Sequential ROMP of cyclooctenes as a route to linear polyethylene block copolymers

Article abstract of DOI:10.1039/c2dt32695g

AB diblock copolymers were prepared by sequential ring-opening metathesis polymerization of cyclooctenes catalyzed by a Ru-based Grubbs catalyst. The relatively slow polymerization of cis-3-phenylcyclooct-1-ene (3PC) or cis-cyclooct-2-en-1-yl acetate (3AC) was first carried out and then followed by the faster polymerization of unsubstituted cis-cyclooctene (COE) from the active Ru-alkylidene chain ends. In contrast, simultaneous polymerization of the two monomers provides copolymers with a statistical monomer distribution owing to extensive chain transfer. The resulting poly(3PC-b-COE) and poly(3AC-b-COE) diblock copolymers were subjected to hydrogenation to selectively saturate the backbone alkenes. The consequences of architectural variance between the materials from simultaneous vs. sequential polymerizations are reflected by the contrasting thermal characteristics.

Full text of DOI:10.1039/c2dt32695g

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Sequential ROMP of cyclooctenes as a route to linear
polyethylene block copolymers†
Cite this: DOI: 10.1039/c2dt32695g
Louis M. Pitet, Jihua Zhang and Marc A. Hillmyer*
AB diblock copolymers were prepared by sequential ring-opening metathesis polymerization of
cyclooctenes catalyzed by a Ru-based Grubbs catalyst. The relatively slow polymerization of cis-3-phenyl-
cyclooct-1-ene (3PC) or cis-cyclooct-2-en-1-yl acetate (3AC) was first carried out and then followed by the
faster polymerization of unsubstituted cis-cyclooctene (COE) from the active Ru-alkylidene chain ends. In
contrast, simultaneous polymerization of the two monomers provides copolymers with a statistical
monomer distribution owing to extensive chain transfer. The resulting poly(3PC-b-COE) and poly(3AC-b-
COE) diblock copolymers were subjected to hydrogenation to selectively saturate the backbone alkenes.
The consequences of architectural variance between the materials from simultaneous vs. sequential poly-
merizations are reflected by the contrasting thermal characteristics.
Received 12th November 2012,
Accepted 18th December 2012
DOI: 10.1039/c2dt32695g
www.rsc.org/dalton
As an alternative, organometallic mediated metathesis
polymerization in concert with hydrogenation can yield com-
Introduction
Linear polyethylene (LPE) has long been the world’s largest pletely linear polyethylene having a wide variety of functional-
volume commodity polymer. LPE constitutes an indispensible ity.21,22 Chain transfer agents (CTAs) employed during
part of daily life, being commonly used for everything from ring-opening metathesis polymerization (ROMP) of cyclic
food storage to transportation to biomedical devices. LPE owes olefins have been used to prepare telechelic polyolefins with
its versatility to its durability, ductility, toughness, corrosion functional groups capable of subsequent transformation to
resistance, and broad use temperature range (Tm = 130 °C; RAFT polymerization,23 ATRP,24,25 NMP,26 anionic polymeriz-
Tg < −100 °C). Enhancing the properties of LPE-based plastics ation27,28 and ROTEP,29–31 collectively generating
a rich
by introducing functionality or increasing compositional variety of block copolymers with many potential uses.32 From
complexity remains appealing, driven by the desire to address a practical perspective, however, it is more appealing to
a broader range of applications.1–5
prepare such copolymers in a single-pot, making use of a
Block copolymers typically undergo microphase separation, single polymerization mechanism.
adopting composition-dependent morphologies comprising
Indeed, block copolymers with LPE have been prepared by
domains with nanoscopic dimensions.6 Block copolymers sequential living ROMP using Mo-based catalysts and employ-
embody the physical attributes of each constituent combined ing cyclopentene monomer to generate the LPE segments
with unique properties derived specifically from the micro- (after hydrogenation).33–36 Ru-based catalysts recently described
phase separation. Preparation of block copolymers that by Vougioukalakis and Grubbs37 have emerged as indispensible
contain LPE typically requires elaborate synthetic protocols, tools for making complex, functional, multicomponent block
which most often involve mechanistic transformations.7 The copolymers. The controlled nature of polymerizations using
last decade has witnessed tremendous progress in functional certain catalyst derivatives allows multiple monomers to be
group tolerance during coordination and metallocene sequentially polymerized to form discrete, chemically distinct
polymerization of vinyl monomers,8–10 ultimately allowing blocks.38,39 The isodesmic nature of the ROMP reactions leaves
precise functionalization of LPE (and copolymers) and trans- double bonds in the backbone that are prone to secondary
formation to alternate mechanisms (e.g., ATRP11–14 and ring- metathesis in a chain transfer process. Extensive chain transfer
opening transesterification polymerization or ROTEP15–17) or leads to randomization (i.e., redistribution) of the monomer
coupling reactions to form block copolymers.18–20
sequence. The extent to which the repeating unit sequences
are redistributed depends largely on the steric environment
around the double bond after ring-opening; less sterically
encumbered backbones will undergo more secondary meta-
thesis. Therefore, living polymerizations and block copolymers
are most typically formed using bulky monomers like
Department of Chemistry, University of Minnesota, Twin Cities, 207 Pleasant Street
SE, Minneapolis, Minnesota 55455-0431, USA. E-mail: hillmyer@umn.edu
†Electronic supplementary information (ESI) available: Experimental details,
spectra, and thermographs. See DOI: 10.1039/c2dt32695g
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substituted norbornenes.40,41 Creating block copolymers
by ROMP that contain poly(cyclooctene) (PCOE), a common
precursor to LPE, has been challenging.42
One strategy that has been employed to form block copoly-
mers involves first a bulky monomer followed by a less encum-
bered one. The sequence of polymerization must be in that
particular order such that the second monomer polymerizes
very quickly and any cross-metathesis during polymerization
will be with the second block as opposed to the first bulky
repeating unit.43 For example, cyclooctene and cyclooctadiene
are both consumed very quickly but undergo substantial
cross metathesis during polymerization using conventional
ruthenium catalysts, and therefore give polymers with rela-
tively broad molar mass distribution.44,45 This sequential
polymerization strategy has been employed with the bulky
carbazole-substituted norbornene, for example.43,46 Likewise,
sequential polymerization of exo-N-arylnorbornene-2,3-dicarb-
oximides followed by dioxepine led to blocky polymer
architectures.47–49 Additionally, Coughlin and coworkers
employed different solvents to adjust the relative polymeriz-
ation rates between COE and functionalized norbornenes.
In this way, either short (nearly alternating) sequences or
long sequences of each monomer were accessible, despite the
simultaneous, single pot reaction conditions.50 In this work
we describe a new synthetic approach to block copolymers that
contain LPE by sequential polymerization of cyclooctenes.
Scheme 1 Sequential polymerization of 3PC or 3AC and COE monomers to
form poly(3PC-b-COE) or poly(3AC-b-COE) diblock copolymers.
Results and discussion
resonance (NMR) spectroscopy after precipitation in cold
MeOH (Fig. S1†). Polymerization of 3PC and 3AC catalyzed by
G3 proceeds in a regio- and stereo-specific manner as con-
firmed by NMR spectroscopy (see ESI Fig. S2†).53 At RT the
polymerization of 3PC/3AC progresses slowly relative to the
unsubstituted COE analog using G3.56
Intramolecular cross metathesis during polymerization
manifests itself in the (re)distribution of chain end functiona-
lization. Therefore a small proportion of chains will be non-
functional, having benzylidene groups at both chain termini
and an equal proportion of chains will contain active Ru at
both chain termini. Upon subsequent monomer addition, this
scenario will result in a mixture of homopolymer poly(3PC)
(or poly(3AC)), diblock copolymer, and symmetric triblock
copolymer assuming the absence of interblock chain transfer
in the second polymerization step.
Chain extension was performed to synthesize block copoly-
mers from the polymerization solution containing poly(3PC)
(or poly(3AC)) with metathesis-active Ru-alkylidene chain ends
(Scheme 1). After one hour of 3PC (or 3AC) homopolymeri-
zation, the solution was cooled to ∼5 °C using an ice bath and
unsubstituted COE was added to the polymerization solution
under an argon atmosphere. The polymerization solution
became extremely viscous within 5 seconds, consistent with
fast polymerization of COE using G3 compared to 3PC (or
3AC). Prior to termination, chain transfer with the poly(3PC)
(or poly(3AC)) block during COE polymerization is minimal
The synthesis of block copolymers from sequential polymeriz-
ation of two monomers by ROMP was achieved by using the
3-bromopyridine ligated derivative of the ruthenium-based
Grubbs catalyst (G3) (Scheme 1).51 The catalyst G3 gives high
initiation and propagation rates during metathesis, and as
such is conducive to living polymerizations of certain strained
and sterically bulky cyclic olefins.51,52 Either cis-3-phenyl-
cyclooct-1-ene (3PC) or cis-cyclooct-2-en-1-yl acetate (3AC) was
polymerized first.53,54 After nearly complete monomer con-
sumption, the second monomer cis-cyclooctene (COE) was
added and rapidly consumed. The reaction was quenched with
ethyl vinyl ether, and the polymer isolated by precipitation.
The unsaturated diblock copolymers poly(3PC-b-COE) and
poly(3AC-b-COE) were saturated by chemical hydrogenation
with p-tosyl hydrazide to form a diblock copolymer containing
a phenyl- or acetoxy-substituted polyethylene block and a com-
pletely linear polyethylene block, h-poly(3PC-b-COE) or h-poly-
(3AC-b-COE), respectively.
The 3PC monomer was prepared by brominating COE in
the allyl position followed by CuI catalyzed Grignard substi-
tution using phenyl magnesium bromide (Scheme S1, ESI†).55
3AcCOE was synthesized by hydrolyzing the brominated COE
followed by acetylation of the resulting 3-hydroxy COE
(Scheme S1†).54 The first monomer was polymerized in THF at
ambient temperature after initiation with G3. An aliquot was
removed and analyzed by 1H and 13C nuclear magnetic
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Table 1 Molecular and composition targets and experimental values for poly(3PC-b-COE) and poly(3AC-b-COE) block copolymers and the precursorsa
Target
Experimental
Mn, target
b
c
b
c
Sample
Ntarget
(kg mol−1
)
wCOE
nCOE
wCOE
nCOE
poly(3PC)-170
poly(3PC25-b-COE75
poly(3PC)-950
poly(3PC40-b-COE60
poly(3PC)-100
poly(3PC50-b-COE50
poly(3AC)-100
poly(3AC25-b-COE75
poly(3AC)-200
poly(3AC50-b-COE50
poly(3AC)-300
170
950
950
1600
100
250
100
550
200
500
300
450
31.6
117
177
)
)
)
0.73
0.50
0.50
0.75
0.50
0.25
0.82
0.63
0.63
0.82
0.60
0.34
0.76
0.62
0.52
0.74
0.45
0.16
0.84
0.73
0.65
0.81
0.56
0.23
353
18.6
35.1
16.8
66.4
33.6
66.7
50.5
67.0
)
)
)
poly(3AC75-b-COE25
a The samples are labeled in accordance with the approximate measured compositions by 1H NMR spectroscopy; the labels are poly(3PCx-b-COEy)
or poly(3ACx-b-COEy), where x and y are the wt% of the respective blocks (i.e., x + y = 100). b wCOE is the weight fraction of cyclooctene monomer.
c nCOE is the mole fraction of cyclooctene.
such that polymer structure contains predominantly a diblock
architecture. Intrablock chain transfer within the PCOE
block as COE polymerization proceeds has been established
for the Grubbs ruthenium-benzylidene catalyst systems having
N-heterocyclic carbene ligands and presumably occurs in this
chain extension process, leading to a broadened molar mass
distribution (Ð → 2). Nevertheless, the architecture should be
blocky with the number-average molar mass dictated by the
initial G3 concentration.
Several samples were prepared by the method described.
The targeted degrees of polymerization for a series of homo-
polymer precursors and the corresponding diblock copolymers
are shown in Table 1 along with the experimentally deter-
mined values for composition by 1H NMR spectroscopy
(Fig. S3 and S4†).
As an example, analysis of the first poly(3PC)-170 and
the resulting copolymer poly(3PC25-b-COE75) block by size-
exclusion chromatography (SEC) after chain extension indi-
cates an increase in molar mass (Fig. 1, top). The SEC chro-
matograms for poly(3AC)-100 and the corresponding block
Fig. 1 Comparison of SEC chromatograms for (top) homopolymer poly(3PC)-
170 and the block copolymer poly(3PC25-b-COE75) and (bottom) homopolymer
copolymer poly(3AC25-b-COE75) are also indicative of substan-
tive increase in molar mass after chain extension (Fig. 1,
bottom). Collectively these suggest that the active catalyst
remains on the chain ends of the first block, and that reinitia-
tion occurs with high fidelity after the second monomer is
added. The relative elution volumes are consistent with the tar-
geted degrees of polymerization (see Table 1).57 The larger
breadth of the peak attributed to the block copolymer poly-
(3PC25-b-COE75) compared with the homopolymer poly(3PC)-
170 suggests a larger molar mass distribution, which may be
poly(3AC)-100 and the block copolymer poly(3AC25-b-COE75).
incomplete consumption; the degree of conversion of 3PC for
the sample poly(3PC40-b-COE60) was lower than in the sample
poly(3PC25-b-COE75). Nonetheless, adjusting the monomer
feed ratios allows control over the polymer composition.
It is the same scenario in the case of 3AC, for which the
presence of acetoxy group at the allylic position reduces
both the polymerization rate and the secondary metathesis
rate relative to the unsubstituted COE. Three poly(3AC-b-COE)
diblock copolymers that have similar molar mass but different
weight fractions of the two blocks were synthesized in
the same manner as for poly(3PC-b-COE) (Table 1). In
all cases, 1H NMR analysis was consistent with nearly
complete consumption of monomer in both stages of the
a
consequence of substantial chain transfer during
chain extension or homopolymer contamination from non-
functional poly(3PC) (see ESI†).
The block copolymers containing 3PC monomer each
had slightly higher PCOE content than originally targeted with
the monomer feed compositions assuming quantitative
monomer consumption. The diluted poly(3PC) block suggests
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polymerization, and all diblock copolymers exhibit a single targeted. Two samples using 3PC were prepared by adding 1,4-
and unimodal predominate peak in SEC. diacetoxy-2-butene (DAB) with a given molar ratio to target
We synthesized three additional copolymers by simul- Ntotal = ([3PC] + [COE])/[DAB] = 500.58 A third sample using
taneous polymerization of either 3PC or 3AC with COE, in an 3AC monomer was also prepared, with targeted degree of
attempt to understand how critical the sequential polymeriz- polymerization equal to 400. Table 2 contains the targeted
ation protocol is for clean formation of block copolymer. and experimental molecular characteristics as measured by
Similar compositions as contained in the block copolymers 1H NMR spectroscopy (Fig. S5†). The SEC chromatograms for
(i.e., those prepared via sequential monomer addition) were the statistical copolymers are all unimodal and nearly identical
with respect to molar mass distribution and elution volume,
suggesting effective chain transfer and molar mass control
with the CTA addition (Fig. S6†).
Table 2 Molecular characteristics of statistical copolymers poly(3PC-s-COE) and
13C NMR spectroscopy provides compelling evidence that
poly(3AC-s-COE)
the monomer sequence depends on the polymerization proto-
Target
Experimental
col (Fig. 2). The top two spectra in Fig. 2 (part a and d) show
the respective homopolymers of poly(3PC) and poly(3AC), indi-
cating a high degree of regio- and stereoregularity during
ROMP with ruthenium-based catalysts. Only two olefinic
signals are observed for both polymers, consistent with the
previously reported structural assessment.53 The block copoly-
mers exhibit identical signals to the homopolymers along with
two additional signals which are attributed to the cis and trans
olefinic carbons from the PCOE blocks. These spectra suggest
Sample
Na
wPCOE
nPCOE
Nb
wPCOE
nPCOE
poly(3PC25-s-COE75
poly(3PC40-s-COE60
poly(3AC60-s-COE40
)
)
)
500
500
400
0.75
0.60
0.40
0.84
0.72
0.50
484
491
420
0.76
0.59
0.39
0.84
0.71
0.48
a Calculated assuming quantitative monomer consumption, and
taking the respective repeating units associated with the monomers.
b Measured by 1H NMR spectroscopy by comparing the integral ratios
between the end groups and repeating units.
Fig. 2 Olefinic (and aromatic) region of 13C NMR spectra for polymers (a) poly(3PC)-170; (b) poly(3PC60-b-COE40); (c) poly(3PC60-s-COE40); (d) poly(3AC)-100; (e)
poly(3AC50-b-COE50) (f) poly(3AC60-s-COE40).
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minimal contribution from a randomized repeating unit Likewise, the proximity of Tg to pristine PCOE suggests a
sequence distribution that would arise from extensive inter- microphase separated structure. The crystallinity of the poly-
block chain transfer. The spectra from two representative (3PC25-b-COE75) is calculated as 27% (ΔHm = 43.4 J g−1; wPCOE
statistical copolymers show additional olefinic signals (Fig. 2c = 0.76), commensurate with a block architecture possessing
and 2e). This contrast can be attributed to various different only slightly less crystallinity within the majority PCOE matrix
segment lengths of repeating units with distinct electronic compared with the corresponding homopolymer. Similarly,
environments; this is a direct consequence of the polymeriz- poly(3PC40-b-COE60) shows a melting endotherm centered at
ation protocol.
35 °C. While this value is lower than PCOE, the crystallinity is
Thermal characteristics can also provide an indication indicative of significantly long segments of PCOE in a blocky
of molecular architecture in copolymers. PCOE is a semi- structure. The melting endotherm had an enthalpy ΔHm of
crystalline polymer with a melting temperature near 50 °C.59 16.1 J g−1, corresponding to a crystallinity within the PCOE
The sequence length influences the capacity to organize into a block of 12.4%.
crystalline lattice. The block and statistical architectures have
contrasting thermal characteristics by differential scanning behavior despite the nearly identical compositions (Fig. 3).
calorimetry (DSC) (Fig. 3). The statistical copolymer poly(3PC25-s-COE75) exhibits a rela-
The statistical copolymers show contrasting thermal
The homopolymer PCOE (Mn = 50 kg mol−1; Ð = 1.8) pre- tively weak melting endotherm with Tm = 22 °C and ΔHm
=
pared by ROMP shows a melting transition with peak Tm of 19.9 J g−1 corresponding to 12.3% crystallinity. Moreover, the
58 °C and a melting enthalpy (ΔHm) of 69 J g−1, corresponding poly(3PC40-s-COE60) has a pronounced Tg that is substantially
to a calculated crystallinity of 32% (ΔH0m = 216 J g−1).60 A weak depressed compared with homopolymer poly(3PC).
glass transition was observed for the PCOE homopolymer near
Similar thermal behavior was found apparent in poly(3AC-
−80 °C. As a result of the bulky aryl substituents, homopoly- b-COE) (Fig. 3b). Homopolymer poly(3AC) is amorphous, exhi-
mer poly(3PC) is a sticky, liquid-like amorphous polymer biting a Tg at −33 °C.54 A nearly equivalent glass transition
with a low Tg (−20 °C; Fig. 3a). The block copolymer poly- occurs for each of the poly(3AC-b-COE)s. Additionally, melting
(3PC25-b-COE75) likewise shows a strong melting endotherm endotherms associated with the PCOE block were observed
with Tm = 51 °C and a weak Tg at −69 °C. The slightly for all three samples. The melting temperature decreases
depressed Tm compared with PCOE is typical of block co- with increasing content of poly(3AC) from 54 °C for poly-
polymers that undergo microphase separation in the melt. (3AC25-b-COE75) to 51 °C for poly(3AC50-b-COE50) and to 43 °C
Fig. 3 DSC thermograms for (a) polycyclooctene (PCOE) homopolymer, block copolymers poly(3PC-b-COE)s, statistical copolymers poly(3PC-s-COE)s and poly(3PC)
homopolymer and (b) PCOE homopolymer, block copolymers poly(3AC-b-COE)s, statistical copolymers poly(3AC-s-COE)s and poly(3AC) homopolymer. Individual
thermograms have been shifted vertically for clarity. The left and right axes are at different scales to accentuate the low temperature glass transitions.
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the original composition. The NMR spectra of all the block
copolymers required high temperatures (100 °C in 1,1,2,2-
tetrachloroethane), owing to the prohibitive solubility of LPE
in all conventional solvents at ambient temperature. However
the statistical copolymers h-poly(3AC-s-COE) were soluble in
CHCl3 at room temperature, a testament to the influence of
polymerization protocol on sequence distribution and thus
physical properties.
A sample of pristine PCOE (Mn = 50 kg mol−1; Ð = 1.8) was
hydrogenated to generate LPE homopolymer for comparison
with the block and statistical copolymers. Several samples of
LPE (h-PCOE) show melting endotherms with relatively consist-
ent Tm = 130–134 °C and ΔHm = 165–200 J g−1 which
corresponds to crystallinities in the vicinity of 60–72% (ΔHm0
=
277 J g−1) (Fig. 4).62 Homopolymer poly(3PC) and poly(3AC)
were also hydrogenated to give h-poly(3PC) and h-poly(3AC)
using an identical protocol (see ESI Fig. S7†). Samples of
h-poly(3PC) are amorphous with Tg = −21 °C (Fig. 4a). The
saturated block polymer h-poly(3PC25-b-COE75) exhibits
a
similar melting endotherm to pristine LPE with a slightly
depressed Tm of 120.5 °C, consistent with the decrease in
melting temperature associated with block copolymers in
which a semi-crystalline block is tethered to an amorphous
block. The melting enthalpy ΔHm = 101 J g−1 corresponds to
49% crystallinity within the LPE matrix component. A weak Tg
can be observed near −20 °C (Fig. 4a). Likewise, the hydrogen-
ated derivative h-poly(3PC40-b-COE60) exhibits a glass transition
at Tg = −18 °C, suggesting a block structure that undergoes
microphase separation. The sample h-poly(3PC40-b-COE60) also
exhibits a melting endotherm at Tm = +115 °C and the melting
enthalpy ΔHm = 53 J g−1 corresponds to a crystallinity of
approximately 32%.
The hydrogenated homopolymer of 3AC h-poly(3AC) is
semi-crystalline with a Tm near 53 °C and a Tg ≈ −36 °C.54
These thermal features are found to be retained in the hydro-
genated diblock copolymers h-poly(3AC-b-COE). In all three
samples, there is a weak Tg in the range −30 to −35 °C and a
Tm in the range 52 to 58 °C, while a second and more intense
melting endotherm appears in the range 125 to 130 °C
(Fig. 4b). The higher temperature Tm is attributed to the semi-
crystalline LPE block and is slightly depressed compared with
the Tm of pristine LPE. As the LPE content decreases in the
diblock copolymer, the normalized melting enthalpy associ-
ated with the second melting peak drops from 185 to 138 J g−1
corresponding to a concomitant decrease in crystallinity from
67% to 50%. Distinct thermal behavior was observed with the
saturated derivatives of the statistical copolymer (h-poly(3AC60-
s-COE40)); it exhibits a broad endothermic transition with
maximum close to 50 °C (Fig. 4b).
Scheme 2 Chemical hydrogenation of unsaturated block and statistical copoly-
mers to form linear polyethylene copolymers (a) h-poly(3PC-b-COE) or h-poly-
(3AC-b-COE) and (b) h-poly(3PC-s-COE) or h-poly(3AC-s-COE).
for poly(3AC75-b-COE25). Meanwhile, the melting enthalpy also
decreases significantly, from 55.8 J g−1 to 30.5 J g−1 and to
10.4 J g−1, which corresponds to a decrease of crystallinity
within the PCOE block from 35% to 31% and to 30%, respecti-
vely. This behavior is consistent with microphase separation
between the poly(3AC) and the PCOE domains in the poly(3AC-
b-COE) samples. Comparatively, the statistical copolymer poly-
(3AC60-s-COE40) displays a single Tg at −61 °C (Fig. 3b), which
is intermediate between the two respective homopolymers
suggesting a homogeneous microstructure.
The backbone double bonds in the copolymers were all
saturated using p-tosyl hydrazide (p-TsNHNH2) in xylenes at
130 °C (Scheme 2). Saturation extent of the copolymers was
determined by 1H NMR spectroscopy. The backbone alkene
bonds are selectively hydrogenated to give LPE segments
having sequential –CH2– repeating units that have essentially
equivalent chemical shifts at 1.3 ppm (Fig. S7†).61 Importantly,
the aryl substituents remain unsaturated as evidenced by the
aromatic proton signals at a chemical shift of 7.3–7.1 ppm.
The ratio of integrated signals associated with the aryl substi-
tuents and linear methylene repeat units agree within 5% of
One dimensional X-ray scattering profiles taken at both
small and large scattering angles were generated for LPE
homopolymer sample63 at ambient temperature after cooling
from 160 °C (Fig. S9†). The small angle X-ray scattering (SAXS)
plot shows the scattering intensity as a function of the spatial
scattering vector q = 4π(sin θ)λ−1, where 2θ is the scattering
angle and λ is the wavelength of incident radiation (1.3776 Å).
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Fig. 4 DSC thermograms for (a) linear polyethylene (LPE) homopolymer, hydrogenated block copolymers h-poly(3PC-b-COE)s, statistical copolymers poly(3PC-s-
COE)s and hydrogenated h-poly(3PC) homopolymer and (b) LPE homopolymer, hydrogenated block copolymers h-poly(3AC-b-COE)s and hydrogenated statistical
copolymers h-poly(3AC-s-COE)s. Individual thermograms have been shifted vertically for clarity. The left and right axes are at different scales to accentuate the low
temperature glass transitions.
The LPE sample shows two broad reflections located at observed for h-poly(3AC). This result suggests that both the
approximately q = 0.28 and 0.60 nm−1, respectively correspond- LPE block and the saturated poly(3AC) block maintain crystal
ing to dimensions (d) equal to 22 and 10 nm according to structures found in the corresponding homopolymers, an indi-
d
=
2π/q. These reflections presumably arise from the cation of efficient microphase separation. Furthermore, the
periodic arrangement of alternating crystalline and scattering intensity of the LPE peaks decreases substantially
amorphous lamellae (q2 : q1 ≈ 2 : 1). with decreasing LPE content, which is in accordance with the
The wide-angle X-ray scattering (WAXS) profile contains two decreased crystallinity within the LPE block indicated by the
sharp peaks corresponding to the (110) and (200) planes at DSC result.
q = 15.4 and 17.1 nm−1, respectively, associated with the
SAXS profiles for the block copolymer poly(3PC50-b-COE50)
orthorhombic crystal lattice conventionally adopted by LPE below the Tm reveal a distinct yet broad reflection centered at
chains.64 One-dimensional WAXS profiles for h-poly(3PC-b- approximately q = 0.29 nm−1, positioned nearly identically to
COE) and h-poly(3AC-b-COE) reveal Bragg scattering reflections the reflection seen in homopolymer LPE (Fig. 6a; Fig. S9†). It
that are consistent with a substantial degree of crystallinity is evident that the polymer remains highly crystalline below
with identical orthorhombic lattice packing (Fig. 5a and 5b). 100 °C from simultaneously collected WAXS profiles (Fig. 6b).
This suggests considerably long segment lengths of LPE
However, above Tm the broad reflection in the SAXS profiles
resulting from the block copolymerization–hydrogenation disappears, and is replaced by a relatively sharp reflection
protocol, and the reversibility of the crystallization/melting centered at q = 0.12 nm−1. The sharp peak is consistent with a
process is illustrated with profiles after heating above Tm and microphase separated, albeit disorganized morphology. The
subsequent cooling back to ambient temperature.
peak position corresponds to an average interdomain spacing
The WAXS measurement of the h-poly(3AC-b-COE) series d = 52 nm (according to d = 2π/q). These results suggest that
shows the same LPE orthorhombic lattice scattering peaks and (1) the prescribed sequential synthetic protocol does indeed
a small scattering signal at q = 10.1 nm−1 (Fig. 5b). The latter lead to block copolymer architecture and (2) the sample is
signal appears at the identical position as one of the scattering microphase separated at 150 °C.
peaks seen for h-poly(3AC).54 In the WAXS profile of h-poly-
The WAXS profiles for this sample are consistent with the
(3AC75-b-COE25) which has fairly high content of poly(3AC), the DSC results, suggesting that the crystalline portions associated
predominate signal splits into two peaks, a feature also with LPE have melted at 150 °C, and that the crystallization/
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Fig. 6 One-dimensional scattering profiles for the block copolymer h-poly-
(3PC50-b-COE50) at (a) small angle and (b) wide angle beginning at 150 °C (>Tm,
LPE) and cooling back to ambient temperature (rate > 50 °C min−1). Profiles have
been vertically shifted for clarity.
Fig. 5 One-dimensional WAXS profiles for the block copolymers (a) h-poly-
(3PC25-b-COE75) beginning at 25 °C, after heating to 150 °C (>Tm,LPE), and again
after cooling back to ambient temperature (rate > 50 °C min−1) and (b) h-poly-
(3AC25-b-COE75), h-poly(3AC50-b-COE50), and h-poly(3AC75-b-COE25) at 25 °C.
Individual profiles have been shifted vertically for clarity.
fast polymerization of unsubstituted cis-cyclooctene to give
copolymers having a blocky distribution of repeating units. In
contrast, simultaneous polymerization of the two monomers
provided copolymers with essentially statistical monomer dis-
tribution owing to extensive chain transfer. The architectural
variance between the materials from simultaneous and
sequential polymerizations was reflected by the thermal and
molecular characteristics. The resulting poly(3-phenylcyclo-
octene-block-cyclooctene) diblock copolymers were subjected
to hydrogenation to selectively saturate the backbone alkenes,
leaving the pendent aryl substituents for further functionaliz-
ation. Furthermore, the hydrogenated poly(3-acetoxycyclo-
octene-block-cyclooctene) diblock copolymers comprise two
semi-crystalline blocks which display distinct melting tem-
peratures and crystal structures. The saturated LPE backbone
offers flexibility, strength, and resistance to chemical
corrosion, and the potentially interesting consequences of
melting phenomena are reversible (Fig. 6b). These results cor-
roborate the SAXS results, and the postulated relationship
between crystal formation and microphase separation.
Conclusions
The synthetic strategy described here is a straightforward route
to block copolymers with segments comprising high density
polyethylene and aryl or acetoxy pendent groups. A diverse
array of further possibilities can be imagined using this strat-
egy. LPE imparts its superior qualities on hybrid materials
with various functionalities by designing comonomers (e.g.,
3-substituted cyclooctenes). The relatively slow polymerization
of cis-3-phenylcyclooct-1-ene was conducted followed by the
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molecular architecture on mechanical performance is 18 H. K. Kim, M. Zhang, X. Yuan, S. N. Lvov and
currently under investigation.
T. C. M. Chung, Macromolecules, 2012, 45, 2460.
19 T. Li, W. J. Wang, R. Liu, W. H. Liang, G. F. Zhao, Z. Li,
Q. Wu and F. M. Zhu, Macromolecules, 2009, 42, 3804.
20 Y. Zhao, L. Wang, A. Xiao and H. Yu, Prog. Polym. Sci.,
2010, 35, 1195.
Acknowledgements
The authors acknowledge financial support from the National 21 C. W. Bielawski and R. H. Grubbs, Prog. Polym. Sci., 2007,
Science Foundation under contract DMR 1006370 as well as 32, 1.
the Petroleum Institute of Abu Dhabi through the ADMIRE 22 T. W. Baughman and K. B. Wagener, Adv. Polym. Sci., 2005,
(Abu Dhabi-Minnesota Institute for Research Excellence) 176, 1.
partnership. L.M.P. gratefully acknowledges support from the 23 M. K. Mahanthappa, F. S. Bates and M. A. Hillmyer, Macro-
Doctoral Dissertation Fellowship awarded by the University molecules, 2005, 38, 7890.
of Minnesota Graduate School. We are indebted to Prof. 24 C. W. Bielawski, T. Morita and R. H. Grubbs, Macro-
Shingo Kobayashi for invaluable input. SAXS was performed at molecules, 2000, 33, 678.
Beamline 5 (DND-CAT) at the Advanced Photon Source (APS). 25 C. W. Bielawski, J. Louie and R. H. Grubbs, J. Am. Chem.
DND-CAT is supported by the Dow Chemical Company, Soc., 2000, 122, 12872.
E. I. DuPont de Nemours & Co., and the State of Illinois. Use of 26 S. M. Banik, B. L. Monnot, R. L. Weber and
the APS was supported by the U.S. Department of Energy, M. K. Mahanthappa, Macromolecules, 2011, 44, 7141.
Office of Science, Office of Basic Energy Sciences, under 27 K. A. Switek, K. Chang, F. S. Bates and M. A. Hillmyer,
Contract No. DE-AC02-06CH11357.
J. Polym. Sci., Part A: Polym. Chem., 2006, 45, 361.
28 S. B. Myers and R. A. Register, Macromolecules, 2008, 41,
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29 L. M. Pitet, M. A. Amendt and M. A. Hillmyer, J. Am. Chem.
Soc., 2010, 132, 8230.
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