Free-radical polymerization and ring-expansion of a cubane acrylate: A unique low-shrink polymer

Article abstract of DOI:10.1071/CH08484

A pendant cubane substituent has been incorporated into an acrylate polymer side chain to give poly[methyl 4-(acryloyloxymethyl)cubane carboxylate]. Treating this polymer with a rhodium(i) salt triggers a catalytic, ring-opening rearrangement of the cubane substructure to cyclooctatetraene, with a concomitant expansion in molecular volume. This system offers a unique opportunity to reverse the shrinkage associated with polymerization.

Full text of DOI:10.1071/CH08484

Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2009, 62, 145–149
Free-Radical Polymerization and Ring-Expansion of a Cubane
Acrylate: a Unique Low-Shrink Polymer
A
A,B
Alison E. McGonagle and G. Paul Savage
A
CSIRO Molecular and Health Technologies, Private Bag 10, Clayton South, VIC 3169, Australia.
Corresponding author. Email: paul.savage@csiro.au
B
A pendant cubane substituent has been incorporated into an acrylate polymer side chain to give poly[methyl 4-
acryloyloxymethyl)cubane carboxylate]. Treating this polymer with a rhodium(i) salt triggers a catalytic, ring-opening
(
rearrangement of the cubane substructure to cyclooctatetraene, with a concomitant expansion in molecular volume. This
system offers a unique opportunity to reverse the shrinkage associated with polymerization.
Manuscript received: 6 November 2008.
Final version: 16 December 2008.
Introduction
Rh(I)
60°
Addition polymerization at constant temperature normally
occurs with an increase in density and hence reduction in the
Cubane
Tricyclooctadiene
Cyclooctatetrene
(r 1.29 g cmϪ3)
(r 0.925 g cm
Ϫ3
)
volume of the reactants.[1] A primary cause of contraction in
volume during polymerization is due to the change from van
der Waals distances to covalent bond lengths between monomer
molecules.An additional factor is the relative packing efficiency
of the monomer and polymer, which may vary greatly between
monomers and polymers that are liquids, amorphous solids, or
crystalline materials.
Scheme 1.
One conclusion from this body of work is that to minimize poly-
mer shrinkage, the monomer should occupy the smallest volume
possible compared with the same unit in the final polymer. Mul-
tiple ring-opening strategies using spiro and bicyclic structures
are effective in this approach and we reasoned that additional
ring-opening would enhance the effect.
Shrinkage during polymerization frequently diminishes the
performance, mechanical properties, and durability of bulk poly-
[
2,3]
meric materials.
Shrinkage in optical adhesives induces
internal stress, leading to optical distortion. In adhesives, sur-
face coatings, and encapsulating materials, voids are created and
these serve as channels for contaminants to infiltrate the poly-
mer. Matrix polymers in composite materials or coatings may
retract from reinforcing fibres and fillers, thereby reducing the
bond area between them. Excessive shrinkage in bulk polymers
used for precision casting products may cause the product to
improperly conform to the mould.
Among polycyclic hydrocarbons, the cubane (pentacyclo
[4.2.0.02 .0 .0 ]octane) structure is particularly fascinat-
ing by virtue of its symmetry and high strain, and because
it has among the highest density of any stable hydrocarbon
,5
3,8 4,7
−
3 [16]
(1.29 g cm ).
Despite the enormous internal strain, cubane
has a surprisingly high kinetic stability due to the lack of
readily available thermal decomposition pathways.[ Neverthe-
17]
less, the cubane skeleton is rapidly ring-opened by rhodium(i)
catalysts[
18,19]
to the corresponding tricyclooctadiene, which
There have been several strategies employed for reducing
or eliminating polymerization shrinkage, the most well known
of which is ring-opening polymerization, pioneered (for this
◦
in turn thermally rearranges to cyclooctatetraene at 50–60 C
(Scheme 1). Cyclooctatetaraene by comparison has the same
molecular formula as cubane but a density of only 0.925 g cm .
purpose) by Bailey.[ During the ring-opening polymerization
of cyclic monomers, one or more rings are opened in each
monomeric unit, and thus one or more bonds are broken for
each new bond formed. Cyclic monomer structures occupy a
smaller volume than their ring-opened counterparts, and hence
the degree of shrinkage is lessened during ring-opening poly-
merization. Bicyclic and spiro monomers that ring-open on
polymerization enhance this effect because two bonds are bro-
ken for every new bond formed. Significant effort in this area has
led to the development of a wide variety of cyclic and bicyclic
4]
−3
We anticipated that a cubane-containing monomer could be ring-
opened during or after polymerization to greatly decrease the
final polymer density, and hence minimize polymer shrinkage.
There are few reported examples of cubane-containing poly-
merssinceEatonenvisionedtheconceptina1992review.[ The
first example of the formation of a cubane-containing polymer
was reported by Chauvin,[ in which a metathesis polymeriza-
tion of 1,4-bis(homoallyl)cubane afforded an oligomer with six
to seven repeat units. The authors attributed the lack of further
chain elongation to poor solubility. Other examples where the
20]
21]
[
5,6]
[7]
monomers, such as spiro orthoesters,
bicyclo orthoesters,
[
8]
[9]
[10,11]
spiroketals, cyclic ethers, polycyclic ketal lactones,
cubanestructureformspartofthepolymerbackbonebypolycon-
spiro orthocarbonates,[
12–14]
and cyclic allylic sulfides
[15]
that
densation followed,
[22–24]
but poor solubility again limited the
ring-open on polymerization to minimize polymer shrinkage.
extent of polymerization to oligomers of at best 68 repeat units.
©
CSIRO 2009
10.1071/CH08484 0004-9425/09/020145

1
46
A. E. McGonagle and G. P. Savage
NaOH, MeOH
THF, RT
MeO2C
BH ·SMe
3 2
MeO C
2
THF, 0°C to RT
CO Me
CO H
2
2
89%
90%
1
3
2
O
MeO C
MeO C
2
2
Cl
O
O
i
OH
Pr NEt, CH Cl , RT
2 2 2
67%
4
Scheme 2.
a
f
e
n
O
O
c
b
d
a
CO Me
2
H O
2
c
b
d
f
e
4.0
3.5
3.0
2.5
2.0
1.5
ppm
Fig. 1. 1H NMR spectrum of poly-4.
Harpp[25] took an alternative approach by tethering cubane via
an ester linkage to norbornene and polymerizing the monomer
using Grubbs catalyst in a ring opening metathesis polymeriza-
NMR spectrum of 4, the three vinyl protons each appeared
as doublet of doublets, with the characteristic pattern of
geminal–cis, geminal–trans, and cis–trans coupling. The C3
axial symmetry of the 1,4-substituted cubane means that there
are only two non-equivalent sets of three cubane protons, and
these appeared as triplets centred at 3.90 ppm and 4.14 ppm,
respectively.
Polymerization of methyl 4-(acryloyloxymethyl)cubane-
carboxylate 4 was carried out under thermal conditions. A
solution of 4 and azobisisibutyronitrile (AIBN) (0.5 wt-%) in
benzene was degassed by three freeze–pump–thaw cycles to
[
26]
4
tion reaction. Polymers up to a molecular weight of 3.6 × 10
were obtained (approximately 143 repeat units), and these were
soluble in low-polarity organic solvents.
Pendant attachment appears to have the most promise for
incorporating cubane into a polymer structure.We now report the
synthesis, polymerization, and cubane ring-opening of methyl
4
-(acryloyloxymethyl)cubanecarboxylate.
0
.1 Pa and the reaction tube was flame-sealed. The reaction was
Results and Discussion
◦
suspendedinanoilbathat70 Cfor6 h.Theisolatedpolymerwas
amalleableglassysolidandNMRanalysisshowedonlyatraceof
starting material, and broad signals for the polymer (Fig. 1). Gel
permeation chromatography (GPC) analysis of the polymer gave
Synthesis and Polymerization of Methyl
4-(Acryloyloxymethyl)cubanecarboxylate
The synthesis of acrylate ester 4 is outlined in Scheme 2. Cubane
dicarboxylate 1 can be conveniently prepared by the method of
Bliese andTsanaktsidis.[ Careful half-hydrolysis of the diester
−
1
a numberaverage molecularweight(M )of64700 g mol , indi-
n
27]
cating ∼260 units of monomer. The weight average molecular
−
1
1
using methanolic sodium hydroxide afforded carboxylic acid 2
weight (M ) was 227400 g mol , for a polydispersity of 3.5.
w
[
28]
in good yield. The most efficient reduction of the carboxylic
acid moiety was found to be using BH3·SMe2 with careful mon-
itoring of the reaction time to avoid over-reduction. Alcohol 3
was acylated with acrylolyl chloride under basic conditions to
afford the target monomer 4.
Rhodium(I) Catalyzed Cyclooctatetraene Formation
Various rhodium(i) and silver(i) salts are known to catalyze
ring expansion of cubanes but [Rh(norbornadiene)Cl]2 is one
of the more efficient.[ A solution of [Rh(norbornadiene)Cl]2
and the cubane polymer poly-4 was heated at 60 C in toluene
(Scheme 3). The reaction was followed by TLC. After 2 h, the
19]
The colourless crystalline methyl 4-(acryloyloxymethyl)
cubanecarboxylate 4 was a stable, easily handled compound,
which could be purified by column chromatography. In the H
◦
1

A Unique Low-Shrink Polymer
147
3
3
2
2
1
1
5
0
5
0
5
0
5
0
5
a
n
O
[Rh(nor)Cl]2
n
O
b
O
O
PhMe, 60°C
CO Me
2
CO Me
c
2
d
Poly-4
5
f
g
e
Scheme 3.
Ring expansion
Table 1. Comparison of monomer and polymer densities and calcu-
lated shrinkages
h
Density [g cm 3]
−
Shrinkage [%]
Monomer
؊
Monomer Polymer
0.0
5.0
10.0
15.0
20.0
1
000/Monomer molecular weight [mole gϪ1
]
Acrylonitrile[3]
Methacrylonitrile[
0.797
0.800
0.940
0.911
0.889
0.902
1.084
1.17
1.10
1.19
1.11
1.05
1.06
1.252 (1.058)A
31.9
27.3
21.0
17.9
15.3
14.9
3]
[
3]
Fig. 2. Relationship of observed polymerization volume shrinkage and
reciprocal molecular weight of the monomer: (a) acrylonitrile; (b) metha-
crylonitrile; (c) methyl methacrylate; (d) ethyl methacrylate; (f) n-propyl
methacrylate; (g) methyl 4-(acryloyloxymethyl)cubanecarboxylate 4;
(h) cyclooctatetraene polymer 5.
Methyl methacrylate
Ethyl methacrylate
[
3]
n-Butyl methacrylate[
3]
n-Propyl methacrylate[
3]
A
4
13.4 (−2.5)
AAfter ring-opening to give polymer 5.
change in density from monomer 4 to ring-opened polymer 5
is a slight decrease (1.084 to 1.058 g cm ), which represents a
marginal expansion in volume of 2.5%.
−
3
starting material was completely consumed and a precipitate
had formed. After cooling, the orange-brown amorphous solid
polymer was collected, which proved to be insoluble in organic
solvents. Typical NMR solvents such as chloroform, DMSO,
tetrachloroethane, toluene, chlorobenzene, and trifluoro-
acetic acid failed to dissolve the polymer. NMR analysis of the
filtrate revealed only a trace of the original cubane-containing
polymer, indicating that the reaction had proceeded to give an
insoluble material, most probably the cyclooctatetraene poly-
acrylate 5. The insolubility of the material is unlikely to be
due to cross-linking because a variety of cubanes have been
ring-expanded under these conditions without significant side
reactions. The orange-brown colour may be due to some for-
The shrinkage that accompanies polymerization decreases
roughly linearly with increasing molecular weight of the
monomer, for a given series of similar functional monomers.[
Relationships between the reciprocal of the monomer’s molec-
ular weight and the shrinkage of some vinyl monomers are
presented in Fig. 2. The shrinkage values, including for the
cubane monomer, are roughly linear. However, on rearrange-
ment from the cube skeleton to the cyclooctatetraene skeleton,
there is a dramatic reversal of polymer shrinkage.
31]
Conclusions
Incorporation of a pendant cubane substituent into the side chain
of a polymer offers a unique opportunity to minimize poly-
mer shrinkage by a catalytic, ring-opening rearrangement of the
cubane to cyclooctatetraene. We have demonstrated such a ring-
opening expansion after polymerization. The cubane 1,4-diester
offers convenient functionality to construct other monomers
for free radical polymerization, and even alternative polymer
systems including substrates for condensation and addition poly-
merization. These alternatives may alleviate the insolubility
issues with polymer 5 and future work will focus on these areas.
[29]
mation of cyclooctatetraene–rhodium complexes.
Fourier-
transform (FT)-IR spectroscopy of this solid in a KBr disk
−
1
revealed a medium band at 1634 cm , which is indicative
of the carbon–carbon double bond stretching frequency of the
cyclooctatetraene ring.[30]
Volume Shrinkage
Density gradient columns of aqueous sodium bromide solu-
◦
tions at 20 C were set up according to British standards BS
2
782–620D, and calibrated using a set of alkyl bromides. For
Experimental
the present report, polymerization volume shrinkage was cal-
culated relative to the initial volume of monomer using the
formula [(ρpolymer − ρmonomer)/ρpolymer] × 100%. The densities
of the monomer 4, its polymer, and the ring-opened polymer 5,
along with some representative methacrylates and acrylonitriles,
are presented inTable 1.The shrinkage associated with polymer-
ization of 4 was consistent with polymerization of other similar
vinyl monomers (13.4%), as expected.Treatment of this polymer
with rhodium catalyst triggered a rearrangement of the cubane
skeleton to the much larger cyclooctatetraene structure, with a
General
General experimental details and instrumentation have been
described previously[ except that proton and carbon nuclear
32]
1
13
magnetic resonance ( H and C NMR) spectra were recorded
at room temperature on a Bruker AV400 operating at 400 and
100.6 MHz, respectively, using CDCl3 as solvent and internal
reference. Commercially available reagents and analytical grade
solvents were obtained from Aldrich Chemical Co. and used
as received. FT-IR spectra were recorded on a Bruker Equinox
55/S FT-IR spectrometer with samples in KBr disks. Densities
were determined in gradient density columns using sodium bro-
−
3
concomitant decrease in density from 1.252 to 1.058 g cm
,
which represents an expansion of 18.3% from the cubane poly-
mer (poly-4) to the cyclooctatetraene polymer 5. The overall
◦
mide solutions at 20 C calibrated with a set of alkyl bromides of

1
48
A. E. McGonagle and G. P. Savage
known density (British standard BS 2782–620D). GPC was car-
ried out on a Waters Associates liquid chromatograph equipped
with differential refractometer and a set of six Ultra-Styragel
(400 MHz, CDCl3) 3.69 (3H, s, Me), 3.85–3.91 (3H, m, H3),
4.13–4.17 (3H, m, H2), 4.32 (2H, s, CH2OR), 5.83 (1H, dd,
J 10.4, 1.5 Hz, H6 cis), 6.14 (1H, dd, J 17.3, 10.4 Hz, H5), 6.40
(1H, dd, J 17.3, 1.5 Hz, H6 trans). δC (101 MHz, CDCl3) 45.0
(C3), 46.5(C2), 51.5(Me), 56.2(C1orC4), 56.3(C1orC4), 64.4
(CH2OR), 128.3 (C5), 130.8 (C6), 166.4 (C=O acryloyl ester),
172.5 (C=O methyl ester). m/z (ES+) (int) 269.0 (15), 223.0
6
5
4
3
columns (10 , 10 , 10 , 10 , 500, and 100 Å). Tetrahydrofuran
◦
−1
was used as eluent at 20 C and a flow of 1.0 mL min . The
system was calibrated using narrow-distribution polystyrene
standards (Waters).
(
10), 199.2 (10), 142.8 (10), 114.9 (100). (Calc. for C14H14O4:
4
-Methoxycarbonylcubane Carboxylic Acid 2
A solution of 1,4-bis(methoxycarbonyl)cubane 1 (10.0 g,
5.4 mmol) in anhydrous THF (240 mL) was treated with a
freshly prepared solution of NaOH (2.0 g, 50.0 mmol) in MeOH
60 mL) dropwise. The pale orange precipitate was stirred at
C 68.28, H 5.73. Found: C 68.35, H 5.78.) m/z (EI) Calc. for
−
1
C14H14O4: 246.0887. Found: 246.0898. νmax(KBr)/cm 2997,
964, 1741, 1718, 1290, 1202, 973.
2
4
Poly-4
(
A solution of methyl 4-(acryloyloxymethyl)cubane carboxylate
room temperature for 16 h and then was concentrated under vac-
uum. The residue was taken up in H2O (150 mL) and this was
extracted with CHCl3 (3 × 80 mL). The aqueous layer was care-
fully acidified to pH 3 with conc. HCl and this was extracted
with chloroform (3 × 80 mL) and these organic extracts were
dried (MgSO4), filtered and concentrated to afford the title com-
−
1
4
(0.20 g, 0.8 mmol) was treated with a 2 mg mL solution of
AIBN in benzene (0.5 mL, 0.006 mmol). The resultant solution
wasdegassedusingthefreeze–thawmethod(×3)andthensealed
in a tube using a blowtorch. The reaction mixture was heated at
◦
7
0 C for 6 h and then allowed to cool to room temperature. The
◦
[33]
tube was cracked open and a small sample was removed, con-
centrated, and analyzed by NMR. GPC analysis of the polymer
pound as a colourless solid (8.3 g, 89%). mp 178–180 C (lit.
◦
1
82–183 C). δH (400 MHz, CDCl3) 3.71 (3H, s, Me), 4.26–4.28
−
1
gave a number average molecular weight (Mn) of 64700 g mol
,
(
6H, m, H2 and H3), CO2H not observed. δC (101 MHz, CDCl3)
indicating ∼260 units of monomer. The weight average molec-
4
5
7.05 (C2 or C3), 47.15 (C2 or C3), 51.7 (Me), 55.5 (C1 or C4),
5.8 (C1 or C4), 171.9 (C=O ester), 177.2 (C=O acid). m/z
−
1
ular weight (Mw) was 227400 g mol , for a polydispersity of
.5. The reaction mixture was evaporated to dryness and the
3
(
Electron ionization (EI)) (intensity (int)) 206.1 (20), 160.1 (18),
−
1
remaining solid was triturated with pentane, and collected by
filtration. δH (400 MHz, CDCl3) 1.38–1.70 (2H, br, CH2 back-
bone), 2.25 (1H, br, CH backbone), 3.69 (3H, s, OMe), 3.85
1
46.0 (40), 102.0 (100). νmax(KBr)/cm 3003, 2562, 1717,
1
690, 1623, 1442, 1100, 849.
(
3H, br, cubane CH), 4.13 (3H, br, cubane CH), 4.21 (2H, br s,
4
-Methoxycarbonyl-1-(hydroxymethyl)cubane 3
−
1
OCH2). νmax(KBr)/cm 2964, 1721, 1441, 1325, 1223, 1183,
092.
A solution of 2 (5.0 g, 24.2 mmol) in anhydrous THF (250 mL)
1
◦
wascooledto0 CandtreateddropwisewithBH3·SMe2 (3.7 mL,
◦
3
9.4 mmol). The reaction mixture was stirred at 0 C for 30 min
Cyclooctatetraene Polyacrylate 5
and then at room temperature for a further 30 min before quench-
ing with H2O (10 mL). The mixture was extracted with Et2O
A solution of polymethyl 4-(acryloyloxymethyl)cubane car-
boxylate (poly-4) (200 mg, 0.81 mmol cubane equivalents)
and [Rh(norbornadiene)Cl]2 (30 mg, 0.07 mmol) in anhydrous
(60 mL) and the organic layer was washed successively with
brine(30 mL), saturatedaqueousNaHCO3 solution(30 mL), and
H2O (30 mL).The ethereal solution was dried (MgSO4), filtered,
and concentrated to afford the title compound as a colourless
◦
toluene (10 mL) was heated to 60 C for 2 h and a precipi-
tate formed. TLC analysis (Et2O/hexane, 1:1) of the solution
indicated complete consumption of the starting material. The
resulting orange solid was collected by filtration and washed
with toluene (2 × 10 mL). The solid was insoluble in organic
◦
[34]
◦
86–90 C).
crystalline solid (4.2 g, 90%). mp 84–86 C (lit.
δH (400 MHz, CDCl3) 1.59 (1H, br s, OH), 3.70 (3H, s, Me),
.77 (2H, s, CH2OH), 3.85–3.91 (3H, m, H3), 4.12–4.17 (3H, m,
H2). δC (101 MHz, CDCl3) 44.5 (C3), 46.3 (C2), 51.5 (Me), 56.4
C1), 58.8 (C4), 63.2 (CH2OH), 172.7 (C=O ester). m/z (elec-
trospray ionization (ES)+) (int) 230.9 (40), 215.1 (30), 170.9
3
−
1
solvents, water, and trifluoroacetic acid. νmax(KBr)/cm 3434
(
1
H2O), 2987 (C=CH), 2950, 1724 (C=O), 1634 (C=C), 1437,
(
256, 1212, 1157, 1087, 967, 732.
−
1
(
1
20), 131.1 (20), 115.0 (100). νmax(KBr)/cm 3298br, 2985,
717, 1442, 1332, 1219, 1029, 842.
Acknowledgements
The authors thank John Tsanaktsidis, Richard Evans, and Ezio Rizzardo for
helpful discussions, Bill Chong for GPC assistance, and Wendy Tian for
FT-IR spectra.
Methyl 4-(Acryloyloxymethyl)cubane Carboxylate 4
A solution of 4-methoxycarbonyl-1-(hydroxymethyl)cubane 3
(
0
0.34 g, 1.8 mmol) in anhydrous CH2Cl2 (18 mL) was cooled to
C and treated with diisopropylethylamine (1.6 mL, 8.9 mmol),
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