects (2g1 defects) being the most common.38
These 2g1 defects have been proposed to be large enough for
ethyl branches.36a,39 In fact, the gtg (2g1) conformation can be
observed using IR and assigned the 1366 and 1305 cm-1
absorptions.
As previously observed in our model EP model copolymers,
the utility of IR to observe and understand changes in structure
is invaluable. In the past, Tashiro et al.28 carried out a detailed
study on the IR response of differing polyethylene crystal
structures. In Figure 6, the saturated ADMET EB copolymer
models clearly exhibit the characteristic shapes and absorption
values (two single peaks at 1461 and 720 cm-1), which suggests
an unorganized packing structure. The limited correlation
between IR and X-ray data in EB copolymers means that the
exact structure cannot be determined from the absorbance
spectra alone. However, following the vibrational analysis made
for n-alkenes and disordered polyethylene we can make certain
observations using the 1366, 1305, and 1352 cm-1 bands.29
Previously, the bands observed at 1366 and 1305 cm-1 were
assigned to a kink and the 1352 cm-1 to a double gauche defect
in PE materials.28 The overall concentration of gauche and kink
methylene sequences for our ADMET EB copolymers is reduced
with the decrease in branch defect content. This trend can be
observed by the comparison of these defect bands versus the
-CH2- scissoring at 1461 cm-1. Also, the ratio of these defect
bands (1366, 1305, and 1352 cm-1) relative to the methylene
wagging vibration at 1261 cm-1 shows a unique pattern. Close
inspection reveals that the ratio of all three disordered vibrations
are equal relative to each other; however, their ratio to 1461
cm-1 changes depending on the branch content and crystallinity.
Of course, both HPEB9 and HPEB15 are amorphous at the
recorded spectra temperature, while HPEB21 is semicrystalline.
The observed disordered ratio trend can also be observed using
the 801 and 769 cm-1 vibrations (visible with EB copolymers30).
The peaks most likely originate from a methylene rock and
proceed with a similar up/down ratio when the defect content
and crystallinity change. While at present the exact cause of
these vibrations and variable intensities cannot be correlated to
structural information in our EB copolymers, we can compare
them to our precise methyl-branched copolymers as well as
theoretical models (Figure 6b).
As mentioned earlier, our EB copolymers exhibit high
concentrations of the kink (tttgtgttt) defect. The concentration
of these defects relative to the double gauche (1351 cm-1
)
remains constant throughout the branch content. Also, it would
seem that the distorted trans segments (shoulder of the 1461
cm-1 absorption) hold this constant relationship as well. The
(31) (a) Baker, C. H.; Mandelkern, L. Polymer 1966, 7, 71. (b) Pe´rez, E.;
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1977, 18, 1005. (b) Vile, J.; Hendra, P. J.; Willis, H. A.; Cudby, M. E. A.;
Bunn, A. Polymer 1984, 25, 1173. (c) France, C.; Hendra, P. J.; Maddams,
W. F.; Willis, H. A. Polymer 1987, 28, 710. (d) McFaddin, D. C.; Russell,
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The higher content of these defects is in accord with the
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A R T I C L E S
Sworen et al.
nearly constant ratio of these bands versus defect content for
our EB copolymers suggests that defect equilibrium is inde-
pendent of methylene sequence length. The thermal analysis of
these polymers, discussed later, shows that HPEB9 is amor-
phous most likely because of the short sequences of trans
methylenes between branch kinks. As the branch frequency
decreases, the resulting longer run length of trans segments
enables the polymer to crystallize, depending on the temperature
as seen for HPEB21.
The larger steric demand has brought about a large increase
in the gauche and kick defects relative to ADMET EP
copolymers (Figure 6b) as well as a new observed vibration
(wag) at 769 cm-1. These defects are small and only slightly
observed for the methyl-branched HPEP21. There is also a shift
in the methylene scissoring vibration from 1472 cm-1 for
HPEP21 to 1461 cm-1 for HPEB21. Although the band
positions are slightly different from a typical orthorhombic
crystal of PE,28 observed at 1472 and 1463 cm-1, our EP and
EB copolymers encompass both peaks, respectively. To further
delineate the copolymers structure, the thermal behavior of these
materials was explored.
(D) Thermal Analysis. Numerous thermal behavior studies
have been performed on commercially produced LLDPEs41 and
EB copolymers containing a statistical distribution of ethyl
branches.4c,5,6a,b,8a,b,27a,42-48 Similar to the results obtained for
EP copolymers, studies on randomly branched EB copolymer
systems have shown that the density, enthalpy, degree of
crystallinity, and peak melting/crystallization points all decrease
as the amount of defect content (ethyl branches) is increased.
Like EP copolymers, the melting behavior of EB systems is
influenced by the amount of SCB; however, the SCB distribution
(SCBD) is by far the most determinant factor on the final
physical properties of a given material. The major problem
arising during modeling studies on ethylene-based polymers is
the compositional heterogeneity normally encountered for these
statistically branched materials. In this way, ADMET EB
copolymers present the advantage of being well-defined materi-
als with a homogeneous distribution of defects along the
backbone. Thus they make excellent substances with which to
model the effect that SCB and SCBD have on the final materials
response of ethylene-based materials.
Figure 7. DSC comparison of (1, top) HPEP9 (ADMET model ethylene/
propylene copolymer with a methyl on each 9th carbon) and (2, bottom)
HPEB9 (ADMET model ethylene/butylene copolymer with an ethyl on
each 9th carbon.
distribution along the hydrocarbon backbone. Previously studied
model EB copolymers, made from hydrogenated poly(buta-
dienes), have exhibited ill-defined melts for branch contents as
high as 106 ethyls/1000 carbons.44 In contrast, the ADMET
model EB copolymer HPEB9, possessing 111 ethyls/1000
carbons, shows no detectable melting point in the range studied
here, suggesting a completely amorphous behavior. This result
is interesting when compared to the narrow melting point
exhibited by the ADMET model EP copolymer (HPEP9) with
the same branch content (Figure 7).
The only viable explanation for this difference is that methyl
branches are readily incorporated into the crystal lattice, whereas
the steric demands of the ethyl branch preclude its taking part
in the crystallization process at this level of precise branch
distribution. In an effort to produce EB copolymers containing
crystalline segments the average methylene run length (MSL)
was increased to produce both model polymers HPEB15 and
HPEB21. Our modeling polymerization chemistry, ADMET,
lends itself perfectly for this task.
As shown previously we are able to modify the backbone of
our model polymers by simple monomer manipulation. Figures
2 and 3 illustrate that we have developed a routine synthesis to
create monomers containing an ethyl branch and any level of
branch content (‘R’/1000 carbons). In fact, the model copoly-
mers containing an ethyl branch on every 21st backbone carbon
were synthesized with two different methodologies. Utilizing
the Wittig reaction along with the appropriate ruthenium
polymerization catalyst, we were able to make the total synthesis
viable for modeling PE on a large scale. Overall this new
synthetic approach allows for modeling material properties and
perhaps polymer blends on industrial scales. Of course, for the
Wittig methodology to be a viable method for monomer
synthesis, the trisubstituted olefin must remain inactive through-
out the polymerization. If at any time the trisubstituted olefin
engages in metathesis, even after all terminal olefins have
reacted, the polymer primary structure would be altered. To
determine if the pendent olefin in the Wittig monomers were
actually inactive in the ADMET polymerization cycle, both
monomers 2 and 3 were synthesized and compared (Figure 8).
To prove the Wittig method of producing monomers was
adequate for modeling EB-branched polyolefins, HPEB21 was
synthesized from a purely R,ω diene monomer obtained by the
Figure 7 shows a calorimetric comparison between the
saturated ethyl (HPEB9) versus fully saturated methyl-branched
polymer (HPEP9). Both materials possess precise branch
(41) (a) Wilfong, D. L.; Knight, G. W. J. Polym. Sci., Part B: Polym. Phys.
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Xu, X. R.; Chen, L. S.; Feng, L. X. J. Mater. Sci. Lett. 2000, 19, 1541. (c)
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Xu, J.; Xu, X.; Feng, L. Eur. Polym. J. 1999, 36, 685. (e) Van Ekeren, P.
J.; Ionescu, L. D.; Mathot, V. B. F.; Van Miltenburg, J. C. Thermochim.
Acta 2002, 391, 185. (f) Starck, P.; Rajanen, K.; Lo¨fgren, B. Thermochim.
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11244 J. AM. CHEM. SOC. VOL. 126, NO. 36, 2004

Modeling Branched Polyethylene
A R T I C L E S
Figure 8. (a) DSC comparison for HPEB21, HPEB15W, and HPEB21W. (b) 13C NMR of HPEB21 and HPEB21W.
procedure outlined in Figure 3. The polymer was then used as
the reference and compared to the Wittig produced copolymer
(HPEB21W). The easiest and most effective method to make
a structure comparison between HPEB21 and HPEB21W was
close inspection of both 13C NMR and differential scanning
calorimetry (DSC). This comparison would lead to an identical
response to the external stimuli if both polymer’s macromo-
lecular behavior were the same. Further, the thermograph and
carbon spectra of both polymers would be equal if the primary
structures were equal. In fact, if at anytime throughout the
metathesis cycle the trisubstituted olefin in UPEB21W becomes
metathesis active, one or both of these techniques would detect
the structural change. Figure 8 reveals that both polymers have
Figure 9. DSC thermograph of (1, bottom) HPEB21W, (2, middle)
annealed HPEB21W for 14 days at 28 °C, and (3, top) annealed HPEB21W
for 28 days at 28 °C.
an identical carbon spectrum and exhibit the same nearly
monomodal melting profile containing two distinct peak melting
temperatures. The enthalpy ratio between these two melting
peaks is the same regardless of the polymer synthetic methodol-
ogy.
branches per 1000 total carbons. The crystal formation and/or
the crystallization kinetics also change when the overall branch
content is increased. For example, when going from the
amorphous HPEB9 to the semicrystalline HPEB21, the melting
profile becomes bimodal. These two different melting crystals
may originate from the same source when comparing HPEB15
and HPEB21, but regardless of crystal origin the enthalpy ratio
increases when going from HPEB15 to HPEB21, favoring the
higher melting crystal. We have used annealing experiments
on HPEB21W in an effort to force the copolymer to prefer a
single crystal form. The same experiments could have been
conducted with HPEB15W; however, we focused on HPEB21W
primarily because of its higher melting temperature (above room
temperature).
The precise nature of the branch location or constant MSL
produces a semicrystalline polymer favoring a single crystalline
region (Tm ) 34.3 °C). Comparison of model EB copolymers
synthesized using either metallocene42,45 or hydrogenated
polybutadienes,8b,44 at the same level of branch content, shows
very ill-defined endotherms. However, when HPEB21 (ethyl
branch) is compared to the precise methyl-branched model
HPEP21, studied previously, the latter has a higher peak melting
temperature (62 °C) and a higher melting enthalpy (103 J/g).
The EP model copolymer also exhibits a sharp melting profile
with no premelting in contrast to the rather large premelting
thermograph seen for HPEB21. To ascertain the semicrystal-
linity limit in our model EB copolymers, the methylene sequence
length was reduced to produce HPEB15W. This model
copolymer, containing an ethyl branch on every 15th carbon,
was synthesized using the Wittig approach only.
Figure 9 illustrates that the EB copolymer HPEB21W can
be manipulated by annealing the sample. The sample was
initially annealed in the DSC at the leading edge of the higher
melting crystal; in the case of HPEB21 the ideal temperature
was found at 28 °C. Upon annealing the sample for 14 days
(Figure 9), the reduction of the lower melting crystal was
substantial and produced a polymer with a precise narrow peak
melting temperature of 34.4 °C. In an effort to reduce the
premelting in HPEB21W, the annealing experiment was carried
outside in an isothermal bath at 28 °C for 28 days (Figure 9).
Figure 9 illustrates that the polymer’s small endotherms and
premelting observed after 14 days have been eliminated,
The reduction of the methylene sequence length causes the
melting point to drop below room temperature. The polymer
still exhibits semicrystallinity, but increasing the defect content
results in a bimodal melting profile. Similar results have been
observed for continuously cooled hydrogenated polybutadienes
at much higher branch content.44 Apparently, precisely placed
ethyl-branched EB copolymers have a single sharp melting
profile for copolymers containing approximately 50-60 ethyl
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A R T I C L E S
Sworen et al.
producing a distinct, narrow single melting peak for HPEB21-
(W). The melting profile for our copolymer containing an ethyl
branch precisely placed on every 21st carbon exhibits the same
behavior as our model EP copolymers, albeit at lower temper-
atures.
chain-addition polymerization. Further studies are being con-
ducted to investigate the packing of these unique materials and
delineate correlations to previously synthesized EP model
copolymers.49
Conclusions
In this light, consensus has it that melting temperatures,
crystallinity, and lamella thickness are a function of branch
content and are relatively independent of branch size (excluding
methyl). Since the identity of the branch (again, excluding
methyl) has little effect on the crystal nature of LLDPE due to
branch exclusion, the comonomer incorporation is the most
important factor influencing the polymer’s behavior. These
issues are a direct result of catalyst choice and polymerization
conditions; for example, a metallocene EB copolymer (homo-
geneous) has a lower Tm (26 °C) than a polymer with the same
average branch content (30 SCB/1000C) but synthesized with
a heterogeneous catalyst system.45 These melting differences
have been suggested to originate from the thick unbranched
crystal formation witnessed in all heterogeneous systems. The
more homogeneous branched system lacks the longer un-
branched sequences and as a result thinner crystals and lower
melting temperatures. Our ADMET-produced polymers have
perfect and constant MSL through every repeating sequence so
a comparison between homogeneous, heterogeneous, and our
copolymers is important.
Correlations between branch content and melting temperature
in EB copolymers have been investigated for metallocene,
Ziegler-Natta (Z-N), and hydrogenated polybutadienes
(HPB).44,45,47,48 From the data reported, the peak melting
temperature decreases with more controlled polymerization
conditions. For the same branch content Tm follows the trend
95, 88, and 62 °C for HPB, Z-N, and metallocene, respectively.
Of course, ADMET copolymers have more control of the
polymer’s primary structure, even more than those observed
for any metallocene catalyst. Having theoretically no hetero-
geneity in branch incorporation (or MSL), ADMET copolymers
should exhibit lower melting points than metallocene copoly-
mers, most likely resulting from smaller crystallite formation.
Relating the branch content of our ADMET copolymers to the
derived chain-addition models agrees with this assumption.
Indeed, if HPEB21 was produced by chain addition, the level
of branch content (43.5 SCB/1000 carbons) would predict a
melting point of ∼60 °C (actual Tm ) 34.5 °C).45 Furthermore,
the enthalpy of fusion of our ADMET EB copolymers is much
higher (∼57 J/g) than either metallocene (∼30 J/g) or Z-N
(∼42 J/g) copolymers. These results are consistent with ADMET
producing more homogeneous sequence lengths relative to any
Acyclic diene metathesis polymerization has proven to control
the primary structure of ethylene/1-butene copolymers resulting
in linear polyethylene containing only ethyl branches. In this
effort, a simple synthetic method has been developed to produce
exact linear model polymers on an industrial scale using
ketodienes. The inherent ability of metathesis to control the
polymers branch identity and placement has profound effects
on the thermal and crystal behavior of these distinct EB materials
resulting in a new class of LLDPEs.
The structural investigation has shown that these ADMET
EB copolymers favor ethyl branch inclusion, producing a similar
crystal structure obtained for our EP model copolymers.
Moreover, these copolymers exhibit distorted methylene se-
quences with high concentrations of kink, gauche, and double
gauche defects. The inter- and intrahomogeneity of the sequence
length distribution in these materials produces lower melting
points and higher melting enthalpies when compared to chain-
propagated EB polymers. In fact, the thermal behavior of these
materials concludes that the MSL and its distribution are more
controlled versus chain-addition chemistry and single-site met-
allocene systems. Step polymerization also produces narrow
monomodal melting profiles when branch content reaches
approximately 45 ethyl branches/(1000 carbons) or 9 mol %
1-butene.
We are currently continuing this branched polyethylene
research by gathering a better base of scattering data and
understanding the differences between random and precise
branch content. In addition, we are investigating precisely
branched linear, low-density materials containing hexyl and
ultimately longer defects.
Acknowledgment. We wish to thank the National Science
Foundation (Grant No. DMR9806492) for financial support of
this research.
Supporting Information Available: Detailed characterization
and experimental data for all synthesized monomers and
polymers. This information is available free of charge via the
Internet at http://pubs.acs.org.
JA047850P
(49) Lieser, G.; Wegner, G.; Smith, J. A.; Wagener, K. B. Colloid Polym. Sci.
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