Polysilylether: A Degradable Polymer from Biorenewable Feedstocks

Article abstract of DOI:10.1002/anie.201606282

The synthesis of polysilylethers (PSEs) using a monomer derived from a biorenewable feedstock is reported. The AB-type monomer was synthesized from undecenoic acid through hydrosilylation and reduction, and the polymerization was catalyzed by earth-abundant metal salts. High-molar-mass products were achieved, and the degree of polymerization was controlled by varying the amount of an AA-type monomer in the reaction. The PSEs possess good thermal stability and a low glass-transition temperature (T_g≈?67 °C). To demonstrate the utility of the PSEs, polyurethanes were synthesized from low-molar-mass hydroxy-telechelic PSEs.

Full text of DOI:10.1002/anie.201606282

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International Edition: DOI: 10.1002/anie.201606282
German Edition: DOI: 10.1002/ange.201606282
Biomass
Polysilylether: A Degradable Polymer from Biorenewable Feedstocks
Chen Cheng, Annabelle Watts, Marc A. Hillmyer,* and John F. Hartwig*
Abstract: The synthesis of polysilylethers (PSEs) using
a monomer derived from a biorenewable feedstock is reported.
The AB-type monomer was synthesized from undecenoic acid
through hydrosilylation and reduction, and the polymerization
was catalyzed by earth-abundant metal salts. High-molar-mass
products were achieved, and the degree of polymerization was
controlled by varying the amount of an AA-type monomer in
the reaction. The PSEs possess good thermal stability and
a low glass-transition temperature (Tg ꢀ À678C). To demon-
strate the utility of the PSEs, polyurethanes were synthesized
from low-molar-mass hydroxy-telechelic PSEs.
acidic or basic conditions to give the corresponding alcohols
and siloxanes.[4–6] The rate of hydrolysis depends on the steric
properties of the substituents on the silicon and carbon atoms
a to the oxygen atom.[7] In addition, the relatively large Si O
À
À
and Si C bond lengths can increase the flexibility of the
polymer backbone, thus leading to low glass-transition
temperatures, a property that is important for creating
elastomers or tougheners for other plastic materials.[8,9]
À À À
À À À À
Polymers containing either C Si O C or C O Si O C
linkages in the repeating unit have been synthesized by
several methods: 1) uncatalyzed melt-condensation of aryl- or
biaryldiols with dianilino- or diphenoxysilanes;[10–12] 2) reac-
tions of dichlorosilanes with either bis(epoxide)s or bis-
(oxetane)s catalyzed by quaternary ammonium salts,[13–15] thus
resulting in polymers with reactive pendant chloromethyl
groups; 3) hydrosilylation of aliphatic and aromatic ketones
or benzoquinones with hydrosilanes catalyzed by ruthenium
and palladium complexes;[16–18] and 4) dehydrogenative cou-
pling of alcohols with hydrosilanes catalyzed by palladium
and rhodium complexes.[19,20] Polysilylethers (PSEs) bearing
aryl and biaryl backbones are typically solids with softening
temperatures above 50 and 1508C, respectively,[10,11] whereas
those bearing aliphatic backbones typically have glass-tran-
sition temperatures (Tg) below À808C.[16] These studies have
shown, as expected from the chemistry of silyl protective
groups, that PSEs synthesized from secondary alcohols or
from silanes bearing bulky groups (e.g., Ph)[15,20] are much
more resistant to either hydrolysis or methanolysis than are
PSEs made from either primary alcohols or from silanes
bearing unhindered groups (e.g., Me).[16,20] Most recently,
T
he global production of plastics reached 300 million metric
tons in 2013.[1] The vast majority of these materials are
sourced from non-renewable fossil fuels.[2] For many reasons,
it is beneficial to develop polymer materials sourced from
renewable feedstocks. Currently, several renewable polymers
have been developed and commercialized, including poly-
(lactic acid) (PLA), poly(hydroxyalkanoate)s (PHA), poly-
amide 11, and bio-polyethylene. However, the total volume of
biorenewable polymers represents a very small fraction of the
global plastic production, and less than half of those materials
are biodegradable.[2]
Issues concerning the disposal of plastics must also be
addressed. The thermal, oxidative, and hydrolytic stability of
most synthetic polymers leads to their accumulation in the
biosphere. It is estimated that a quarter of plastic worldwide is
disposed in landfills, and tens of millions of metric tons of
plastics accumulate in the oceans, causing damage to aquatic
ecosystems.[1] Thus, it is important to design, synthesize, and
evaluate polymers that can be degraded under mild, ambient
conditions into low-molecular-weight monomers or oligomers
and can either be further metabolized by microorganisms or
otherwise assimilated.[3] Usually, biodegradable polymers
contain heteroatom linkages in the backbone (such as
polyesters), allowing degradation through hydrolysis or
enzymatic chain scission.
À À À À
polymers containing C O Si O C linkages were synthe-
sized through silicon acetal metathesis polymerization cata-
lyzed by a strong acid.[21] The methanol byproduct was
actively removed during the reaction to drive the equilibrium
to the polymer product.[21]
For several reasons, we considered undecenoic acid
derivatives to be a desirable starting material to prepare
polysilylethers (PSE). Undecenoic acid is derived from
pyrolysis of rincinoleic acid, a principal component of castor
oil.[22] Undecenoic acid contains a terminal alkene and
a terminal carboxylic acid which allow sequential functional-
ization to construct an AB-type bifunctional monomers.[23]
Once functionalized on both termini, the molecule contains
11 CH2 units, which should further increase the flexibility of
the resulting polymer chain.
To develop new renewable and degradable polymers, we
À
sought to incorporate Si O linkages into the polymer back-
bone. Silyl ethers are common protecting groups used in
organic synthesis and can be cleaved by hydrolysis under
[*] C. Cheng, Prof. Dr. J. F. Hartwig
Department of Chemistry, University of California
Berkeley, CA 94720 (USA)
We report the synthesis of a novel, bifunctional monomer
E-mail: jhartwig@berkeley.edu
À
containing Si H and OH functionalities from an undecenoic
A. Watts, Prof. Dr. M. A. Hillmyer
Department of Chemistry, University of Minnesota
Minneapolis, MN 55455 (USA)
acid derivative and polymerization of this monomer to afford
PSEs with controlled molar mass. The PSEs undergo con-
trolled degradation in neutral to moderately acidic (pH 2)
aqueous media and are suitable for constructing polyur-
E-mail: hillmyer@umn.edu
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201606282.
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ethanes (PU) using PSEs as a macromolecular diol creating
soft segments.
We envisioned a method to combine commodity silanes
with undecenol by hydrosilylation of the alkene, and subse-
À
À
quent polycondensation forming Si O bonds. Although Si O
À
bonds can be formed through the direct reaction of Si Cl and
À
R OH moieties, this reaction requires a stoichiometric
amount of base and separation of the stoichiometric corre-
sponding salt byproduct. In addition, a monomer containing
À
a Si Cl moiety will be sensitive to moisture. Thus, we sought
Figure 2. Dehydrogenative polymerization of 1 by various catalysts.
coe=cyclooctene.
À
to use the catalytic dehydrogenative condensation of Si H
and OH groups[24–26] as an alternative strategy to form the
À
À
Si O bonds. The advantage of this strategy is that Si H bonds
are hydrolytically stable and do not readily react with OH
groups in the absence of a catalyst. In addition, the only
byproduct of this coupling process is H2, which is easily
removed.
insoluble material. Thus, we conducted the polymerization
with the more defined, single-site iridium catalysts reported
by Luo and Crabtree.[34] However, the molar mass of the
resulting polymeric products was relatively low.
To synthesize a bifunctional molecule, containing one
Si H and one OH moiety, from undecenoic acid, we
Although many other transition metals could be eval-
uated for this process, these iridium and ruthenium catalysts
are among the most active for dehydrogenative silylation.
Thus, we investigated reactions catalyzed by alkali-metal
alkoxides. The ring-opening polymerization of octamethylte-
trasiloxane (D4) catalyzed by alkali-metal hydroxides[35] and
the dehydrogenative silylation of alcohols catalyzed by strong
inorganic bases are known.[36–39] Thus, we investigated the
polymerization of neat 1 with CsOH (0.2-1 mol%) as the
catalyst (Figure 2). The polymerization proceeded rapidly at
1408C under these conditions to afford a transparent viscous
oil. KH was also studied as the catalyst, and in this case the
reaction proceeded at 1008C with 1 mol% of KH. The NMR
spectrum of the product consisted of well-resolved signals for
the silicon methyl groups, the methylene group a to silicon,
and the methylene groups a and b to the oxygen atom. Signals
corresponding to the monomer were not observed. These data
provided preliminary evidence for formation of the targeted
polymer and high conversion of the monomer.
À
conducted the hydrosilylation of methyl 10-undecenoate
with Me2SiClH catalyzed by 10 ppm of Karstedtꢀs catalyst[27]
À
À
(Figure 1). The Si Cl moiety was used as a masked Si H
Figure 1. Synthesis of the bifunctional monomer 1. THF=tetrahydro-
furan.
group because Me2SiH2 is a catalyst poison.[28–30] One-pot,
À
consecutive reduction of both the Si Cl and the ester groups
The functionality at the terminus of the polymer chains
was deduced by analysis of the end groups (Figure 3). The
À
with LiAlH4 (which ensures that the Si Cl and OH moieties
À
are not present in the same pot because the Si Cl reduction is
much faster than that of the ester reduction) furnished the
novel bifunctional monomer 1 on a decagram scale. The
monomer was distilled to obtain material with a purity
(99.5% by GC analysis) suitable for synthesis of high-molar-
mass polymers by step-growth polymerization.
Several methods for the dehydrogenative silylation of
alcohols are known, including reactions catalyzed by tran-
sition-metal complexes[20,31] and by boranes.[24] We first
attempted the dehydrogenative polymerization of 1 with
[{Ir(coe)2Cl}2] as the catalyst, based on prior work on the
iridium-catalyzed dehydrogenative silylation of alcohols by
Simmons and Hartwig.[31] However, polymerization of 1 cata-
lyzed by [{Ir(coe)2Cl}2] at temperatures ranging from 20 to
808C led to insoluble products (Figure 2). This material was
tentatively assigned to be a cross-linked polymer because the
reaction of a hydrosilane with an iridium precursor lacking
strongly coordinating ligands has been shown to form a silane-
bridged dimeric iridium species containing multiple silyl
groups bound to iridium.[32,33] Similarly, polymerization con-
ducted with [{Ru(p-cymene)Cl2}2] as the catalyst led to an
Figure 3. Detailed structure of the PSE with signals corresponding to
the end groups labeled as Hd, Hc, and SiOSi(Me)2.
2
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1H NMR spectrum of the polymer contained a triplet at d =
3.57 ppm corresponding to the CH2OSi group, a multiplet at
d = 0.60 ppm corresponding to the SiOCH2 group, and
a singlet at d = 0.09 ppm corresponding to the Si(CH3)2. No
Table 1: The effect of catalyst loading on the molar mass of the
polymers.[a]
À
signals from the Si H functionality were observed, thus
indicating complete reaction at these bonds. The H NMR
[b]
[c]
[c]
[d]
[d]
Entry
x
Mn
Mn
Mw
ꢀ[c]
Mn
Mw
ꢀ[d]
1
1
2
3
4
0.2
0.5
1
30
21
16
7
49
38
24
9
85
63
43
16
1.74
1.65
1.80
1.88
37
23
17
8
65
40
29
11
1.72
1.69
1.73
1.40
spectrum revealed a group of small triplets (d = 3.66–
3.60 ppm) that resonate 0.08 ppm downfield of the major
CH2OSi signal (d = 3.56 ppm) of the polymer chain, a small
multiplet (d = 0.51 ppm) that resonates 0.09 ppm upfield of
the major CH2SiO signal (d = 0.60 ppm), and a small singlet
(d = 0.04 ppm) that resonates 0.05 ppm upfield of the major
Si(CH3)2 signal (d = 0.09 ppm). Together, these signals indi-
cate the presence of Si O Si linkages within the polymer
chain. Assuming the Si O Si linkage results from reaction of
two Si H ends of a monomer with H2O (most likely formed
from reaction of CsOH with the OH groups of the monomer),
an excess of OH groups would be present in the reaction.
Thus, both ends of the polymer should be terminated by OH
groups.
This hypothesis is supported by comparing the chemical
shifts of the aforementioned protons to those of the model
compound 2 (Figure 4). The Si(CH3)2 and CH2SiO signals of
2.5
[a] dn/dc=0.049. dn/dc of the polymer was determined using a sample
with a Mn of 10 kgmolÀ1 (by SEC). Molar masses are in kgmolÀ1
[b] Determined by NMR spectroscopy. [c] Determined by SEC with RI.
[d] Determined by SEC with LS.
.
À À
À À
polymerization,[40] we hypothesized that the degree of poly-
merization could be controlled by varying the amount of
catalyst (and thus H2O) in the system.[41] To test this
hypothesis, we conducted the polymerization with 0.5–2.5%
CsOH and examined the molar masses by both NMR analysis
and by SEC equipped with a differential refractive index (RI)
and light scattering (LS) detectors. Indeed, the molar masses
of the polymer formed from reactions conducted with larger
quantities of H2O (from CsOH) were lower than those from
reactions with smaller quantities of H2O (Table 1).
À
À À
the proposed Si O Si linkages in the polymer overlap with
those of 2, and the signal from the CH2OH unit in the
To avoid side reactions associated with excess,
unquenched hydroxide catalysts, we conducted the polymer-
ization with a constant (1 mol%) amount of KH and
controlled the molar mass by adding various amounts of
AA-type monomer 1,10-decanediol as opposed to various
amounts of CsOH.[42] The molar mass of the polymer, formed
from a series of polymerizations with 1–8 mol% 1,10-
decanediol, decreased with increased loading of 1,10-decane-
diol (Table 2).
Table 2: The effect of 1,10-decanediol on the molar masses of PSE.[a]
ꢀ[c]
Mn
Mw
ꢀ[d]
[b]
[c]
[c]
[d]
[d]
Entry
x
Mn
Mn
Mw
1
2
3
4
1
2
4
8
14
23
11
5.6
3.6
43
15
9.0
6.3
1.9
1.4
1.6
1.8
16
33
2.0
8.1
4.9
2.6
n.d.
n.d.
3.1
n.d.
n.d.
4.6
n.d.
n.d.
1.5
Figure 4. Comparison of the 1H NMR spectrum of PSE to that of the
model compound 2.
[a] Molar masses are in kgmolÀ1. [b] Determined by NMR spectroscopy.
[c] Determined by SEC with polystyrene standards. [d] Determined by
SEC with light scattering. n.d.=not determined.
proposed OH end groups in the polymer overlap with those of
the CH2OH end of 2. Consistent with this assignment, the
ratio of the integrations between the major and the minor
signals resulting from the CH2O, CH2SiO, and Si(CH3)2
groups are consistent with this assignment.
The molar mass of the polymer can be estimated from the
ratio of the CH2OH and CH2OSi by NMR spectroscopy
(Mn = 21k; see the Supporting Information) assuming exactly
two end groups per chain. These data agree with the molar
mass measured by light scattering size exclusion chromatog-
raphy analysis (Mn = 23 kgmolÀ1; Table 1).
The PSEs produced by the based-catalyzed polymeri-
zation were, in general, colorless, viscous oils or soft solids,
depending on the molar mass. Analysis of the PSE with an
Mn value of about 23 kgmolÀ1 by thermogravimetric analysis
showed that this material lost 1% of its weight at 2448C and
5% of its weight at 2788C. Thus, the PSEs are significantly
more thermally stable than polysilicon acetals containing
aliphatic backbones[21] but less stable than PSEs containing
aromatic backbones.[10,17,20] The Tg of the polymer is À678C,
Because H2O can effectively act as a dihydroxy AA-type
monomer which introduces a stoichiometric imbalance during
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and no significant melting or crystallization transitions were
observed by differential scanning calorimetry.
Having designed the polysilylethers to undergo hydrolysis
À
at the Si O bonds, we tested the stability of the PSE materials
toward mixtures of aqueous and organic solvents, and water
alone under hydrolytic degradation conditions. Dissolution of
a PSE sample (Mn ꢀ 23 kgmolÀ1) in a common organic
solvent, such as THF, followed by addition of an equal
volume of neutral water led to complete degradation to 2
under these biphasic conditions after either 24 hours at 508C,
or after 2 days at 238C. In addition, complete degradation of
the same polymer at 508C occurred within 7 days in water at
pH 2 and within 50 days at pH 4 to give 2 as a white foam in
the aqueous layer, despite the low solubility of the starting
polymer in water.[43] At room temperature, complete degra-
dation at pH 2 occurred over 22 days. Degradation even
Figure 6. Overlay of the SEC traces of PU (solid line) and PSE (dashed
line).
than that of the main peak (Figure 6, solid line). This lower-
molecular-weight material most likely corresponds to a small
amount of PSEs lacking hydroxy end groups. The Mn and Mw
determined by light scattering are 45 kgmolÀ1 and
70 kgmolÀ1, respectively. The polymer exhibited good ther-
mal stability (1% weight loss at 2098C and 5% weight loss at
3098C) and a glass transition at À688C.
À
occurred at neutral pH at 508C, and 10% of the Si O bonds
hydrolyzed after 50 d (Figure 5).
In conclusion, we have synthesized a novel polysilylether
which is sourced from a renewable feedstock and can be
hydrolyzed under mild reaction conditions. The molar mass of
the polymer can be controlled by varying the amount of AA-
type monomer in the reaction system. In addition, we have
synthesized a polyurethane from methylene diphenyl diiso-
cyanate (MDI) and a dihydroxy-telechelic polysilylether.
Further studies on the relationship between the length of the
PSE and the physical and mechanical properties of the
resulting polyurethane, and further studies on synthesizing
additional copolymers with PSE structures are underway.
Acknowledgments
We thank NSF Center for Sustainable Polymers (CHE-
1413862) for funding. M.A.H. thanks Dr. Yanzhao Wang for
helpful input.
Figure 5. Degradation of PSE in water. Conversion of the silylether
linkages as determined by NMR spectroscopy.
Keywords: biomass · polymers · reduction · silicon ·
To exploit the low Tg value of the polysilylethers and the
hydroxy termini, we synthesized polyurethanes from a hy-
droxy-telechelic PSE (ca. 3 kgmolÀ1) and a diisocyanate. The
polymerization was conducted with methylene diphenyl
diisocyanate (MDI) in THF at 658C for 4 hours with
2 mol% of Sn(Oct)2 (relative to PSE, 1 mol% per OH end)
as the catalyst. The formation of polyurethanes was evidenced
by the NMR spectral data and SEC data of the material
obtained after precipitation in MeCN to give a colorless and
sustainable chemistry
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4
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Received: June 28, 2016
Published online: && &&, &&&&
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Communications
Biomass
Cradle to grave: Castor oil derivatives
were converted into polysilylethers
(PSEs) by step-growth polymerization.
The hydrolytically labile silyl ether link-
ages allowed facile degradation of the
polymers into a degradable diol, thus
completing the life cycle of the polymer.
The PSEs possess a low glass-transition
temperature (Tg), tunable molecular
weight, and two hydroxy termini, which
enable the construction of polyurethanes
from the PSEs.
C. Cheng, A. Watts, M. A. Hillmyer,*
J. F. Hartwig*
&&&& — &&&&
Polysilylether: A Degradable Polymer
from Biorenewable Feedstocks
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
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