Journal of the American Chemical Society
Article
a
Table 2. Photocatalytic Depolymerization Reactions of PMP-x and OTBS-x Copolymers
Copolymer
1 mol %
5 mol %
10 mol %
20 mol %
33 mol %
50 mol %
PMP-x
43%
78%
63%
17%
79%
34%
>99%
29%
>99%
73%
88%
77%
a
See SI for more details on reaction scale and optimal conditions for each copolymer. Mol% were for the loading of 1a/1b in the copolymers with
1
respect to cyclooctene. Reported yields were for monomeric, oligomeric and polymeric aldehyde products, quantified via H NMR analysis of the
crude reaction mixtures at high temperature (100 °C) relative to an internal standard, and the average of two experiments.
and 1,8-octanediol were obtained in overall yields of 34% and
2% from M and M through similar reaction sequences,
Baeyer−Villiger oxidation (see SI for details). In addition, 1,8-
octanedinitrilea precursor to 1,8-octanediamineand 1,8-
octanedioic acid (i.e., suberic acid) were also produced from
tion offers a straightforward strategy to control and tune the
thermomechanical properties of the recycled condensation
polymers.
2
OH‑3
4
In addition to influencing the chain length of the
degradation products, the loading of functionalized cyclo-
octenol monomers also modulates the physical properties of
these materials. Specifically, the quantity and identity of these
cyclooctenols (i.e., 1a or 1b) have a significant impact on the
materials’ thermomechanical properties. We first studied the
thermal properties of these materials through differential
scanning calorimetry (DSC) and thermogravimetric analysis
(TGA). Both PMP and OTBS homopolymers (Figure 5A,B,
respectively) are amorphous materials with glass transition
M
in excellent yields (62% and 94%, respectively). Upon
OH‑2
repolymerization, these α,ω-difunctional compounds would
provide access to a wide range of condensation polymers (e.g.,
polyester, polyamide, and polycarbonate).
Having demonstrated the degradability of PMP and OTBS
homopolymers, we sought to investigate how the concen-
tration of hydroxyl groups within the polymer backbone
influences the material’s degradability. We thus prepared a
variety of copolymers (targeted total degree of polymerization
temperatures slightly above room temperature (T = 43 and 31
g
°C, respectively). In contrast, all PMP-x and OTBS-x
copolymers exhibited a melt transition temperature (Tm),
suggesting that they are semicrystalline materials akin to
unfunctionalized polyethylene (i.e., 0 mol % cyclooctenol). We
also observed that for both copolymer series, Tm decreases
linearly and crystallinity (χ) decreases exponentially with
increasing loading of 1a or 1b. These trends are not impacted
by the identity of the cyclooctenol monomer; both 1a and 1b
=
%
2000, referred to as PMP-x and OTBS-x, where x is the mol
of 1a or 1b, respectively) (Table S2) by polymerizing 1a or
1
b with cyclooctene under the same conditions used for the
1
synthesis of homopolymers. H NMR analysis of the
copolymerization reaction indicated the formation of statistical
copolymers, as cyclooctene and the functionalized cyclo-
polymerization (Figure S4). We then studied the degradation
of these copolymers under photocatalytic conditions and
lower T and χ to the same extent at similar incorporation
m
levels (mol %). This observation is in agreement with the Flory
exclusion model, which posits that the quantity rather than the
identity of substituents of sufficient size along polyethylene
1
assessed the reaction efficiency via H NMR quantification of
the long-chain aldehyde products at high temperature (100
3
5−39
°
C) due to their poor solubility at ambient temperature. We
found that PMP-x copolymers with at least 10 mol % of 1a
PMP-10, PMP-20, PMP-33, and PMP-50) underwent
backbone impacts the crystallinity of the material.
Notably, all the homopolymers and copolymers studied here
are thermally stable as assessed by TGA, with most polymer
samples exhibiting a decomposition temperature (Td,5%) above
The mechanical properties of these materials were also
investigated through uniaxial tensile elongation testing. The
PMP homopolymer (Figure 5C, purple trace) exhibited a
(
efficient degradation (i.e., 79−100%) (Table 2). Even
copolymer with as little 1 mol % 1a was also significantly
depolymerized (i.e., 43%). OTBS-x copolymers could also be
degraded. While OTBS-1 was cleaved in good yield of 78%,
depolymerization of OTBS-5 afforded the expected products
in only 17% yield. Nonetheless, copolymers with at least 10
mol % 1b all degraded appreciably (29−77% yield). In
principle, the long-chain aliphatic products of copolymer
degradation can be derivatized into a wide range of α,ω-
bifunctional compounds in a similar manner as described for
monomers M -M . These macromonomers can subsequently
distinct yield point (strain at yield (ε ) = 6.1 ± 0.1%, stress at
Y
yield (σ ) = 40 ± 3 MPa) that is accompanied by formation of
Y
a neck within the gauge length of the dog-bone specimen. A
plateau in stress was observed following yielding until
approximately 100% strain, during which the neck propagated
throughout the entire gauge length. Further elongation of the
specimen resulted in an increase in stress until break (strain at
break (ε ) = 170 ± 20%, stress at break (σ ) = 33 ± 2 MPa).
2
6
be used for condensation polymerization to produce
degradable materials that might exhibit similar thermomechan-
B
B
33,34
ical properties as polyethylene.
It is worth noting that the
In contrast, the OTBS homopolymer (Figure 5D, purple trace)
exhibited “rollover” yielding (ε = 8 ± 2%, σ = 1.3 ± 0.2
aliphatic chain length of these macromonomers is dependent
on the original ratio of monomers present in the copoly-
merization reaction; copolymers with lower cyclooctenol
loadings will produce longer-chain aliphatic products upon
depolymerization. The statistical nature of this copolymeriza-
Y
Y
MPa) that was not accompanied by any neck formation and
broke at a significantly higher strain and lower stress (ε = 580
B
± 20%, σ = 20 ± 3 MPa) than the PMP homopolymer.
B
Despite being elongated to over 500% strain, the OTBS
1
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J. Am. Chem. Soc. 2021, 143, 12268−12277