Renewable Isohexide-Based, Hydrolytically Degradable Poly(silyl ether)s with High Thermal Stability

Article abstract of DOI:10.1002/cssc.201801123

Several degradable poly(silyl ether)s (PSEs) have been synthesized by dehydrogenative cross-coupling between bio-based 1,4:3,6-dianhydrohexitols (isosorbide and isomannide) and commercially available hydrosilanes. An air-stable manganese salen nitrido complex [Mn^VN(salen-3,5-tBu₂)] was employed as the catalyst. High-molecular-weight polymer was obtained from isosorbide and diphenylsilane (M_n up to 17000 g mol^?1). Thermal analysis showed that these PSEs possessed high thermal stability with thermal decomposition temperatures (T_{?5 %}) of 347–446 °C and glass transition temperatures of 42–120 °C. Structure–property analysis suggested that steric bulk and molecular weight have a significant influence to determine the thermal properties of synthesized polymers. Importantly, these polymers were degraded effectively to small molecules under acidic and basic hydrolysis conditions.

Full text of DOI:10.1002/cssc.201801123

Accepted Article
Title: Renewable Isohexides-Based, Hydrolytically Degradable
Poly(silylether)s with High Thermal Stability
Authors: Srikanth Vijjamarri, Marianne Hull, Edward Kolodka, and
Guodong Du
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ChemSusChem
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Renewable Isohexides-Based, Hydrolytically Degradable
Poly(silylether)s with High Thermal Stability
Srikanth Vijjamarri, [a] Marianne Hull, [a] Edward Kolodka, and Guodong Du*
[b]
[a]
Abstract: Several degradable poly(silylether)s (PSEs) have been
successfully synthesized by dehydrogenative cross-coupling between
bio-based 1,4:3,6-dianhydrohexitols (isosorbide and isomannide) and
commercially available hydrosilanes. An air-stable manganese salen
have been growing due to their wide diversity of applications in
biomedical fields; representative examples include orthopedic
implants, tissue engineering, drug delivery systems, and imaging
agents. [5]
v
t
nitrido complex [Mn N(salen-3,5-Bu
High molecular weight polymer was obtained from isosorbide and
diphenylsilane (M up to 17000 g/mol). The thermal analysis showed
2
)] was employed as the catalyst.
Driven by these considerations,
isohexides (1,4:3,6-
dianhydrohexitols) namely isosorbide (IS; 1,4:3,6-dianhydro-D-
glucitol), isomannide (IM; 1,4:3,6-dianhydro-D- mannitol), and
isoidide (II; 1,4:3,6-d-dianhydro-L-iditol) have received significant
attention in polymer chemistry (Figure 1). [ 6 ] Isohexides are
commonly prepared from carbohydrate sources via multistep
reactions including hydrogenation and dehydration. [7] Among the
three isomers, IS, a C2 exo, C5 endo diol and IM, a C2/C5 endo
endo diol are readily available due to their high yield production
from D-sorbitol and D-mannitol, respectively, whereas the C2/C5
exo exo isomer, II is rare due to the negligible existence of its
precursor, L-idose in plants. [8] These bifunctional isohexides are
chiral and non-toxic; [9] the unique aliphatic bicyclic structure with
two fused tetrahydrofuran rings provides high strength and rigidity
to the molecule. [10] Consequently, the incorporation of isohexides
into the polymer backbone can dramatically enhance the thermal
and mechanical properties of polymers, which makes them
attractive for high thermal applications such as flame retardants.
n
that these PSEs possessed high thermal stability, with thermal
decomposition temperatures (T-5% of 347–446 °C) and glass transition
temperature of 42–120 °C. Structure-property analysis suggested that
steric bulk and molecular weight have significant influence in
determining the thermal properties of synthesized polymers.
Importantly, these polymers were effectively degraded to small
molecules under acidic and basic hydrolytic conditions.
Introduction
Petrochemical based synthetic polymers have played a significant
role in the chemical industry due to their attractive properties like
durability, elasticity, and other valuable features. However,
depletion of fossil fuel feedstock and environmental persistence
of most petrochemical polymers limit their sustainability
perspective.[1] To overcome these challenges, naturally originated
and eco-friendly polymers have emerged as a major sustainable
alternative.[ 2 ] In this regard, a variety of biobased renewable
monomers and their derivatives have been extensively reported
in the polymer synthesis. [3] Yet, the low thermal stability of most
of the bio-based polymers, particularly low glass transition
[
11]
Figure 1. Chemical structures of (a) isosorbide (exo endo), (b) isomannide
endo endo), (c) isoidide (exo exo). Dotted lines in (a) and (b) represent
(
g d
temperature (T ) and low thermal decomposition (T ) are a major
intramolecular hydrogen bonds
limitation for high temperature industrial processes and
applications. [4] On the other hand, design and development of
various degradable polymers, hydrolytically or enzymatically,
Interestingly, a variety of reactive functionalities such as amine,
azide, isocyanate, carboxylic acid, and ester groups can be
installed to replace or extend the original hydroxyl groups for new
starting materials, which further broadens the scope of accessible
polymers. [12] Hence, a wide range of thermally stable polyesters,
[
[
a]
b]
S. Vijjamarri, M. Hull, G. Du*
Department of Chemistry
University of North Dakota, 151 Cornell Street Stop 9024, Grand
Forks, North Dakota 58202, United States
E-mail: guodong.du@und.edu
[
13]
polyamides, [14] poly (ester amide)s, [15] poly(ether imide)s, [16]
polymethacrylates, [17] polyurethanes, [18] polycarbonates, [19] and
poly(isosorbide fatty alkylates) [20] have been synthesized through
E. Kolodka
Department of Chemical Engineering
University of North Dakota, 241 Centennial Dr. Stop 7101, Grand
Forks, North Dakota 58202, United States
either chain or step growth polymerization. Notably, poly (ester
amide)s derived from isohexides have been shown to be
degradable in bioanalogous studies and can be successfully used
in biological applications. [21]
Supporting information for this article is given via a link at the end of
the document.
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the resulting PSE (i) still possesses the C2 exo and C5 endo
configurations. Besides, attribution of four peaks in 13C NMR
between 128.0 and 140.0 ppm and a single peak in 29Si NMR at -
31.48 ppm confirms the presence of diphenylsilyl moiety in the
However, isohexide based poly(silylether)s (PSEs) have not been
reported though there is a significant industrial and academic
attention for them. PSEs are potentially susceptible to hydrolytic
degradation due to the presence of a labile silyl ether (Si–O–C)
linkage. [22] A variety of synthetic methods have been reported to
produce silylethers from various available resources. [3a,23] One of
the most commonly used methods is transition metal catalyzed
condensation polymerization of alcohols with silanes. [24] Recently,
we have reported the synthesis of degradable PSEs from
biobased furan derivatives and silanes catalyzed by an air-stable
resultant polymer. The GPC analysis showed the M
n
as 3300
g/mol with dispersity (Đ) of 1.31.
Scheme 1. Polymerization of IS with Different Silanes
v
t
manganese nitrido salen complex [Mn N(salen-3,5- Bu
2
)] (Mn-1).
[
22a]
Mn has great potential as a sustainable alternative to precious
Table 1. Polymerization of IS with Different Hydrosilanes
metals such as platinum, rhodium, and ruthenium, because it is
inexpensive, earth abundant, nontoxic and biocompatible. [25] Use
of readily available hydrosilanes as co-monomer over moisture
(g/mol) [b]
Đ [b]
Polymer #
Structure (time h)
M
n
Yield %
#
PSE (i)
PSE (ii)
PSE (iii)
PSE (iv)
ISPh
2
2
-(24)
-(40)
3300
5700
4000
3500
1.31
1.66
1.44
1.57
78
86
70
72
1
2
3
4
ISPh
2
sensitive chlorosilanes is also advantageous as nontoxic H is the
ISPhNp-(40)
ISPhMe-(40)
only by-product in this dehydrogenative coupling reactions. [26]
Motivated by these results, herein we present the synthesis and
characterization of IS and IM based PSEs via polycondensation
method catalyzed by Mn-1. Remarkably, PSEs derived from
isohexides exhibit high glass transition and thermal stabilities, yet
are hydrolytically degradable.
[
a] Reaction conditions: substrate, 0.87−0.90 mmol; silane, 1.0 equiv; and Mn
catalyst 1, 1.0 mol %. Conversion is >95% as judged by the consumption of
hydrosilanes by NMR. [b] Determined by gel permeation chromatography (GPC)
calibrated with polystyrene standards
In a new reaction between the same monomers for 40 h of
n
reaction time, the molecular weight (M ) of the resulting PSE (ii)
was increased to 5700 g/mol (Table 1, entry 2). When compared
with PSEs derived from 2,5-bis(hydroxymethyl) furan under
identical conditions,[22a] IS has produced lower molecular weight
Results and Discussion
The IS and IM based PSEs were synthesized through
polycondensation process by reacting them with secondary
hydrosilanes (Scheme 1). The polymerization reactions were
performed according to our recent work, and toluene was again
chosen as the solvent in this study because of the formation of
low molecular weight polymers in other solvents such as benzene,
mesitylene, xylene, and dioxane (data presented in SI, Table S1).
The reactions were monitored at regular time intervals by NMR
spectroscopy. The resulting PSEs were purified through the
precipitation method, and the details are described in the
d
c
b
a
2 2
experimental section. The reaction between Ph SiH and IS went
smoothly and the conversion reached >95% in less than 10 h,
nevertheless, the reaction time was prolonged to 24 h (Table 1,
entry 1). The disappearance of IS’s hydroxyl peak at 2.96 ppm
Figure 2. Comparison of 13C NMR for IS and IS-derived PSEs. (a) IS
monomer, (b) PSE (i), (c) PSE (iii), (d) PSE (iv)
1
and silane hydrogens (SiH) at 4.92 ppm in H NMR confirmed the
PSEs. Although direct comparison of GPC-based molecular
weights of different polymers should be taken with caution, we
attribute it to the secondary hydroxyl groups of isohexides being
consumption of starting materials (Figure S1). As illustrated in
Figure 2 (b), the appearance of six characteristic peaks at 71.3,
3.9, 76.0, 78.0, 82.0, and 88.3 ppm in 13C NMR indicates that
7
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relatively less reactive than primary hydroxyl groups, as observed
Considering the low reactivity of isohexides and the step-growth
polymerization pathway of current polycondensation reactions,
we further increased the reaction time to synthesize polymers with
high molecular weight. Thus, a new reaction between IS and
in similar reactions.[25c,
26a]
Additionally, the intramolecular
hydrogen bonding between the proton of endo hydroxyl group and
an endocyclic oxygen may also decrease the reactivity of the
endo hydroxyl group (see Figure 1). [27-28]
Ph
analysis displayed that high molecular weight polymer PSE (v)
was indeed achieved, and M was observed to be 17000 g/mol
2 2
SiH was performed under optimized conditions for 80 h. GPC
To explore the reactivity of hydrosilanes in manganese catalyzed
polycondensation
naphthylphenylsilane (PhNpSiH
reaction. A low molecular weight PSE (iii), M
reactions,
sterically
) was next employed in the
= 4000 g/mol was
hindered
1-
n
2
with Đ of 2.08 (Table 2, entry 1). Therefore, the reaction time was
kept as 80 h in further reactions. Similarly, the longer reaction time
boosted the molecular weight of the PSE (vi) from IS and sterically
n
produced at 40 h (Table 1, entry 3). The appearance of fourteen
peaks between 125.0 and 145.0 ppm in 13C NMR confirms the
incorporation of naphthylphenyl moiety into the polymer structure
hindered PhNpSiH
2, entry 2). In another reaction between a less active silane,
PhMeSiH and IS, the molecular weight of the resulting PSE (vii)
2
up to 14000 g/mol with the Đ of 2.00 (Table
(
(
Figure 2 (c)). When a less hindered silane, methylphenylsilane
PhMeSiH ) was used, PSE (iv) with even lower molecular weight
2
2
was observed as 13300 g/mol with the Đ of 1.92 (Table 2, entry
was obtained (Table 1, entry 4), presumably due to the lower
3).
reactivity of PhMeSiH
2
than Ph
2
SiH
2
in the reaction.[25c]
Table 2. Polymerization of IS and IM with Hydrosilanes
Polymer #
PSE (v)
Structure (time h)
ISPh -(80)
M
n
(g/mol) [b]
17000
14000
13300
13000
9500
Đ [b]
2.08
2.00
1.92
2.25
1.56
2.05
Yield %
90
PSE (iv)
#
1
2
2
3
4
5
6
PSE (vi)
PSE (vii)
PSE (viii)
PSE (ix)
PSE (x)
ISPhNp-(80)
ISPhMe-(80)
82
PSE (iii)
PSE (i)
73
2
IMPh -(80)
71
IMPhNp-(80)
IMPhMe-(80)
72
6100
70
Si-H stretching
OH stretching
Hydrosilane
[
1
a] Reaction conditions: substrate, 1.78 mmol; silane, 1.0 equiv; and catalyst Mn-
1.0 mol %. Conversion is >95% as judged by the consumption of hydrosilanes
by NMR. [b] Determined by GPC calibrated with polystyrene standards.
IS
Encouraged by these results, IM, a symmetric isohexide was
examined as monomer and polymerization reactions were
performed with the three hydrosilanes mentioned above.
Figure 3. ATR FT-IR spectra of Isosorbide (bottom), hydrosilane, PSE (i), PSE
iii), and PSE (iv)
Reaction with Ph
2
SiH
2
under the typical polymerization conditions
of 13000 g/mol with Đ of 2.25
(
resulted in PSE (viii), exhibiting M
n
The appearance of silylmethyl peak at -3.50 ppm in 13C NMR
(Table 2, entry 4). In contrast to IS-derived PSE (v), IM-derived
PSE (viii) have displayed significantly different NMR spectra.
confirms the formation of PSE (iv) (Figure 2 (d)). The formation of
low molecular weight polymers with hindered and alkyl
hydrosilanes in Mn-1 catalyzed reactions are consistent with our
previous observations. These polymers were further
characterized by FT-IR. As shown in Figure 3, the disappearance
1
Appearance of only three peaks at 3.70, 4.20, and 4.40 ppm in H
NMR for the eight alicyclic protons and three peaks at 72.0, 75.3,
and 82.0 ppm in 13C NMR demonstrates the structural difference
of IS and IM, which is in agreement with the symmetric nature of
IM. We next carried out two polymerization reactions under
of hydroxyl (OH) and Si–H stretching frequencies around 3300
cm-1 and 2100 cm-1 respectively in FT-IR also supports the
2 2
standard conditions with PhNpSiH and PhMeSiH . As depicted in
Table 2 (entries 5 and 6), M of the resulting PSEs (ix) and (x)
n
formation of PSEs between isohexide and hydrosilanes. The
entire series of PSEs exhibit similar FT-IR absorption patterns.
were 9500 g/mol and 6100 g/mol, respectively. Comparatively,
IM-derived PSEs (viii-x) showed lower molecular weights than the
corresponding IS-derived PSEs (v-vii). These results suggest that
Synthesis of PSEs from Isosorbide and Isomannide at
extended reaction times
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Table 3. DSC and TGA Data of PSEs
Final residue %
#
Polymer #
Structure (time)
T
g
/°C [b]
T
-5%/°C [c]
T
-50%/°C [c]
T
max /°C [c]
(
800 °C) [c]
1
2
3
4
5
6
7
8
9
PSE (i)
PSE (ii)
PSE (iii)
PSE (iv)
PSE (v)
PSE (vi)
PSE (vii)
PSE (viii)
PSE (ix)
PSE (x)
ISPh2-(24)
ISPh2-(40)
75
78
382
408
419
347
410
446
397
384
432
367
487
488
495
482
498
509
486
494
505
484
479
481
482
470
484
489
476
484
480
476
32
18
25
26
30
27
27
27
30
27
ISPhNp-(40)
ISPhMe-(40)
ISPh2-(80)
115
38
85
ISPhNp-(80)
ISPhMe-(80)
IMPh2-(80)
IMPhNp-(80)
IMPhMe-(80)
120
43
76
116
42
10
[
a] Reaction conditions: substrate, 1.78 mmol; silane, 1.0 equiv; and catalyst Mn-1, 1.0 mol %. [b] Determined by DSC. [c]
Determined from TGA. T-5% and T-50% refer to the temperatures at which 5% and 50% weight losses were observed,
respectively, whereas Tmax refers to the temperatures at which maximum rate of weight loss occurs
100
ISPh2-(24 h)
ISPh2-(40h)
1.2
0.8
0.4
0
80
60
40
20
0
ISPhNp-(40h)
ISPhMe-(40h)
ISPh2-(80 h)
ISPhNp-(80h)
ISPhMe-(80h)
IMPh2-(80h)
IMPhNp-(80h)
IMPhMe-(80h)
200
700
Temp ºC
0
200
400
Temperature ºC
600
800
2
Figure 4. TGA thermograms of isohexide-derived PSEs, recorded from 30 to 800 °C at a heating rate of 20 °C/min under a N atmosphere. Inset is the DTG
curve of the same data set.
orientation of hydroxyl groups of isohexides has a significant
influence on their reactivity: the two endo hydroxyl groups of IM
could both participate in intramolecular hydrogen bonding with
adjacent oxygen atom, whereas only one endo hydroxyl group for
intramolecular hydrogen bonding in IS (Figure 1).
90% (Table 2). Also, the FT-IR spectra of all the synthesized
PSEs are presented in the SI.
Thermal Analysis
The thermal properties of these PSEs have been comparatively
examined by techniques including differential scanning
calorimetry (DSC) and TGA, and the results are summarized in
Table 3.
2 2
For a direct comparison of IS and IM, reactions wiht Ph SiH were
performed under identical conditions (Figure S21), which
demonstrated that IS reacted faster than IM in the reaction,
though the difference was small. Nevertheless, all these
polymerization reactions gave reasonable isolated yields of 70-
The thermal stabilities of the PSEs were evaluated by TGA from
30 to 800 C under inert (nitrogen) atmosphere, and results are
illustrated in Figure 4. All the synthesized PSEs have displayed a
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values (480-490 °C)of Ph
2
SiH
2
-derived PSEs (i, ii, v and viii) were
17
18
19
20
21
22
23
2 2
in between the values of PhNpSiH and PhMeSiH derived PSEs.
IMPh2-(80h)
This observed trend illustrates that steric bulk around the silicon
has played an important role in enhancing the thermal stability of
the PSEs. In this context, it is worth noting that these PSEs exhibit
ISPh2-(24h)
high thermal stabilities (as judged by T-5%, T-50%, or Tmax /°C) when
ISPh2-(80h)
compared to other isohexide-based polymers such as polyesters,
polyamides, and poly(ether amides). [14b,16,30,31,32 ]
ISPh2-(40h)
70
Furthermore, molecular weights of polymers also can impact the
thermal stability of PSEs, which is evident when comparing the
60
80
90
100
110
120
Temperature ºC
M
n
and T-50% of PSEs (i), (ii), and (v). The PSE (i) with low M
300 g/mol has displayed T-50% of 487 °C, whereas PSE (v) with
high M of 17000 g/mol has displayed T-50% of 492 °C.
n
of
1
9
0
1
2
3
4
3
ISPhNp-(40h)
2
2
2
2
2
n
IMPhNp-
Glass transition data acquired from DSC analysis are listed in
Table 3, and thermograms from the second heating cycles are
displayed in Figure 5. It can be seen that the T
g
values of these
was observed
ISPhNp-(80h)
PSEs were in the range of 38–120 °C, and only T
g
without any melting or crystallization transitions, which suggests
they are amorphous in nature. As noted above, these PSEs
90
100
110
120
130
140
150
Temperature ºC
g
generally feature higher T ’s than other isohexide-based polymers
including polyesters, polyurethanes, and poly(ester urethanes),
[
2c,9,32,33,34]
though a few polyesters, poly(ether imides), and IS
incorporated poly(butylene terephthalate)s have greater ’s
detailed data in the SI, Table S2). [16, 32, 35] In this study, the
19
20
21
22
T
g
(
PhNpSiH
2
g
-derived PSEs (iii), (vi), and (ix) have displayed the
IMPhMe-(80h)
ISPhMe-(40h)
highest T
’s at 115, 120 and 116 ºC, respectively, which could be
attributed to the presence of a highly hindered naphthyl group in
the polymer backbone. Notably, polyesters based on isohexides
ISPhMe-(80h)
60
g
with rigid aromatic diacids can reach comparable T ‘s (up to 105
30
40
50
Temperature ºC
70
ºC).[31] Similar to the trend in thermal degradation temperatures
Figure 5. DSC thermograms from the second heating cycles of the isohexide-
based PSEs. Top) PSEs of IS and IM with Ph SiH . Middle) PSEs of IS and IM
with PhNpSiH . Bottom) PSEs of IS and IM with PhMeSiH
120
100
2
2
2
2
.
80
60
40
20
0
single-stage decomposition profile. Irrespective of the molecular
weights and polymer backbone structure, majority of PSEs have
shown a remarkable thermal stability as indicated by T-5% around
ISPSEs
BHMFPSEs
3
47-446 ºC and T-50% between 470-509 ºC, which is at the high
Ph SiH -PSEs PhNpSiH -PSEs PhMeSiH -PSE
2
2
2
2
end of the PSEs reported to date. [23a,24a,29]
Figure 6. Comparison of Tg’s of IS-PSEs with BHMF-PSEs
PSEs derived from PhNpSiH
exhibited the highest T-50%, while PhMeSiH2- derived PSEs such
as PSE (iv) , (vii), and (x) showed the lowest T-50%. The T-50%
2
, namely PSE (iii), (vi), and (ix) have
observed above, the PhMeSiH
2
-derived PSEs (iv), (vii) and (x)
with a less hindered methyl group have exhibited comparatively
.
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lowest T
and (viii) have shown modest T
addition, high molecular weight PSEs have shown higher T
over low molecular weight PSEs (i, ii, v). All these observations
are in accord with the literature report that the T values of
g
’s (38-43 ºC), while the Ph
’s in between (75–85 °C). In
’s
2
SiH
2
-derived PSEs (i), (ii), (v)
molecular weight change over time (Figure 7). A high molecular
weight PSE (v) was chosen as a representative. As anticipated,
no or little degradation occurred at neutral conditions, but a
considerable decrease in the molecular weight was observed in
g
g
g
2
the first few hours when 2 vol % HCl/H O (pH 2) in THF was used
polymers can be enhanced by high molecular weight and
steriacally hindered groups. [36-37]
as the medium at room temperature (RT). However, the
degradation slowed down and resulted in partial degradation with
To better understand the role of the rigidity of isohexides in
affecting the thermal properties of PSEs, we have compared the
glass transition temperatures of IS-derived PSEs with that of 2,5-
bis(hydroxymethyl)furan (BHMF) derived PSEs (Figure 6). It is
M
n
of 6900 g/mol in 24 h. This may suggest that the extent of
degradation depends on the amount of acidic hydrogens available.
[
22]
2
In accord with this, when the amount of HCl/H O (pH 2) was
increased to 10 vol %, a rapid decrease in the molecular weight
was observed and the total degradation was completed in less
than 10 h. Raising the temperature greatly accelerated the
degradation process, as a complete degradation occurred in less
than an hour when the temperature was raised to 50 °C from RT.
In addition, PSE (v) also underwent hydrolysis in basic medium,
clear that IS-derived PSEs have higher
corresponding BHMF-derived PSEs, and in both series, the T
increase with the steric bulk at the silicon, i.e. in the order of
SiPhMe SiPh SiPhNp, in line with the literature
observations.[34,38] Furthermore, the difference in T
’s between the
two series are observed to be 75, 92, and 35 ºC respectively, for
Ph SiH , PhNpSiH , and PhMeSiH derived PSEs. In other words,
T
g
’s than the
g
’s
<
2
<
g
2
as partial degradation was observed when 2 vol % KOH/H O (pH
2
2
2
2
11) in THF was used as the medium at RT. The extent of
degradation apparently correlated with the amount of available
hydroxide ions, as the resultant polymer had a higher molecular
the same bulky SiPhNp group would impart a larger influence
when combined with a rigid, secondary diol than with a primary
diol. This suggests a synergy between the steric bulk and rigidity
of two co-monomers in affecting the thermal properties of the
polymers.
weight (~8200 g/mol) compared to that of 2 vol % HCl/H
condition.
2
O (pH 2)
To explore the effect of steric hindrance in degradation, IS-derived
PSEs with Ph SiH (v), PhNpSiH (vi), and PhMeSiH (vii) were
Hydrolytic Degradation
2
2
2
2
Environmental degradability is one of the key considerations in
the sustainable development and applications.[39] Hydrolytically
sensitive polymers are of special interest owing to their potential
applications in biomedical fields such as in the delivery of protein
based vaccines and nucleic acids, and treatment of inflammatory
related diseases. [ 40 ] We have investigated the degradation
behaviour of PSEs under various conditions by monitoring the
compared in the degradation processes. As expected, all three
PSEs exhibited a rapid initial degradation under the similar
2
conditions using 2 vol. % HCl/H O (pH 2) acidic medium (Figure
8). Obviously, PSE (vi) with highly hindered naphthyl group has
shown some hydrolytic stability over the other two PSEs. It should
be noted that complete degradation was not observed even after
six days. Similar dependence on the availibility of acidic
1
1
1
8000
5000
2000
2
vol % pH 7 solution, RT
2
vol % pH 2 solution, RT
9
6
3
000
000
000
0
10 vol % pH 2 solution, RT
2
vol % pH 11 solution, RT
1
0 vol % pH 2 solution, 50 ºC
0
5
10
15
20
25
Time (h)
Figure 7. Degradation plots of PSE (v) (Mn vs time) under neutral, acidic, and basic conditions. Conditions employed are 0.02 mL of HCl/H2O (pH 2) in 0.98 mL of
THF (2 vol % pH 2) and 0.10 mL of HCl/H2O (pH 2) in 0.90 mL of THF (10 vol % pH 2) at RT or 50 ºC and 0.02 mL of KOH/H O (pH 11) in 0.98 mL of THF (2 vol
pH 11. ■ 2 vol % pH 7 solution, RT.  2 vol % pH 2 solution, RT. ● 10 vol % pH 2 solution, RT. ► 10 vol % pH 2 solution, 50 ºC.  2 vol % pH 11 solution, RT.
2
%
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ChemSusChem
10.1002/cssc.201801123
[
Type here]
hydrogens has been mentioned in the degradation of PSEs.[22d]
Additionally, gas chromatography-mass spectrometry (GC/MS)
analysis on the fully degraded samples of PSE (v) clearly showed
the presence of the starting monomer IS (m/z 146.1 g/mol) and
silanol derivatives, such as diphenylsilanediol (m/z 216 g/mol),
particularly in consideration of their low toxicity and partially
biobased origin. Further studies on improving the properties of
PSEs and their potential applications are underway in our lab.
Experimental Section
Materials, Methods, and Instrumentation
hexaphenylcyclotrisiloxane
octaphenylcyclotetrasiloxane (m/z 792.3 g/mol). Also, another
compound having two IS and one Ph SiH units (m/z of 472.1
g/mol) was observed (Figure S19). Quantification by NMR with an
internal standard (PhMe Si) indicated nearly quantitative
(m/z
594
g/mol),
and
Isosorbide, isomannide and hydrosilanes were purchased from
Sigma Aldrich. Deuterated solvents were purchased from the
Cambridge Isotope Laboratories. Solvents were degasified and
dried over molecular sieves (4 Å) overnight prior to use.
Glassware was dried overnight prior to use. H, 13C, and 29Si NMR
2
2
1
3
regeneration of the diol monomer IS (Figure S20). Since no IS
degradation products was detected, we assume the mechanism
of hydrolytic degradtion relied on the labile Si-O-C linkage. It is
worth mentioning that the degradation products of PSEs, mainly
alcohols and silanol derivatives, are nontoxic and neutral, which
would lead to no extreme pH changes in the surrounding
environment upon degradation. [41]
spectra were recorded on
a
Bruker AVANCE 500 NMR
with
spectrometer in deuterated chloroform (CDCl
3
)
tetramethylsilane (TMS) as the internal reference. FT-IR was
performed on a Thermo Scientific Nicolet iS5 FT-IR instrument,
and analyzed with OMNIC 8.2 software. The samples for analysis
were directly loaded as a thin film using an iD5 ATR accessory.
GPC analysis was performed on a Varian Prostar system
equipped with a PLgel 5 μm Mixed-D column and a Prostar 355
refractive index (RI) detector. THF was used as the eluent and
elution rate was maintained at 1 mL/min (20 °C). Polystyrene
standards were used for the instrument calibration. TGA were
performed on a SDT Q 600 instrument with Advantage software.
18000
15000
12000
PSE (vii)
9000
6000
3000
0
PSE (vi)
2 3
The samples in Al O cups were heated in the 30 to 800 °C range
PSE (v)
25
with a ramp rate of 20 °C/min and a flow rate of 100 mL/min of
nitrogen (furnace purge gas). DSC thermograms were collected
on a PerkinElmer Jade differential scanning calorimeter calibrated
with indium and zinc standards. Samples were analyzed in
0
50
75
100
125
150
Time (h)
Figure 8. Degradation plots of PSE (v, vi, and vii) (M
n
vs time) under acidic
hermetic pans under an inert (N
2
) atmosphere (20 mL/min). Glass
conditions. Conditions employed are 0.02 mL of HCl/H
2
O (pH 2) in 0.98 mL of
transition temperatures (T ’s) of PSEs were determined from the
g
THF (2 vol % pH 2) at RT. ■ PSE (v). ● PSE (vi). ▲ PSE (vii).
Conclusions
second heating cycles from −30 to 350 °C with a heating/cooling
rate of 20 °C/min. Obtained data were analyzed using the Pyris
V9.0.2 software. GC-MS analyses were performed with a 5890
GC with 5972 MS equipped with an autosampler (6890 series,
Agilent Technologies, Santa Clara, CA, USA). Injections were
performed in the split mode with 10:1 ratio and the injection
volume was 1 µL. The temperature program started at 70°C for 1
min, followed by a gradient of 40 °C/min to 140 °C, then a gradient
of 10 °C/min to 350 °C and held for 10 min. The separation was
performed using a 39.7 m long Agilent HP-5MS column. A
constant carrier gas (helium) at a flow rate of 1.5 mL/min (total
flow is 19.5 mL/min) was maintained during the analysis. The MS
data in total ion chromatograms (TIC) were acquired in the mass
range of m/z of 35–1000 at a scan rate of 1.56 scan/s.
Degradable and partially biobased poly(silylether)s (PSEs) have
been synthesized by manganese catalyzed dehydrogenative
cross coupling between isohexides and a series of hydrosilanes.
The PSEs have shown excellent thermal stabilities (T-5% up to
446 °C and T-50% up to 509 °C) and high glass transition
temperatures (T up to 120 °C). This is attributed to the presence
g
of built-in rigidity from isohexide bicyclic rings, as well as the bulky
substituent groups like naphthyl on silicon. The two factors may
interact synergistically in enhancing the thermal stability of PSEs.
At the same time, these PSEs are readily degradable via
hydrolysis under acidic and basic conditions, which can be
modulated by the available active ions (H+ and OH ) and
temperature. This combination of high thermal resistance and
easy hydrolytic degradability may lead to novel applications,
-
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ChemSusChem
10.1002/cssc.201801123
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Type here]
General Procedure for the Synthesis and Purification of PSEs. Degradation of PSEs. To a sample vial PSEs (15-16 mg),
The polymerization reactions were performed under an inert calculated amount of THF, deionized water, and calculated
atmosphere of dry nitrogen (N ). Oil baths equipped with digital amount of HCl/H O or KOH/H O were added. The amounts for
2
2
2
thermometers and controllers were used to set and read the
temperature during the reaction. For a typical synthesis, to a
Schlenk flask (50−100 mL) fitted with a magnetic stirrer, a
the neutral condition were 0.98 mL of THF and 0.02 mL of
deionized water; for acidic conditions 0.98 mL of THF and 0.02
mL HCl/H
(pH 2), for basic conditions 0.98 mL of THF and 0.02 mL
KOH/H O (pH 11). A magnetic stir bar was added and the
2 2
O (pH 2), and 0.90 mL of THF and 0.10 mL of HCl/H O
v
condenser, and nitrogen inlet and outlet, catalyst [Mn N(salen-
t
3
,5- Bu
2
)] (Mn-1) (10 mg, 0.018 mmol,
1
mol %) and
2
stoichiometric equivalents of isohexide or isomannide and
hydrosilane (1.78 mmol of each, 1:1 ratio) were added, followed
by the addition of 2.0-2.5 mL of solvent inside the glovebox (more
details are in the SI). The reaction flask was then taken out,
connected to a Schlenk line under nitrogen flow, and heated to
reflux for specified time. The reaction progress was monitored
periodically taking out a small amount of samples for NMR
analyses. After the reaction, a small amount of DCM (1−2 mL)
was first added to the brown colored crude reaction mixture, and
MeOH was then added portion-wise (10−12 mL) which resulted in
a biphasic mixture. The top layer containing the unreacted
material was removed, and the bottom pale/off-white colored
solid/viscous part was washed with MeOH for a couple of times.
Finally, the resulting polymers were dried under vacuum to a
constant weight for characterization. The spectral data are
compiled in the SI.
homogenized mixture was stirred on a magnetic stirrer at RT or
50 ºC. Samples were taken at specified time intervals and
analyzed by GPC.
Acknowledgement
This material is based upon work supported by the National
Science Foundation under Grant No. NSF EPSCoR Award IIA-
1355466 and REU CHE 1460825. We thank Mr. B. Nespor and
Dr. A. Kubatova for help with GC-MS analysis.
Conflicts of Interest
There are no conflicts of interest to declare.
Key Words: Biomass
.
Isohexides
.
Poly(sillether)s
.
Degradadable polymers . Manganese catalysis
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FULL PAPER
Entry for the Table of Contents
FULL PAPER
Thermally stable and
S. Vijjamarri, M. Hull, E.
Kolodka, G. Du*
hydrolytically degradable
poly(silylether)s
have
been
successfully
from
Renewable Isohexides-
Based, Hydrolytically
Degradable
Poly(silylether)s with High
Thermal Stability
synthesized
hydrosilanes
20
21
22
23
1
00
1.2
.8
and
8
6
4
2
0
0
0
0
0
0
biobased isohexides using
manganese salen
catalyst.
0.4
0
a
90
120
Temp °C
150
300
500
Temperature ºC
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