Functionalized and Degradable Polyphthalaldehyde Derivatives

Article abstract of DOI:10.1021/jacs.9b07508

Polymers that depolymerize back to monomers can be repeatedly chemically recycled, thereby reducing their environmental impact. Polyphthalaldehyde is a metastable polymer that can rapidly and quantitatively depolymerize due to its low ceiling temperature. However, the effect of substitution on the physical and chemical properties of polyphthalaldehyde derivatives has not been systematically studied. Herein, we investigate the cationic polymerization of seven o-phthalaldehyde derivatives and demonstrate that judicious choice of substituent results in materials with a wide range of ceiling temperatures (<-60 to 106 °C) and decomposition temperatures (109-196 °C). We anticipate that these new polymers and their derivatives will enable researchers to access degradable materials with tunable thermal, physical, and chemical properties.

Full text of DOI:10.1021/jacs.9b07508

Communication
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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Functionalized and Degradable Polyphthalaldehyde Derivatives
J. Patrick Lutz,† Oleg Davydovich,§ Matthew D. Hannigan,† Jeffrey S. Moore,§
Paul M. Zimmerman,† and Anne J. McNeil*,†
†Department of Chemistry and Macromolecular Science and Engineering Program, University of Michigan, Ann Arbor, Michigan
48109-1055, United States
§Department of Chemistry and Beckman Institute for Advanced Science and Technology, University of Illinois at
Urbana−Champaign, Urbana, Illinois 61801, United States
S
* Supporting Information
Polyphthalaldehyde (PPA) is among the most thoroughly
studied depolymerizable polymers.8,9 Linear or cyclic PPA
ABSTRACT: Polymers that depolymerize back to
monomers can be repeatedly chemically recycled, thereby
reducing their environmental impact. Polyphthalaldehyde
is a metastable polymer that can rapidly and quantitatively
depolymerize due to its low ceiling temperature. However,
the effect of substitution on the physical and chemical
properties of polyphthalaldehyde derivatives has not been
systematically studied. Herein, we investigate the cationic
polymerization of seven o-phthalaldehyde derivatives and
demonstrate that judicious choice of substituent results in
materials with a wide range of ceiling temperatures (<−60
to 106 °C) and decomposition temperatures (109−196
°C). We anticipate that these new polymers and their
derivatives will enable researchers to access degradable
materials with tunable thermal, physical, and chemical
properties.
(cPPA) can be obtained via anionic or cationic polymerization
(respectively) of o-phthalaldehyde (oPA) below its Tc of
−36 °C.10,11 While end-capped linear PPA and cPPA are
kinetically stable at room temperature, solution-phase exposure
to acid results in complete depolymerization at rates too rapid
to measure using standard analytical techniques (<1 min;
Scheme 1).1 The thermodynamic instability of PPA is key to
its depolymerization, but this same property has led to
challenges in polymer processing.12 For example, PPA requires
plasticizers to improve its processability because its glass
transition temperature (Tg) is above its thermal degradation
temperature.13
A small number of substituted polyphthalaldehydes have
been reported, primarily for the purpose of increasing thermal
stabilities. For example, researchers at IBM found that
polyphthalaldehydes bearing 4-bromo, 4-chloro, and 4-
trimethylsilyl substitution exhibited higher thermal degradation
temperatures than unsubstituted PPA, making them more
suitable for photolithographic applications.14 In related work,
Phillips and co-workers showed that end-capped linear
poly(4,5-dichlorophthalaldehyde) displayed a similar effect.15
Poly(4-methylphthalaldehyde) has also been synthesized as a
mechanistic probe,16 but no other PPA derivatives have been
reported.
This relative lack of PPA derivatives results in part from the
perception that synthesizing substituted o-phthalaldehydes is
prohibitively challenging, relying on a few established, but
often low-yielding, synthetic routes.8 Furthermore, it is still
unclear a priori whether a specific phthalaldehyde is polymer-
izable under experimentally accessible conditions due to a lack
of quantitative data on the ceiling temperatures of known PPA
derivatives. To address these deficiencies, we set out to
synthesize a range of substituted oPA derivatives and evaluate
their Lewis acid-catalyzed polymerizations. As a result of these
studies, we identified four new cPPA derivatives with ceiling
temperatures ranging from −23 to +106 °C and demonstrated
that their thermal degradation temperatures positively correlate
with their Tc. We anticipate that these new polymers will
significantly expand the versatility of the PPA scaffold and that
the structure−property relationships will serve as a roadmap
hile much progress has been made in synthesizing
Wpolymers with diverse structures, considerably less
attention has been paid to their fates after use. Low-ceiling
temperature (Tc) polymers are a class of metastable materials
that are readily triggered to depolymerize back to monomers at
temperatures above their Tc (Scheme 1).1,2 Such materials
have the potential to address a grand challenge in sustainability
by facilitating recycling through repeated depolymerization/
repolymerization cycles, extending their useful lifetimes.3,4
Depolymerizable polymers also have important applications in
areas such as lithography,5 triggered release,6 and transient
electronics.7
Scheme 1. Low-Ceiling Temperature (Tc) Cyclic Polymers
Received: July 15, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.9b07508
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
for researchers to develop other PPA-based materials with
varied physical and chemical properties.
Computational Tc Estimation. To estimate the ceiling
temperatures for substituted phthalaldehyde derivatives, a
literature method17 was adapted. Specifically, the acetalization
of oPA with MeOH was used as the model reaction (Scheme 2,
higher Tc values (M6−M8), in line with these substituents’
relative abilities to impact aldehyde electrophilicity.15,17
Monomer Synthesis and Homopolymerization. We
devised and executed 2−5 step synthetic routes to substituted
oPAs M1−M4 and M6−M8 (SI pp S4−S24). Notably, during
purification, monomers M6−M8 underwent carbonyl hydrate
formation and oligomerization to varying extents on silica,
suggesting that their ceiling temperatures were near or above
room temperature. In each of these cases, the dialdehyde was
obtained via vacuum sublimation.19
Scheme 2. Model Reaction for Phthalaldehyde
Polymerization
We next subjected each monomer to SnCl4-catalyzed
cationic polymerization conditions at −78 °C in DCM. The
reactions were quenched by adding pyridine to sequester the
Lewis acid, and then the polymers were precipitated into
MeOH and isolated by vacuum filtration.10 As a baseline, the
reaction of unsubstituted oPA (M5) produced cPPA (P5) in
35% isolated yield (Table 1, entry 5). Propoxy-substituted M1,
butylthio-substituted M2, and hexyl-substituted M3 all possess
estimated ceiling temperatures <−78 °C and were therefore
not expected to generate polymer under these conditions.
Consistent with the predictions, these monomers failed to
R = H). Density functional theory (B3LYP/6-31++G**)18 was
used to calculate the heat of formation for each reaction
component, enabling us to determine the overall enthalpy
(ΔH) for the equilibrium (SI pp S100−S122). Using our
measured Tc for oPA of −36 °C (vida infra), we computed the
entropy (ΔS) from eq 1:
1
ΔH
ΔS
generate isolable polymers; H NMR spectroscopic analysis
T =
c
(1)
following attempted precipitation from MeOH and solvent
removal revealed mixtures of monomer and the corresponding
dimethyl acetals (entries 1−3; see also Figure S25). In contrast
to M1−M3, the estimated Tc of hexynyl-substituted M4 (−60
°C) suggested that its polymerization was feasible. Indeed, P4
was isolated in 64% yield (entry 4). Methyl ester-substituted
M6, phthalimide derivative M7, and tetrafluorophthalaldehyde
M8 had predicted ceiling temperatures substantially higher
than that of oPA, and all three were effectively polymerized at
−78 °C (entries 6−8).
We next assumed that the ΔS determined for oPA would be
similar for the substituted derivatives, given that the changes in
bonding are similar for each reaction. With this assumption,
the ceiling temperature was estimated by calculating the ΔH
for each oPA derivative and solving eq 1 for Tc. We chose to
focus exclusively on symmetrically substituted derivatives
because nonsymmetrically substituted oPAs would have
electronically distinct acetal linkages, making it difficult to
disentangle the effects of different substituents.
Ceiling Temperature Measurement and Thermal
Analysis. While the results of the polymerizations were
qualitatively consistent with our Tc estimations, we sought to
evaluate these predictions quantitatively. To do so, a variable-
temperature 1H NMR spectroscopic method was adapted from
a protocol by Kohl and co-workers (SI pp 72−97).20 Solutions
with known initial concentrations of monomer ([M]0) and
catalyst were prepared. Then, the monomer concentration
([M]) was measured at different temperatures by integrating
the aldehyde C−H resonance versus an internal standard.
Consistent with the predicted ceiling temperatures, we
observed dramatic differences in monomer conversion based
on the phthalaldehyde substitution: hexyl-substituted M3
showed <5% conversion at −60 °C, whereas tetrafluoro-
phthalaldehyde M8 reached >95% conversion at rt (Figure 1).
In the absence of a Lewis basic quenching agent, the
polymerizations were reversible; warming the reactions to
the starting temperatures regenerated the dialdehyde mono-
mers.
To quantitatively determine the ceiling temperature, the
data from each reaction were plotted as R·ln[M] versus 1/T,
where R is the universal gas constant. The slope and intercept
of the resulting line correspond respectively to the ΔH and ΔS
for the polymerization. From these two values, eq 2 was used
to calculate the experimental Tc.21 Performing this analysis on
oPA provided a ceiling temperature of −36 °C (Table 2, entry
5), in close agreement with the results of Kohl and co-
workers.20 We could not measure Tc values for M1−M3 due to
the limited temperature range of the NMR probe; however,
−60 °C provides an upper limit (entries 1−3).
Our computations suggested that simple changes to oPA
yield monomers with ceiling temperatures ranging from
−122 °C to +98 °C (Table 1). As expected, electron-donating
substituents resulted in a predicted Tc below that of oPA
(M1−M3), while electron-withdrawing substituents led to
a
Table 1. Cationic Polymerization of oPA Derivatives
b
c
monomer
R =
OPr
SBu
n-C6H13
CCBu
H
CO2Me
−
est. Tc (°C) Yield (%) Mn (kg/mol)
Đ
M1
M2
M3
M4
M5
M6
M7
M8
−122
−86
−80
−60
−36
−2
0
0
0
64
35
48
83
53
−
−
−
−
−
−
2.2
2.2
1.7
1.9
2.0
12.3
3.2
11.7
25.1
16.2
d
+43
+98
−
a
b
c
Entries 4−8 represent the average of two runs. At 1 M. Isolated
d
yield. Experimentally determined.
B
DOI: 10.1021/jacs.9b07508
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Figure 2. (A) Plot of decomposition temperature (via DSC) versus
the experimental ceiling temperature for P4−P8 (via 1H NMR
spectroscopy). (B) DSC thermogram for P8.
Following purification,12 the polymers’ thermal decomposi-
tion temperatures (Td) were measured by differential scanning
calorimetry (DSC; SI pp S49−S71).26 A positive relationship
between Tc and Td was apparent, with Td ranging from 109 °C
for M5 to 196 °C for M8 (Figure 2A). Interestingly, polymer
P8 was the only polymer to exhibit a glass transition below its
decomposition temperature (Figure 2B), suggesting that P8
could have processing advantages over unsubstituted PPA.
Cationic Copolymerization. To elucidate the impact of
copolymer composition on thermal stabilities, we copolymer-
ized M5 with M6 at feed ratios ranging from 0 to 100 mol %
M6 (SI pp S54−S62).27 Following purification, the cumulative
Figure 1. Plot of the normalized monomer concentrations versus
temperature in the presence of 10 mol % SnCl4 ([M6]0 = 0.35 M;
[M]0 = 0.70 M for all other monomers).
Table 2. Thermodynamic Parameters for Polymerizing
a
M1−M8
b
c
est. Tc
(°C)
ΔH
ΔS
expt. Tc
(°C)
1
copolymer composition was estimated via H NMR spectros-
monomer
(kcal/mol)
(cal/mol·K)
d
copy. A linear trend between copolymer composition and feed
ratio was evident (Figure 3A), suggesting either an alternating
or statistical sequence. Thermal analysis of the copolymers
revealed a near-linear relationship between copolymer
composition and Td (Figure 3B). The slight nonlinearity is
likely a result of differing levels of residual Lewis acid
remaining in the (co)polymers after purification.12,28 A similar
trend in decomposition temperature has been reported for
copolymers of oPA with ethyl glyoxylate.13 These results
demonstrate that PPA substitution patterns and incorporation
ratios can be rationally designed to obtain copolymers with
targeted thermal properties, though improvements to the
purification process will likely be required to ensure consistent
results.
M1
−122
−86
−80
−60
n/a
−2
+43
+98
−
−
−
−0.90
−3.0
−4.9
−7.0
−7.9
−
−
−
−2.9
−11.9
−14.9
−19.3
<−60
<−60
<−60
−23
−36
+13
d
M2
d
M3
d
M4
d
M5
e
M6
f
M7
+74
+106
f
M8
−20.1
a
b
c
Entries 4−8 represent the average of two runs. At 1 M. Corrected
to 1 M. CD2Cl2 solvent. CDCl3 solvent. 1,1,2,2-Tetrachloroethane-
d
e
f
d2 solvent.
ΔH
T =
c
ΔS + R·ln[M]0
(2)
Microcapsule Fabrication. In previous work, cPPA was
used to form triggerable core−shell microcapsules.29 The new
PPA derivatives described herein could generate capsules that
exhibit different surface functionalities, specific ion coactiva-
tors,29b and rates of payload release. For a preliminary study,
P6 was prepared on gram scale in 76% yield (SI p S71) and
subjected to the previously described microencapsulation
conditions optimized for unsubstituted cPPA (SI pp S98−
S99).29a Briefly, oil-in-water emulsions comprised of P6/jojoba
oil/DCM in aqueous poly(vinyl alcohol) were generated via a
microfluidic flow focusing device. Rapid evaporation of the
DCM followed by filtering and washing furnished micro-
capsules with an average diameter of 228 5 μm. Scanning
electron microscopy (SEM) confirmed these capsules ex-
hibited a core−shell architecture, with an estimated shell-wall
thickness of 11 μm (Figure 4). These P6 microcapsules
represent a potential starting point to generate anionic,
cationic, and labeled depolymerizable capsules via postencap-
sulation functionalization reactions.
Hexynyl-substituted M4 displayed a Tc of −23 °C, slightly
above that of oPA. This result is consistent with the weak
electron-withdrawing nature of alkynyl groups22 (σm = 0.21, σp
= 0.03 for −CCMe).23 Ester-substituted M6 exhibited a Tc
of 13 °C, in line with the stronger electron-withdrawing
abilities of esters (σm = 0.36, σp = 0.45 for −CO2Me).24 Both
phthalimide M7 and tetrafluorophthalaldehyde M8 had ceiling
temperatures significantly higher than room temperature (74
and 106 °C, respectively) due to the even stronger resonance-
and inductive-withdrawing effects of these substituents.
While our estimations of ceiling temperatures were
qualitatively accurate and useful in identifying a range of
substrates to examine, there are substantial quantitative
differences between computed and experimental values.
Examining the thermodynamic parameters in Table 2 reveals
that a contributing factor in this discrepancy was our flawed
assumption that the ΔS would remain approximately constant
for the different monomers. In fact, a ∼7-fold difference in ΔS
was observed for M4 versus M8.25
To summarize, seven substituted o-phthalaldehyde deriva-
tives were synthesized and subjected to cationic polymerization
C
DOI: 10.1021/jacs.9b07508
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Paul M. Zimmerman: 0000-0002-7444-1314
Anne J. McNeil: 0000-0003-4591-3308
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We (A.J.M. and J.S.M.) gratefully acknowledge the National
Science Foundation for supporting this work through a Phase I
Center for Chemical Innovation Grant (CHE-1740597) as
well as the Department of Defense for a National Defense
Science and Engineering Graduate Fellowship for M.D.H.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.9b07508.
■
S
Experimental details, characterization data, synthetic
procedures (PDF)
AUTHOR INFORMATION
Corresponding Author
*ajmcneil@umich.edu
ORCID
J. Patrick Lutz: 0000-0002-5795-5049
Matthew D. Hannigan: 0000-0002-2267-1388
■
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DOI: 10.1021/jacs.9b07508
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
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