Precision polyethylene: Changes in morphology as a function of alkyl branch size

Article abstract of DOI:10.1021/ja907521p

Metathesis polycondensation chemistry has been employed to control the crystalline morphology of a series of 11 precision-branched polyethylene structures, the branch being placed on each 21st carbon and ranging in size from a methyl group to an adamantyl group. The crystalline unit cell is shifted from orthorhombic to triclinic, depending upon the nature of the precision branch. Further, the branch can be positioned either in the crystalline phase or in the amorphous phase of polyethylene, a morphology change dictated by the size of the precision branch. This level of morphology control is accomplished using step polymerization chemistry to produce polyethylene rather than conventional chain polymerization techniques. Doing so requires the synthesis of a series of unique symmetrical diene monomers incorporating the branch in question, followed by ADMET polymerization and hydrogenation to yield the precision-branched polyethylene under study. Exhaustive structure characterization of all reaction intermediates as well as the precision polymers themselves is presented. A clear change in morphology was observed for such polymers, where small branches (methyl and ethyl) are included in the unit cell, while branches equal to or greater in mass than propyl are excluded from the crystal. When the branch is excluded from the unit cell, all such polyethylene polymers possess essentially the same melting temperature, regardless of the size of the branch, even for the adamantyl branch.

Full text of DOI:10.1021/ja907521p

Published on Web 11/05/2009
Precision Polyethylene: Changes in Morphology as a
Function of Alkyl Branch Size
Giovanni Rojas,† Bora Inci, Yuying Wei, 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
Received September 4, 2009; E-mail: wagener@chem.ufl.edu
Abstract: Metathesis polycondensation chemistry has been employed to control the crystalline morphology
of a series of 11 precision-branched polyethylene structures, the branch being placed on each 21st carbon
and ranging in size from a methyl group to an adamantyl group. The crystalline unit cell is shifted from
orthorhombic to triclinic, depending upon the nature of the precision branch. Further, the branch can be
positioned either in the crystalline phase or in the amorphous phase of polyethylene, a morphology change
dictated by the size of the precision branch. This level of morphology control is accomplished using step
polymerization chemistry to produce polyethylene rather than conventional chain polymerization techniques.
Doing so requires the synthesis of a series of unique symmetrical diene monomers incorporating the branch
in question, followed by ADMET polymerization and hydrogenation to yield the precision-branched
polyethylene under study. Exhaustive structure characterization of all reaction intermediates as well as the
precision polymers themselves is presented. A clear change in morphology was observed for such polymers,
where small branches (methyl and ethyl) are included in the unit cell, while branches equal to or greater
in mass than propyl are excluded from the crystal. When the branch is excluded from the unit cell, all such
polyethylene polymers possess essentially the same melting temperature, regardless of the size of the
branch, even for the adamantyl branch.
Introduction
Commercial LLDPE is prepared by chain-growth polymer-
ization using Ziegler-Natta or metallocene chemistry.13 The
Given that polyethylene (PE) is the most widely consumed
macromolecule today, its structure remains of scientific interest
in spite of the vast amount of research already present in the
literature. The ability to produce materials with a wide range
of polymer architectures is a result of systematically altering
the morphology of this polyolefin.1,2 Improvements to the
performance of polyethylene can be achieved by controlling the
mode of polymerization, catalyst type, pressure, and temperature.
These manipulations can alter the amount of short-chain
branching (SCB) and the short-chain branch distribution (SCBD)
to tune the final properties like tensile strength, stiffness,
processability, and melting temperature.1,2 Linear low-density
polyethylene (LLDPE) structures are of particular interest. These
are copolymers of ethylene and R-olefins, where the short chain
branch is delivered by the R-olefin comonomer. Propene,
1-butene, 1-hexene, and 1-octene are the comonomers of choice
due to their low cost and their well understood effects on the
thermal and mechanical properties of the final material.2-12
absence of multisite initiation in single-site metallocene systems
results in the synthesis of copolymers possessing narrower
molecular weight distributions and higher levels of R-olefin
comonomer incorporation.14,15 Due to the well-defined nature
of the catalyst, metallocene systems produce materials incor-
porating linear and bulky R-olefins not compatible with
Ziegler-Natta chemistry. Copolymerization via metallocene
chemistry of ethylene with odd-carbon-number R-olefins (e.g.,
1-pentene or 1-heptene) is also feasible, because such com-
pounds are available using the Fischer-Tropsch olefin synthesis
process. These possibilities have led to the creation of a
significant number of ethylene/R-olefin copolymers with a wide
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† Present address: Max Planck Institute for Polymer Research, Mainz,
Germany, 55128.
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Changes in Morphology of Precision Polyethylene
A R T I C L E S
range of applications.16-24 Furthermore, metallocene copolym-
erization of ethylene with nonlinear, bulkier R-olefins such as
3-methyl-1-butene,25-28 4-methyl-1-pentene,29-32 vinyl-
cyclohexene,33-36 and norbornene28 has created a new class of
materials with better impact strength than that of traditional
ethylene linear R-olefin copolymers.
By its nature, copolymerization of ethylene with R-olefins
via chain-propagation chemistry incorporates structural defects
via head-to-head or tail-to-tail monomer coupling. Further,
inevitable chain transfer or chain walking produces structures
with alkyl branches of varying lengths randomly spaced along
the main chain.37-41
∆hm ) 204 J/g).55 The crystalline unit cell is orthorhombic. A
most probable distribution of molecular weights exists, and Mw
values approaching 80 000 can be achieved in this way.
The key synthetic point in this approach revolves around the
synthesis of appropriate symmetrical diene monomers. In order
to ensure absolute control of branch identity and its precision
placement along the polymer backbone, the branch is “built-
in” the diene monomer itself before polymerization. Homo-
polymerization of the diene monomer (via polycondensation),
followed by exhaustive hydrogenation, generates a symmetrical
repeat unit, which in effect mirrors a precision model ethylene/
R-olefin copolymer under examination.
Precisely placed methyl branches in ADMET PE were the
first to be examined.43 Continuation of this research led to the
synthesis of ethyl-branched polyethylene44 and subsequently to
the creation of butyl- and hexyl-branched polyethylene.45,46 This
paper extends the study to branches of different mass for
comparative purposes, where we have observed a significant
change in polymer morphology as a function of branch size.
We report the synthesis, characterization, thermal behavior, and
X-ray investigation of polyethylene containing 11 different
branches, each placed on every 21st carbon in polyethylene.
In recent years, we have obviated formation of these defects
by turning to step polycondensation chemistry (the ADMET
reaction) rather than chain polymerization chemistry.42 Sym-
metrical diene monomers are condensed into unsaturated
polymers which upon hydrogenation generate what we call
“precision-branched polyethylene”. We are able to create
polymers where both the identity of the branch and its position
along the chain are known without equivocation. We are also
able to systematically “randomize” the position of the branch
to mirror “reality” in commercial polymer systems. Note that
we have made unbranched ADMET polyethylene to serve as a
point of comparison with chain-made commercial polyethylene.
The thermal behavior of this ADMET polyethylene is virtually
the same as that of high-density polyethylene (Tm ) 134 °C,
Results and Discussion
A. Monomer Synthesis and ADMET Polymerization of
Precisely Sequenced Polymers. Monomer synthesis is key in
this research, where earlier efforts employed elaborate meth-
odologies for the synthesis of R,ω-dienes in only moderate
yields.43-45 Lately, we have been able to synthesize symmetrical
R,ω-diene monomers possessing alkyl branches of various
lengths in two steps with nearly quantitative yields via the
alkylation/decyanation of primary nitriles. This approach is
based on the double alkenylation of the carbon R to the nitrile,
followed by the reductive elimination of the nitrile moiety.
Scheme 1 shows the synthetic approach for the preparation of
alkyl R,ω-diene monomers (4a-k) from primary nitriles 2a-k,
where the ease of monomer synthesis has allowed us to expand
the scope of our study significantly. While most of nitriles were
commercially available, nitriles 2i and 2j were synthesized by
cyanation of bromides 1i and 1j.
Alkenylation of nitriles 2a-k in the presence of lithium
diisopropyl amide (LDA) and 11-bromoundec-1-ene produces
the alkylcyano R,ω-dienes 3a-k in quantitative yields. Decyan-
ation of nitriles 3a-k is achieved with potassium metal via
radical chemistry. The resulting tertiary radical after decyanation
is further quenched by abstraction of hydrogen from t-BuOH
to give R,ω-diene monomers 4a-k in quantitative yields.
Scheme 2 illustrates the ADMET polycondensation of alkyl
R,ω-diolefin monomers 4a-k using first-generation Grubbs
catalyst (5). Typical polycondensation procedures are used (no
solvent, high vacuum, preferably using mechanical stirring in
the reactor). The polymerization proceeds efficiently, yielding
11 unsaturated polymers with branches of methyl to adamantyl
on every 21st carbon. Exhaustive hydrogenation of the unsatur-
ated polymers using p-toluenesulfonyl hydrazide, tripropyl
amine, and xylene yields saturated ADMET polymers. As
described further below, the efficiency of hydrogenation can
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A R T I C L E S
Rojas et al.
Scheme 1. Synthesis of 12-Alkyltricosa-1,22-dienes (4a-k)
main chain. Examination of 1H and 13C NMR spectra of
monomers and polymers confirms complete transformation and
control over the primary structure’s precision.
Scheme 2. Synthesis of Precisely Sequenced Polymers
As an example of this level of control, Figure 1 shows the
1H NMR spectra for the ADMET polymer 6i-(tert-butyl) and
its precursors. The transformation begins with the cyanation of
1-bromo-2,2-dimethylpropane (1i), yielding 3,3-dimethyl-
butanenitrile (2i), shown in Figure 1a. Double alkenylation of
nitrile 2i with 11-bromoundec-1-ene yields premonomer 3i,
2-tert-butyl-2-(undec-10-enyl)tridec-12-enenitrile, as shown in
Figure 1b. Decyanation of nitrile 3i forms monomer 4i, 12-
tert-butyltricosa-1,22-diene, in quantitative yield, as shown in
Figure 1c. ADMET polymerization of 4i yields the unsaturated
polymer, 5i-(tert-butyl). Analysis of the olefin region (5-6 ppm)
supports formation of polymer, which is evidenced by the
disappearance of the terminal olefin signals (5.1 and 5.9 ppm
in Figure 1c) and the formation of the internal olefin (5.3 ppm
in Figure 1d). Further hydrogenation of the internal olefins yields
6i-(tert-butyl), a perfectly sequenced polymer, corresponding
to polyethylene with tert-butyl branches on every 21st backbone
carbon, with no observable traces of olefin by 1H NMR (Figure
1e).
Table 1. Molecular Weights and Thermal Data for Precisely
Sequenced Polymers
Mw × 103 (PDI)b
¯
branch identity on
every 21st carbon
Tm, °C (peak)
∆hm, J/g
unsaturateda
saturateda
methyl
ethyl
propyl
butyl
pentyl
hexyl
isopropyl
sec-butyl
tert-butyl
cyclohexyl
adamantyl
63
24
12
12
14
12
11
9
104
65
60
57
58
49
37
43
50
20.2 (1.7)
50.2 (1.9)
41.2 (1.7)
41.5 (1.8)
45.1 (1.8)
44.6 (1.8)
45.5 (1.7)
43.0 (1.6)
30.6 (1.7)
32.5 (1.6)
64.7 (1.7)
20.2 (1.7)
50.7 (1.9)
41.4 (1.7)
40.3 (1.7)
45.8 (1.8)
46.1 (1.7)
46.0 (1.7)
42.6 (1.9)
32.1 (1.7)
33.6 (1.6)
70.8 (1.3)
Figure 2 shows the 13C NMR spectra for the same transfor-
mations. In the spectrum for the 3,3-dimethylbutanenitrile (2i,
Figure 2a), the resonance at 118.67 shows the presence of the
nitrile functionality. After the double alkenylation of nitrile 2i,
the spectrum for 2-tert-butyl-2-(undec-10-enyl)tridec-12-ene-
nitrile (3i, Figure 2b) shows the presence of the nitrile
functionality at 122.95 ppm along with the characteristic
terminal olefin signals at 114.27 and 139.26 ppm, respectively.
The absence of the signal at 122.95 ppm after decyanation
(Figure 2c) demonstrates complete elimination of the CN group,
yielding monomer 4i, 12-tert-butyltricosa-1,22-diene. ADMET
polymerization of 4i yields the unsaturated polymer 5i-(tert-
butyl). Comparison of parts c and d of Figure 2 shows the
disappearance of the signals belonging to the terminal olefin at
114.31 and 139.44 ppm and formation of the new internal olefin
(cis at 130.12 ppm, minor component, and trans at 130.58 ppm,
major component) produced from the effective metathesis
polymerization. Subsequent exhaustive hydrogenation of the
internal olefin yields the saturated polymer 6i-(tert-butyl), whose
13C NMR spectrum (Figure 2e) shows no detectable trace of
olefins. This observation is further supported by the absence of
the out-of-plane C-H bend in the alkene region at 969 cm-1 in
the infrared (IR) spectrum, as described below.
13
9
37
2 and 8
-8 and 17
a Weight-average molecular weight data obtained by GPC in THF (40
°C) relative to polystyrene standards (g/mol). b PDI, polydispersity index
j
j
(Mw/Mn).
1
be followed by the disappearance of the olefin signals in H
NMR (Figure 1) and 13C NMR (Figure 2) and by the absence
of the out-of-plane alkene C-H bend (969 cm-1) using infrared
(IR) spectroscopy (Figure 4).
Table 1 reports the molecular weights for the precisely
sequenced polymers obtained via ADMET polymerization;
weight-average molecular weights were obtained by gel per-
meation chromatography (GPC) versus polystyrene standards
with molecular weights of sufficient mass to allow analysis of
thermal behavior. As expected, most probable molecular weight
distributions are observed. Important to note is that the molecular
weights before and after hydrogenation show that no cleavage
of the polymer chains occurs during hydrogenation.43
B. Structural Data for Precisely Sequenced Polymers. This
synthetic approach produces polymers with precisely known
primary structures,43-45 meaning that the polymers only have
defects (branches) intentionally and evenly placed along the
Upon close inspection of the 13C NMR data for the ADMET
polymers, it can be concluded that the branches are precisely
placed along the polyethylene main backbone with no unwanted
“defects” due to chain transfer typically observed during chain-
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Changes in Morphology of Precision Polyethylene
A R T I C L E S
Figure 1. Comparison of 1H NMR spectra for a typical ADMET polymerization transformation 6i-(tert-butyl): (a) 3,3-dimethylbutanenitrile (2i); (b)
premonomer 3i, 2-tert-butyl-2-(undec-10-enyl)tridec-12-enenitrile; (c) monomer 4i, 12-tert-butyltricosa-1,22-diene; (d) ADMET unsaturated polymer 5i-
(tert-butyl); and (e) ADMET saturated polymer 6i-(tert-butyl).
growth chemistry. Figure 3 shows a portion (10-55 ppm) of
the 13C NMR spectra for some of the precisely sequenced
polymers, 6c-(propyl), 6e-(pentyl), 6g-(iso-propyl), 6i-(tert-
butyl), and 6j-(cyclohexyl). All spectra are dominated by a
singlet at 29.99 ppm corresponding to methylenes on the main
polyethylene chain. Note that the presence of alkyl branches
precisely placed along the main chain affects the chemical shifts
of carbons located within three CH2 units from an individual
branch.47 In the spectrum for 6c-(propyl), which is polyethylene
containing propyl branches on every 21st backbone carbon
(Figure 3a), the resonances belonging to the propyl branch, 1
(δ 14.80 ppm), 2 (δ 20.07 ppm), 3 (δ 36.37 ppm), and the
carbon at the branch point 4 (δ 37.42 ppm) indicate that only
propyl branches are present, in agreement with previously
reported data on chain-growth materials obtained by copolym-
erization of ethylene with 1-pentene.17,48-51 While the 6c-
(propyl) spectrum shows resonances for three carbons at the
propyl branch, the 6e-(pentyl) spectrum, Figure 3b, shows five
resonances corresponding to the pentyl branch precisely placed
on every 21st backbone carbon: a (δ 14.80 ppm), b (δ 22.97
ppm), c (δ 26.62 ppm), d (δ 26.94 ppm), and e (δ 33.89 ppm).
In agreement with the 6c-(propyl) spectrum, which shows a
resonance for the branch point carbon at 37.42 ppm, 6e-(pentyl)
shows the corresponding branch-point peak at 37.64 ppm.21-24
(48) Blitz, J. P.; Mcfaddin, D. C. J. Appl. Polym. Sci. 1994, 51, 13–20.
(49) Randall, J. C. J. Polym. Sci. Part B: Polym. Phys. 1973, 11, 275–
287.
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2000, 38, 3120–3135.
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Polym. Phys. 2002, 40, 2595–2611.
S. P. J. Am. Chem. Soc. 2003, 125, 2228–2240.
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A R T I C L E S
Rojas et al.
Figure 2. Comparison of 13C NMR spectra for a typical ADMET polymerization transformation 6i-(tert-butyl): (a) 3,3-dimethylbutanenitrile (2h); (b)
premonomer 3h, 2-tert-butyl-2-(undec-10-enyl)tridec-12-enenitrile; (c) monomer 4h, 12-tert-butyltricosa-1,22-diene; (d) ADMET unsaturated polymer 5i-
(tert-butyl); and (e) ADMET saturated polymer 6i-(tert-butyl).
In addition to spectra a and b of Figure 3 for polyethylene
with precisely placed linear branches, spectra c-e of Figure 3
present the 13C NMR spectra for polyethylene containing
nonlinear bulkier branches on every 21st carbon. 6g-(iso-propyl),
6i-(tert-butyl), and 6j-(cyclohexyl) are shown; 6h-(sec-butyl)
and 6k-(adamantly) were omitted for better clarity. In all cases,
the resonances belonging to the branches, as well as the
resonance from the carbon at the branch point, indicate that
only intentionally placed branches are present. Peak assignments
were based on previous detailed NMR studies.25-36,52-63
It is not surprising that the presence of a bulkier branch, for
example isopropyl, causes a shift in the resonance corresponding
to the carbon at the branch point, 37.42 and 37.64 ppm for the
carbons at propyl and pentyl branches, respectively, versus 43.96
ppm for the carbon directly attached to an isopropyl branch.
The resonances for the backbone carbon at all nonlinear
branches were shifted, due to the highly branched functional-
ization, as shown in Figure 3.
In addition to the NMR characterization of the precisely
sequenced ADMET materials, infrared (IR) spectroscopy was
used to study all precision polymers. Although X-ray diffraction
techniques provide the absolute crystal structure, IR spectros-
copy is a complementary technique that provides an estimation
of the crystal structure. Tashiro et al. carried out a study of
branching behavior on polyethylene using wide-angle X-ray
diffraction (WAXD), IR, and Raman spectroscopy. They
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Changes in Morphology of Precision Polyethylene
A R T I C L E S
Figure 3. Comparison of 13C NMR spectra for a typical ADMET polymerization transformation possessing bulky branches: (a) 6c-(propyl), (b) 6e-
(pentyl), (c) 6g-(iso-propyl), (d) 6i-(tert-butyl), (e) 6j-(cyclohexyl). 6h-(sec-butyl) and 6k-(adamantyl) were omitted for a better figure presentation.
concluded that the scissoring at 1466 cm-1 and the methylene
rock at 721 cm-1 indicate a hexagonal crystal structure, while
the double methylene rock at 719 and 730 cm-1 and single band
at1471cm-1 correspondtoanorthorhombiccrystalstructure.6,64,65
Figure 4 shows the IR spectra for the 6c-(propyl), 6e-
(pentyl), 6g-(iso-propyl), 6h-(sec-butyl), 6i-(tert-butyl), 6j-
(cyclohexyl), and 6k-(adamantyl) polymers, with the spectra
of previously reported precisely sequenced ones, 6a-(methyl),43
6b-(ethyl),44 6f-(hexyl),45 and 6d-(butyl),46 included for com-
parison. As mentioned above, there is no out-of-plane C-H
bend absorption at 969 cm-1, indicating complete absence of
CdC in the saturated polymers. All spectra are dominated by
two sets of absorption bands (∼2900 and 1472 cm-1) belonging
to C-H stretching and methylene scissoring, respectively. In
addition to the stretching and scissoring, the methylene rocking
band is observed for all copolymers at 718 cm-1, which is often
associated with the monoclinic crystalline structure.66 Although
orthorhombic crystals show the characteristic Davidov splitting
64,65
around 720 cm-1
,
all of our precisely sequenced ADMET
polymers display a single absorption band at 718 cm-1
,
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A R T I C L E S
Rojas et al.
Figure 4. Infrared spectra for the saturated ADMET polymers 6a-(methyl), 6b-(ethyl), 6c-(propyl), 6d-(butyl), 6e-(pentyl), 6f-(hexyl), 6g-(iso-propyl),
6h-(sec-butyl), 6i-(tert-butyl), 6j-(cyclohexyl), and 6k-(adamantyl).
indicating the absence of orthorhombic crystal behavior. This
is a direct result of the branch being present. Moreover, the
two experimental absorption bands at 718 and 1472 cm-1 are
characteristic of a highly disordered phase, similar to the pattern
previously observed.43-45 Comparison of all spectra in Figure
4 suggests that incorporation of linear defects (methyl to hexyl)
and nonlinear bulkier branches (iso-propyl, sec-butyl, tert-butyl,
cyclohexane, and adamantyl) evenly spaced along the polyeth-
ylene chain does not alter the C-H stretching or methylene
scissoring and methylene rocking regions in the IR. However,
there is significant variation in the intensity of the absorption
band at 1378 cm-1, corresponding to the symmetrical bend for
the terminal methyls on the pendant branches. As Figure 4
shows, the intensity of the absorption band at 1378 cm-1 is
smaller when linear alkyl branches are incorporated, while
higher intensity is observed when bulkier branches (iso-propyl,
sec-butyl, and tert-butyl) are placed along the PE chain. On
the other hand, when cyclohexyl and adamantyl groups are
precisely placed on every 21st backbone carbons, the absorption
at 1378 cm-1 almost disappears because of the absence of
methyl groups at the branch; the very weak absorption at 1378
cm-1 is due to the terminal methyl groups of the PE backbone
of 6j-(cyclohexyl) and 6k-(adamantyl).
for ADMET-produced random and precisely sequenced linear
low-density PEs (LLDPE).43-45,47,72-76 Those studies showed
that the melting behavior of ethylene-based copolymers is
influenced by the amount of short-chain branching (SCB), where
the determining factor on the final physical properties of the
resulting LLDPE is the short-chain-branch distribution (SCBD).
Interpretation of such thermal results for chain-growth PE has
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C. Thermal Behavior for Precisely Sequenced Polymers.
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mercially produced chain-growth LLDPE,11,36,67-71 as well as
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Changes in Morphology of Precision Polyethylene
A R T I C L E S
Figure 5. Differential scanning calorimetry curves for ADMET polymers possessing linear branches.
been limited, due to the heterogeneity of the material and the
presence of unwanted defects.
explained by the disruption of the crystal structure due to the
presence of the precisely placed methyl “defects”. The same
trend is observed when the alkyl branch is an ethyl group, 6b-
(ethyl), as shown in Figure 5. Incorporation of an ethyl branch
“defect” on every 21st backbone carbon renders a material with
a melting point below that of 6a-(methyl) (Tm ) 24 °C with a
heat of fusion ∆hm ) 65 J/g). This indicates that the larger ethyl
groups disrupt the crystal structure even more, resulting in
depression of both the melting temperature and the heat of
fusion. Even so, the branch remains in the unit cell, albeit
changing its crystalline lattice. Crystal structures of both 6a-
(methyl) and 6b-(ethyl) obtained using wide-angle X-ray
diffraction (WAXD) and small-angle X-ray scattering (SAXS)
reveal that the chains pack into a triclinic lattice that allows
inclusion of methyl and to some extent ethyl branches as lattice
defects.44,47
The ADMET polymers 6c-(propyl), 6d-(butyl), 6e-(pentyl),
and 6f-(hexyl) also give well-defined endotherms, as shown in
Figure 5. Extension of the branch size from ethyl to propyl
follows the same pattern of decreasing the melting temperature
and heat of fusion (Tm ) 24 °C and ∆hm ) 65 J/g for 6b-
(ethyl), Tm ) 12 °C and ∆hm ) 60 J/g for 6c-(propyl)).
However, at this point the morphology of the polymer changes
significantly. Incorporation of larger branches, even adamantyl
branches, leads to a series precision branched polyethylenes
having very similar - in some cases identical - melting behavior,
regardless of the mass of the branch. Polymers containing linear
branches with three or more carbons on every 21st backbone
carbon [6c-(propyl), 6d-(butyl), 6e-(pentyl), and 6f-(hexyl)]
melt at ∼13 °C with ∆hm ∼ 58 J/g. Three- to six-carbon side
branches are excluded from the crystal lattice, quite a different
situation than for methyl and ethyl branching. It is noteworthy
that the endotherm for the ethyl branched polymer, 6b-(ethyl),
shows a shoulder on the lower melting side. This may indicate
partial exclusion of ethyl branches from the unit cell such that
the material may contain two types of crystalline regions.
The situation is quite different in the case of the precision
branched polyolefins described here. The branch-to-branch
distance is held constant, while the identity of the branch is
systematically increased in size. An immediate observation
becomes obvious: methyl and ethyl branching reduces the
melting point of ADMET polyethylene in a progressing manner;
on the other hand, all further branches, from propyl to
adamantyl, produce polymers that have essentially the same
melting point. There is a clear change in morphology of these
polymers from a situation where the branch (methyl or ethyl)
is included in the polymer’s unit cell to one where branches of
greater mass are excluded from the unit cell.
Figure 5 shows the DSC thermograms for the previously
reported 6a-(methyl), 6b-(ethyl), 6d-(butyl), and 6f-(hexyl)
polymers,43-45 along with the 6c-(propyl) and 6e-(pentyl)
polymer models (refer to Table 1 for physical data). Similar to
previous studies involving ADMET PE,43-45 6c-(propyl) and
6e-(pentyl) (i.e., polyethylene containing propyl and pentyl
branches on every 21st backbone carbon) display well-defined
endothermic transitions, with none of the broadening observed for
analogous copolymers obtained via chain polymerization.9,11,12,77-79
The data in Figure 5 show that incorporation of defects
precisely spaced by 20 backbone carbons causes the melting
temperature of the material to decrease. While high-density
polyethylene with practically no defects along the main chain
shows a melting transition at Tm ) 134 °C with a heat of fusion
∆hm ) 204 J/g,42 incorporation of methyl branches on every
21st backbone carbon, 6a-(methyl), decreases the melting point
to 63 °C and the enthalpy of fusion to 104 J/g. This effect is
(77) Bracco, S.; Comotti, A.; Simonutti, R.; Camurati, I.; Sozzani, P.
Macromolecules 2002, 35, 1677–1684.
(78) Zhang, F. J.; Song, M.; Lu, T. J.; Liu, J. P.; He, T. B. Polymer 2002,
43, 1453–1460.
(79) Pak, J.; Wunderlich, B. Macromolecules 2001, 34, 4492–4503.
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Figure 6. Differential scanning calorimetry curves for ADMET polymers possessing bulkier branches.
Similar observations are made for precision polyethylene
possessing nonlinear, even bulkier branches. Data for such
polymers are found in Figure 6 (also Table 1) for polymers
6g-(iso-propyl), 6h-(sec-butyl), 6i-(tert-butyl), 6j-(cyclohexyl),
and 6k-(adamantyl). Melting points are similar, in the range
of 9-17 °C with similar enthalpies of fusion. These homologous
polymers with nonlinear bulkier branches display sharp, well-
defined endothermic transitions, with none of the broadening
observed for copolymers obtained via chain polymerization.25-36
It is interesting that polymers with bulky, nonlinear branches
display much the same thermal properties as polymers with
linear side chains of three or more carbons; both types of
branches are excluded from the unit cell. There must be a limit
in size where the side branch totally disrupts crystallization of
the main PE chain. We are approaching that limit with the
adamantyl branch, since the crystal lattice is quite disturbed,
forcing the material to crystallize in two different domains, as
evidenced by a bimodal endotherm (Figure 6). Although not
shown here, annealing experiments reveal that it is possible to
force the adamantyl-branched polyethylene to crystallize in one
type of crystalline region by holding the sample at -20 °C for
2 h. Only one endotherm was observed at -19 °C with a ∆hm
of 26 J/g.
D. X-ray Investigation for Precisely Sequenced Polymers.
Wide-angle X-ray diffraction (WAXD) measurements further
support the observation of a change in polymer morphology as
a function of branch size. Six such WAXD diffractograms are
shown in Figure 7; these patterns indicate that the introduction
of branches leads to the lattice distortion and local conforma-
tional disorder, where the type of crystal structure and polymer
morphology is strongly dependent on the branch identity.
For the sake of comparison, ADMET PE is displayed at the
bottom of Figure 7 exhibiting the typical orthorhombic crystal
form with two characteristic crystalline peaks superimposed with
the amorphous halo, exactly the same as for high-density
polyethylene made by chain propagation chemistry. The more
intense peak at scattering angle 21.5° and the less intense one
at 24.0° correspond to reflection planes (110) and (200),
respectively. Upon introducing precisely placed branches of
known identity, the crystal structure loses its symmetry with
the unit cell shifting from orthorhombic to triclinic. Moreover,
in contrast to linear polyethylene, scattering occurs at relatively
lower scattering angles and with broader reflections being
displayed, suggesting the increase of crystallite size and a
decrease of crystallinity.
In the case of the methyl-branched polymers 6a-(methyl),
two reflections representing a triclinic crystal orientation occur
at scattering angle 19° with the Miller index (100) and 22° with
the Miller index (010). Transmission electron microscopy (TEM)
shows the lamellar thickness to be quite small, between 10 and
20 nm,81 a thickness that can only present if the methyl side
chains are incorporated in a triclinic lattice.
Similar changes in crystalline unit cell identity were observed
in the ethyl branched polymer, 6b-(ethyl).80 Two strong
reflections shift to lower scattering angles (18° and 21°)
compared to the methyl-branched polymer (19° and 22°). On
the basis of Bragg’s law, this observation allows us easily to
draw the conclusion that the ethyl branch is incorporated in the
crystal region. In contrast to the 6a-(methyl), the introduction
of the ethyl side chain perturbs the crystal structure more and
requires larger space to be incorporated. As a consequence, the
reflections shift to lower scattering angles, which correspond
to larger d-spacing.
For the polymers possessing bulkier branches (propyl or
larger), the WAXD diffractograms (the top four graphs in Figure
7) show nearly identical scattering patterns, indicating that the
crystal structure is independent of the branch identity. Moreover,
these patterns are obviously different from those of polymers
possessing smaller branches like methyl or ethyl branches,
exhibiting larger scattering angles and even broader reflections.
To understand these observations, the scattering angles of
two strong diffraction peaks as a function of branch identity
are illustrated in Figure 8. We see two trends: the decrease of
scattering angles for smaller branches, followed by leveling-
(80) Sworen, J. C. Modeling Linear-Low Density Polyethylene: Copolymers
Containing Precise Structures. PhD Thesis, University of Florida,
Gainesville, FL, 2004.
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Sci. 2004, 282, 773–781.
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Changes in Morphology of Precision Polyethylene
A R T I C L E S
Figure 8. Scattering angles of two strong reflections for alkyl-branched
precision polymers. The top graph is for the reflection at higher angle, while
the bottom one is for the reflection at lower angle.
Note that the reflection occurring at ∼19.5° is much broader
and less symmetric than peaks for the methyl-branched polymer
6a-(methyl) or ethyl-branched polymer 6b-(ethyl). The asym-
metry suggests the presence of more than one crystal lattice
besides triclinic; broadening is due to a decrease in the degree
of crystallinity.
Of particular interest is the fact that the WAXD scattering
patterns for these precision polyolefins are noticeably different
from those for randomly branched polymers: the precision
structures result in varying crystal lattice identity and relatively
sharper scattering peaks.47,82-85 The random ethylene-propylene
(EB) copolymer with 20% comonomer content exhibits a
dominant hexagonal crystal form,83 compared to the triclinic
form present in our precision 6b-(ethyl) polymer. As for the
random ethylene-butene (EB) copolymer, it shows orthorhom-
bic crystal structure with low crystallinity.83,84 In contrast, the
precision ethylene-octene (EO) polymer (6f-hexyl) exhibits an
additional hexagonal mesophase besides the orthorhombic
crystalline phase.85
Figure 7. Wide-angle X-ray diffraction patterns for seven precision
polymers: ADMET PE at 27 °C (no branch, Tm ) 134 °C), 6a-(methyl)
at 27 °C (methyl branch, Tm ) 63 °C), 6b-(ethyl) at room temperature
(ethyl branch, Tm ) 24 °C),80 6c-(propyl) at 0 °C (propyl branch, Tm ) 12
°C), 6d-(butyl) at 0 °C (butyl branch, Tm ) 12 °C), 6h-(sec-butyl) at 0 °C
(sec-butyl branch, Tm ) 9 °C), 6e-(pentyl) at 0 °C (pentyl branch, Tm) 14
°C). Prior to measurements, all samples were heated to above melting
temperature in order to remove thermal history and then cooled to specific
temperature at a rate of 1 °C/min.
Conclusions
off of the scattering angles for bulkier branches. The former
trend is easily understood. The methyl/ethyl side chains function
as defects in the crystal lattice and result in the increase of
d-spacing, thus decreasing scattering, where the ethyl results
in a smaller scattering angle. Distinct difference is the case for
the bulkier branches, where the angles for bulkier branched
polymers, 6c-(propyl), 6d-(butyl), 6e-(pentyl), and 6h-(sec-
butyl) increase to higher degrees, 19.5° and 22.5°. Recall the
X-ray scattering theory: scattering angles are determined by the
type of unit cell while the peak intensities are based on the atom
arrangements within the lattice. With this in mind, we conclude
that the packing behaviors of precision polymers possessing
sizes equal to or larger than propyl are different from those of
the methyl- and ethyl-branched polymers. In all cases, the
bulkier branch is excluded from the unit cell into the amorphous
region. The morphology and packing behavior of these precision
polymers with bulky branches are independent of the size of
the branch. This assessment is in agreement with the thermal
data in Table 1, where essentially identical melting temperatures
are observed for all the branched polymers.
A universal two-step alkylation-decyanation synthesis pro-
cedure has been assembled to prepare a series of 11 symmetrical
diene monomers where the branch is ‘built-in’ before the
polymerization. The precise primary structures of both mono-
mers and polymers have been confirmed via H NMR, 13C
1
NMR, and IR spectroscopy to prove the purity of the monomers
and the absence of the undesired side reactions during poly-
condensation and hydrogenation steps.
Thermal characterization of the polymers by DSC confirmed
the precise nature of ADMET polymers by displaying sharp
and well-defined endothermic transitions. Methyl and ethyl
branches, being included in the unit cell, decrease the melting
point of ADMET polyethylene in a progressive manner,
(82) Alamo, R. G.; Jeon, K.; Smith, R. L.; Boz, E.; Wagener, K. B.;
Bockstaller, M. R. Macromolecules 2008, 41, 7141–7151.
(83) Hu, W.; Srinivas, S.; Sirota, E. B. Macromolecules 2002, 35, 5013–
5024.
(84) Hu, W.; Sirota, E. B. Macromolecules 2003, 36, 5144–5149.
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molecules 1999, 32, 3735–3740.
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A R T I C L E S
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lowering the melting point and degrees of crystallinity as
compared with an unbranched ADMET polyethylene standard.
On the other hand, increasing the size of the branch to propyl
and larger yields a set of polymers with a completely different
morphology; the branches are excluded from the unit cell,
leaving a crystalline similar for all of them, even for the
adamantyl-branched polymer. All these polymers exhibit es-
sentially the same melting temperature and degree of crystal-
linity. WAXD patterns support this argument. The morphology
of these precisely branched polymers changes dramatically,
depending on the branch mass. Relatively smaller branches
(methyl and ethyl) are included in the polymer’s unit cell,
whereas branches possessing greater mass and steric bulk are
excluded from the unit cell.
Acknowledgment. The authors thank the National Science
Foundation under grant DMR-0703261 and the International Max
Planck Research School for Polymer Materials for financial support.
We also thank Dr. Werner Steffen at Max Planck Institute for
Polymer Research for help with the X-ray measurements.
Supporting Information Available: Experimental procedures
and spectral data for all monomers and polymers. This material
is available free of charge via the Internet at http://pubs.acs.org.
JA907521P
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