Size-Selective Catalytic Polymer Acylation with a Molecular Tetrahedron

Article abstract of DOI:10.1016/j.chempr.2020.05.011

Selective catalysis at the molecular level represents a cornerstone of chemical synthesis. However, it still remains an open question how to elevate tunable catalysis to larger length scales to functionalize whole polymer chains in a selective manner. We

Full text of DOI:10.1016/j.chempr.2020.05.011

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Size-Selective Catalytic Polymer Acylation with
a Molecular Tetrahedron
Mona Sharafi, Kyle T. McKay,
Monika Ivancic, ..., Appala Raju
Badireddy, Jianing Li, Severin T.
Schneebeli
jianing.li@uvm.edu (J.L.)
severin.schneebeli@uvm.edu (S.T.S.)
HIGHLIGHTS
We synthesized a catalytically
active, hydrazone-linked
molecular tetrahedron
The tetrahedron threads over
polymeric substrates to
disentangle them
Short polymers are functionalized
selectively with the catalytic
tetrahedron
Our results suggest untapped
potential for selective polymer
functionalization
We report an original catalytic molecular tetrahedron. By threading through the
cavity of the tetrahedron, polymeric substrates are unfolded or broken apart. Our
catalyst distinguishes between polymer chains of different lengths, functionalizing
the shorter polymers selectively over the longer ones—as a proof of concept for
selective catalysis to modify polymers. Our findings advance the fundamental
understanding of the thermodynamic and kinetic phenomena controlling the
interactions between molecular cages and synthetic polymers, offering valuable
ability to create complex materials in the future.
Sharafi et al., Chem 6, 1469–1494
June 11, 2020 ª 2020 Elsevier Inc.
https://doi.org/10.1016/j.chempr.2020.05.011

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Size-Selective Catalytic Polymer Acylation
with a Molecular Tetrahedron
Mona Sharafi,1 Kyle T. McKay,1 Monika Ivancic,1 Dillon R. McCarthy,1 Natavan Dudkina,1
Kyle E. Murphy,1 Sinu C. Rajappan,1 Joseph P. Campbell,1 Yuxiang Shen,2 Appala Raju Badireddy,2
1,3,
Jianing Li,1, and Severin T. Schneebeli
*
*
SUMMARY
The Bigger Picture
Nature employs sophisticated
catalysts to selectively
Selective catalysis at the molecular level represents a cornerstone of
chemical synthesis. However, it still remains an open question how
to elevate tunable catalysis to larger length scales to functionalize
whole polymer chains in a selective manner. We now report a hydra-
zone-linked tetrahedron with wide openings, which acts as a catalyst
to size-selectively functionalize polydisperse polymer mixtures. Our
experimental and computational evidence supports a dual role of
the hydrazone-linked tetrahedron. To accelerate functionalization
of the polymer substrates, the tetrahedron (1) unfolds the polymer
substrates and/or breaks the polymer aggregates and (2) enables
target sites (amino groups) on the polymers to coordinate with
catalytic units (triglyme) attached to the tetrahedron. With the tet-
rahedron as the catalyst, we find that the reactivity of the shorter
polymers increases selectively. Our findings enable the possibility
to engineer hydrolytically stable molecular polyhedra as organoca-
talysts for size-selective polymer modification.
functionalize biopolymers after
they have already been
synthesized. Yet, despite plenty of
examples in biology, efficient and
selective modification of man-
made polymers is still difficult.
Many functional polymers with
complex topologies cannot be
generated in high yields with
traditional methods. Thus,
catalysts able to derivatize man-
made polymers for desirable
polymeric size, sequence, and
folded topology are in critical
need for future advances in fields
such as medicine, electronics, and
renewable energy. This work now
provides one of the first examples
for selective catalysis with
INTRODUCTION
Selective catalytic chemical modification after synthesis represents1,2 an effective means
to diversify the structures and functions of small molecules and polymers. Yet, despite
various examples of protein posttranslational modifications (PTMs) catalyzed by en-
zymes, it remains a daunting task to create catalysts that can selectively recognize and
modify man-made polymers. Thus, it has been a major goal of synthetic chemists to
create supramolecular catalysts,3–7 which can operate selectively on man-made poly-
mers.8,9 These efforts have led to the successful application of selective supramolecular
catalysts for small-molecule transformations10–18 as well as for growth8,19,20 and
functionalization9,21 of linear polymers in a processive manner. However, selective
post-synthetic polymer modification (hereafter referred to as PSPM) by supramolecular
organocatalysts has, to the best of our knowledge, not yet been reported. We now show
that a hydrazone-linked tetrahedron with large openings can size-selectively functional-
ize (Figure 1B) complex polymeric mixtures. Analogous to previously reported catalytic
transport systems operating with polymeric tracks,21,22 our catalyst is designed to slide
along the polymeric substrates during the reaction, which leads to efficient (Figure 6)
catalytic functionalization of side-chain functional groups.
polymeric substrates. We show
that stable synthetic catalysts with
large openings can distinguish
between polymer chains of
various lengths in a complex
mixture, providing proof of
principle for selective catalytic
polymer functionalization. This
concept will be applied to prepare
otherwise inaccessible polymers
in the future.
This process is enabled—similar to the seminal system of Nolte and co-workers19–21
—
by threading of the polymer chains through the cavity of the catalyst. Yet, in
contrast to Nolte’s catalyst, which was optimized for linear polymers, threading,
and size-selective functionalization of side-chain polymers also become possible
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Figure 1. Substrate-Selective Catalysis: From Small Molecules to Polymeric Substrates
(A) Examples of prior work in the size-selective catalysis arena with small-molecule substrates.
(B) We extend the concept of size-selective catalysis to larger substrates with a hydrazone-linked
molecular tetrahedron as the organocatalyst, which can size-selectively modify a polydisperse
mixture of polymer chains. Catalysts are shown in green, reactive substrates and products in
orange, unreactive substrates in yellow-orange, and reagents in blue/black. All structures are
shown approximately on the same scale and the polymer chains are illustrated schematically in their
fully extended conformations. Please note that—as detailed in the Results and Discussion—the
longer polymer chains have a tendency to not only fold but also aggregate more than the shorter
ones, which can explain the catalyst’s selectivity toward the shorter polymer chains.
with our organocatalytic tetrahedron. With regard to prior work in the field of size-se-
lective catalysis (which has recently been reviewed by Otte17), our system is unique in
that it acts on a polydisperse mixture of polymer chains in a size-selective manner,
whereas mostly pairs of small molecules were used as the substrates for size-selective
catalysis in prior work (see the examples shown in Figure 1A).
1Department of Chemistry, University of Vermont,
Burlington, VT 05405, USA
2Department of Civil and Environmental
Engineering, University of Vermont, Burlington,
VT 05405, USA
3Lead Contact
To achieve activity and selectivity, a PSPM catalyst has to recognize the polymeric
substrates to initiate catalysis. Molecular polyhedra with large openings are well
suited to meet this challenge, with tetrahedral structures providing the largest
*Correspondence: jianing.li@uvm.edu (J.L.),
severin.schneebeli@uvm.edu (S.T.S.)
https://doi.org/10.1016/j.chempr.2020.05.011
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A
B
R
R
R
Shape-directing
hydrogen bonds
R
18 Å
13 Å
R
R
R
R
22 Å
9 Å
R
R
R
Cage-1
R
Tet-1 (this work)
Hydrazone cage by Warmuth and
Hydrazone-linked tetrahedron with
large openings — Td symmetrical
coworkers47 — C2d symmetrical
R
O
O
C3H6-Triglyme
=
=
O
O
Figure 2. Comparison of Tetrameric Hydrazone-Linked Molecular Cages
(A) A C2d-symmetric hydrazone cage reported by Warmuth and co-workers.47
(B) The Td-symmetric hydrazone tetrahedron reported in this work. Its wide openings allow it to act
as a size-selective catalyst for PSPM. Molecular models of both cages were minimized with the
OPLS348 force field.
Color scheme: C, green; polar H, white; N, blue; O, red.
openings of all possible regular polyhedra.4 For instance, the surface area per face of
a regular tetrahedron is 1.8 times larger than the corresponding surface area per face
for a cube with an identical volume. Here, we now show that—due to their wide
openings—tetrahedral cages allow polymers with bulky side chains to thread
through, preparing for further catalytic functionalization. Although molecular poly-
hedra with Td symmetry can be synthesized23–28 with dynamic covalent chemistry
(e.g., with imine linkages),23,29–38 the currently reported systems likely lack sufficient
stability39,40 for organocatalytic PSPM processes.41 While hydrolytic stability can be
imparted onto imine-linked cages by reduction of the imines to amines,37,38,42 the
secondary amines formed upon reduction are good nucleophiles, which might
engage in side reactions under the acylation conditions employed in this work.
Thus, rather than using cages with imine/amine linkages, we first invented a method
to create a porous tetrahedron (Figure 2) with acyl-hydrazone linkages and periph-
eral glycol chains required for the PSPM catalysis. The primary reasons for utilizing
a hydrazone-linked cage43–46 as our organocatalyst were the following: (1) hydra-
zone linkages generally display enhanced hydrolytic stability, which renders them
excellent candidates for catalysis, (2) the NH group available in acyl-hydrazones rep-
resents a suitable hydrogen bond donor, which can direct the assembly of hydra-
zone-linked polyhedra, and (3) the hydrogen-bonding capabilities of hydrazones
can assist PSPM catalysis by binding and unfolding polymeric substrates.
However, due to the hydrogen-bonding capabilities of hydrazones, hydrazone-
linked cages with large openings47 (see Figure 2 for models minimized with the
OPLS348 force field) have a high propensity to form interlocked structures.49,50
This feature renders it difficult to synthesize hydrazone-linked cages without interca-
tenation, which is one reason for why hydrazone-linked polyhedra with large open-
ings and Td symmetry are challenging to synthesize. The closest design to a
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OMe
XO
OMe
MeO
MeO
H
O
MeO
DMF, POCl3
MeO
MeO
98%
OX
OMe
XO
H
MeO
syn-1
H
O
O
syn-2, X = –Me
syn-3, X = –H
(1) 89%
(2) 85%
O
O
O
syn-4, X = –C4H8-Triglyme
=
C3H6-Triglyme
NaClO2
NaH2PO4
DMSO
CH3CN
Triglyme-C4H8O
Y
OMe
79%
O
O
MeO
MeO
R
R
R
OC4H8-
Triglyme
Triglyme-C4H8O
Y
Y
O
O
R
R
syn-5, Y = –OH
R
(3) 70%
R
R
syn-6, Y = –OMe
syn-7, Y = –NHNH2
(4) 81%
O
R
O
TFA, CH2Cl2
R
Tet-1
R = –C3H6-Triglyme
R
R
80%
Scheme 1. Synthesis of the Hydrazone-Linked Molecular Tetrahedron Tet-1
Reagents and conditions: (1) BCl3, CH2Cl2; (2) Br-C4H8-triglyme, K2CO3, DMF, D; (3) MeOH, H2SO4,
D; (4) H2NNH2, MeOH, H2O, D.
tetrahedral hydrazone-linked cage is (see Figure 2A) the C2d symmetric hydrazone
cavitand Cage-1, invented by Warmuth and co-workers.47 Yet, with ca. 9–13 A (Fig-
˚
ure 2A), the openings of Cage-1 are relatively narrow, which prevents intercatena-
tion but would likely also prevent side-chain polymers from threading through as
required for PSPM catalysis.
For an efficient synthesis of hydrazone-tetrahedra (Figure 2B) with large openings,
we designed a special vertex (syn-7) to encode tetrahedron formation (Scheme 1).
Syn-7 combines hindered rotation51 around three Ph-Ph s-bonds with three intra-
molecular [NH.OR]-hydrogen bonds, which direct44 the growth of the tetrahedron
upon hydrazone formation. This strategy allowed us to create a hydrazone-linked
Td-symmetric molecular tetrahedron. The tetrahedron not only binds to side-chain
polymers, but it also catalyzes the PSPM of amine-functionalized polymers with rela-
tively long side chains in a size-selective manner.
RESULTS
Synthesis of a Hydrazone-Linked Molecular Tetrahedron
The synthesis (Scheme 1; see Figures S30–S61 for spectroscopic characterization data)
of syn-7 started with a trifold formylation of syn-1, which is readily created on a multigram
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O
O
O
NH2
OH
O
n
OH
O
(1)
(2)
Simplified Representation of NH2-POA
73%
O
O
O
O
O
CO2H
NH
HN
HN
NH
HN
r
r
b
a
c
NH2-POA
Scheme 2. Synthesis of an Amine-Functionalized Polymeric Substrate for Size-Selective Catalysis
Reagents and conditions: (1) H2N-n-C8H17, DMF, D; (2) H2N-n-C6H12-NH2, DMF, D. Based on
elemental analysis and the Mw (6.3 kDa), the average number of repeating units a, b, and c were
a = 12.1, b = 3.7, and c = 4.7.
scale with a solid-state-driven amplification of the syn-atropoisomer of 1 as we reported
previously.51 The methoxyl groups ortho to the aldehydes of the resulting syn-2 were
then removed selectively with BCl3 to afford the tris-phenol syn-3. Finally, syn-3 was con-
verted into the vertex syn-7 through (1) alkylation of the phenolic ꢀOH groups, (2) Pinnick
oxidation52 ofthealdehydestocarboxylicacids,(3)esterification, and(4)hydrazinolysisof
the resulting methyl esters. To form the tetrahedron Tet-1, we mixed two equivalents of
the vertex syn-7 with three equivalentsof terephtalaldehyde aswell astrifluoroacetic acid
(TFA) as the catalyst in CH2Cl2. After 48 h of stirring at room temperature, Tet-1 formed in
80% yield, as confirmed (see Supplemental Experimental Procedures; Figures S2, S44,
and S45) by high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) as
well as 1H, 13C, and diffusion-ordered nuclear magnetic resonance spectroscopy (1H
DOSY NMR). The 1H NMR spectrum of Tet-1 (Figure S44) illustrates its Td symmetry
with only five resonances (all singlets) appearing in the region between 6 and 12 ppm.
˚
Based on a minimized molecular model (Figure 2B), the height of Tet-1 is 22 A, which
1
˚
agrees well with the solvodynamic diameter (21 A) measured by H DOSY NMR spectros-
copy (Figure S2). The tetrahedron is not only soluble in organic solvents but also in
aqueous mixtures with polar organic solvents. To test its hydrolytic stability, we therefore
dissolved Tet-1 in a 90:10 vol % mixture of DMSO-d6 and D2O and heated the resulting
clear solution to 80ꢁC for 5 h. No degradation was observed (Figure S1) under these con-
ditions by 1H NMR spectroscopy, which illustrates the excellent hydrolytic stability of Tet-
1, even at elevated temperatures.
Polymer Recognition with Tet-1
With Tet-1 isolated, we embarked on investigating the tetrahedron’s ability to
recognize amine-functionalized polymers with side chains, since binding to such
macromolecules is a prerequisite for the catalytic polymer functionalization
described below. To create amine-functionalized polymers, we first condensed
1-aminooctane (90 mol %) with commercial poly(isobutylene-alt-maleic anhydride)
and then reacted (Scheme 2) the remaining anhydride units with 1,6-diaminohexane.
As shown by (1) elemental analysis, (2) by Fourier transform infrared (FTIR)
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Figure 3. 1H DOSY and 1H-1H NOESY NMR Spectra Demonstrate Complex Formation and
Threading of the Polymers through the Cavity of the Tet-1 Catalyst
(A–C) Partial 1H DOSY NMR spectra (500 MHz, CD2Cl2, 298 K) of (A) Tet-1 (0.28 mM), (B) a mixture of
Tet-1 (0.28 mM) and NH2-POA (3.0 mg mL–1; ꢂ0.45 mM), and (C) pure NH2-POA (3.0 mg mL–1). The
spectra show a decrease of the diffusion constants for both NH2-POA and Tet-1 upon mixing. This
finding establishes complex formation between the two compounds. The diffusion constant of the
residual CHDCl2 signal at 5.32 ppm of the CD2Cl2 solvent remained constant (32.5 3 10ꢀ10 m2 s–1 in
all cases; see Figure S3), indicating that the viscosities of the NMR solutions were not affected
significantly by the presence of the polymers at the low concentrations employed.
(D) Partial 1H-1H NOESY NMR spectrum (500 MHz, CD2Cl2, 298 K) of Short-NH2-POA (3.0 mg mL–1
)
in complex with Tet-1 (0.28 mM). Short-NH2-POA represents the shorter chains (Mw = 2.6 kDa) of
the NH2-POA polymer sample, which were isolated (see the Experimental Procedures) by running
an additional size-exclusion column on parts of the original NH2-POA sample. NOE cross peaks
between the aliphatic polymer resonances Poly-aliphatic and signals corresponding to Tet-1 are
circled. Notably, a NOE cross peak between Poly-aliphatic and the c protons on Tet-1 (pointing
inward) is observed, which is consistent with the NH2-POA polymers threading through the cavity of
Tet-1. An analogous NOESY spectrum has been obtained (see Figure S7) for the longer chains in
NH2-POA, i.e., Long-NH2-POA (Mw = 9.9 kDa), which shows similar (although slightly weaker) NOE
cross peaks.
spectroscopy, and (3) by measuring the percentage of free amino groups through
complete acylation at elevated temperature, this procedure (detailed in the Exper-
imental Procedures) leads to full and partial amidation of the maleic anhydride units
to afford a polydisperse mixture of amine-functionalized poly(isobutylene-alt-n-octyl
maleamide) (NH2-POA) polymers with an Mw (1H DOSY NMR)53 of 6.3 kDa. The poly-
meric mixture is soluble in a variety of organic solvents and contains 3.7 repeat units
(out of 20.5) with free NH2-containing sidechains on average.
Next, binding between Tet-1 and the polymeric NH2-POA substrate was confirmed by
1
1
(1) H DOSY NMR spectroscopy (Figures 3A–3C), (2) H-1H nuclear Overhauser effect
spectroscopy (NOESY) NMR (Figure 3D), (3) all-atom molecular dynamics (MD) simula-
1
tions (Figure 4), and (4) 1H NMR-based host-guest titrations (Figure 5). The H DOSY
NMR spectrum of the NH2-POA/Tet-1 mixture shows two diffusion bands for the
[NH2-POA@Tet-1] complex at different diffusion coefficient (D) values (1.7 3 10–10 and
s
2.5 3 10ꢀ10 m2 ꢀ1). These two diffusion bands of the complex likely arise from two
different complexation geometries, which interconvert slowly on the NMR timescale.
Based on a direct comparison with MD simulations (vide infra), the larger complex,
with an average D value of 1.7 3 10–10 m2 s–1, is the one where NH2-POA is threaded
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Figure 4. MD Simulations Showing an NH2-POA Polymer Chain Unfolding When Binding to Tet-1
The MD simulations help explain the increase of the polymer’s solvodynamic diameter, which is
observed (Figure 3) upon complexation with Tet-1 by 1H DOSY NMR spectroscopy.
(A) Time average bar charts showing the radius of gyration (rgyr) of the NH2-POA polymer chain by
itself (pattern-free bar) and in complex (patterned bar) with Tet-1. Error bars represent standard
deviations. The time averages and standard deviations were calculated from the last 400 of 800-ns
MD simulations in explicit solvent.
(B and C) Representative snapshots of 800-ns MD simulations of NH2-POA by itself (B) and NH2-
POA threaded through the cavity of Tet-1 (C). The solvent (CH2Cl2) is hidden for clarity.
through the cavity of Tet-1. For this binding geometry, we clearly observed (see Figures
3D and S7) nuclear Overhauser effect (NOE) cross peaks between the c proton reso-
nances of Tet-1 (which point straight into the cavity of the molecular tetrahedron) and
the aliphatic proton resonances of the polymer chains (Poly-aliphatic). This finding sup-
ports threading of the polymer chains through the cavity of Tet-1. In general, the NOE
cross peaks between the proton resonances of the polymers and Tet-1 (especially with
H-d) are stronger (cf. Figures 3D and S7) with the shorter polymers (Short-NH2-POA).
Thus, since the longer polymers bind the strongest (Figure 5B) to Tet-1, they very likely
interact(seeFigure4C for asimulated structure ofthe complex witha longpolymer chain)
not onlywith the inside but also withthe outside of Tet-1. Inturn, the shorter polymersare
not long enough to effectively wrap around the tetrahedron, while at the same time
threading through Tet-1’s cavity, which leads (vide infra) to a more active catalytic confor-
mation with the shorter polymer chains.
In the threaded binding geometry, the solvodynamic diameter of NH2-POA in-
creases by ꢂ50%, compared with the solvodynamic diameter of the NH2-POA poly-
mer on its own (Figure 3). To confirm this finding, we ran MD simulations (Figure 4) of
the polymer with and without Tet-1 for 800 ns. These simulations demonstrated that
the observed increase in polymer size (ꢂ20% size increase is predicted by the MD
simulations) is indeed caused by unfolding of the polymer, when it threads through
Tet-1 (see Video S1 for the trajectories). As also indicated by the MD simulations, the
polymer unfolding is driven (Figure S29) by the cage breaking up the intramolecular
hydrogen bonds in the polymer and replacing them with polymer-to-cage hydrogen
bonds. Further support for this finding is provided by (1) Figure S10, which shows
that when DMSO-d6 (a well-known hydrogen bond disruptor) is added to a solution
of [NH2-POA@Tet-1] complex in CD2Cl2, rapid decomplexation occurs as well as (2)
by Figure S28, which demonstrates that Tet-1 no longer serves as an effective cata-
lyst in a polar solvent like DMSO-d6. In the second diffusion band, the diffusion
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Figure 5. Long-NH2-POA Binds Stronger to Tet-1 Than Short-NH2-POA, but the Short Polymers
Decomplex Faster
(A) Stacked partial 1H NMR spectra (500 MHz, CD2Cl2, 298 K) demonstrating how key Tet-1 and
polymer resonances change upon titration of Short-NH2-POA into a 0.28 mM solution of Tet-1.
See Figure S11 for the titration of Tet-1 with itself and Figure S12 for the titration of NH2-POA
with itself.
(B) Titration curves fitted to a 1:1 binding model in the intermediate-slow NMR exchange regime for
threading of both Short- and Long-NH2-POA into the cavity of Tet-1. Although the longer polymer
chains bind stronger to Tet-1 (which leads to weaker catalysis with the longer chains, as illustrated
in Figure 9C), the shorter polymer chains dethread faster (with an off rate, koff, nearly three times as
fast as the longer polymers). Ka = Complex association constants. The titration curves were fit with a
custom python script, making use of the LineshapeKin 4.0 NMR simulation software54 in
combination with the curve-fit algorithm implemented in the SciPy 0.18 package55 with the trust
region reflective (TRF) algorithm. Error bars represent standard deviations calculated from the
covariance matrix of the best fit obtained with the SciPy curve-fit algorithm. See Figures S8 and S9
for a full comparison of the experimental and predicted spectra and the Supplemental
Experimental Procedures for the custom python code used to implement the parameter fitting.
(C) Zoomed-in, representative snapshot from the 800-ns MD simulation of the [NH2-POA@Tet-1]
complex (see Figure 4C for a zoomed-out view). The snapshot shows that the threaded polymer
chains preferentially pass by the side of the aromatic linkers in Tet-1. We hypothesize that
classical56 aromatic ring-current effects operating in this binding geometry are primarily
responsible for the observed downfield57 shifts, which occur (Figure 5A) for the polymer resonances
in the range of 3.05–2.55 ppm upon complexation with Tet-1.
coefficient (D) of the polymer decreases only slightly, compared with the D of the
polymer by itself, from 2.8 3 10–10 m2 s–1 (unbound polymer, Figure 3C) to 2.5 3
10–10 m2 s–1 (bound polymer, Figure 3B). Thus, we conclude that in the second bind-
ing conformation, the polymer gets unfolded to a lesser extent than when it is
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threaded through the cage, likely because the polymer and Tet-1 interact with side-
on coordination modes in the second binding conformation.
To gain further insight into the nature of the polymer recognition abilities of Tet-1, we
separated and isolated (as detailed in the Experimental Procedures) the shortest
(Short-NH2-POA, Mw = 2.6 kDa) and longest (Long-NH2-POA, Mw = 9.9 kDa) fractions
contained in the original NH2-POA sample with an additional size-exclusion chromatog-
raphy run. We then titrated both the short and long polymer samples into 0.28 mM so-
lutions of Tet-1 in CD2Cl2 while recording the 1H NMR spectra of the mixtures at each
titration point. The titration data obtained (Figure 5A) show that the polymer resonances
shift downfield57 when the polymer threads through the cavity of Tet-1. This finding is
explained (Figure 5C) by the detailed structure and associated aromatic ring-current ef-
fects56 of the threaded binding geometry, wherein the polymer chain passes by the side
(and not the face as in most small-molecule pseudo-rotaxanes/rotaxanes58,59) of the
aromatic linkers in Tet-1.
AnalysisoftheNMR titration curves(Figure5B)withtheLineshapeKin4.054 software(fitto
a simple 1:1 binding model) in combination with the curve-fit algorithm implemented in
the SciPy 0.18 package55 provided further information regarding the complexation ther-
modynamics and kinetics between the NH2-POA polymer substrates of different lengths
and the Tet-1 catalyst. First, we discovered thatthe shorter polymer chains (Mw = 2.6 kDa)
decomplex nearly three times as fast as the longer (Mw = 9.9 kDa) ones (see Figure 5B for
the decomplexation rate constants, koff, measured for Short- and Long-NH2-POA poly-
mers). This result represents further evidence for pseudorotaxane formation between
Tet-1 and the NH2-POA polymers, as dethreading from a pseudorotaxane-like confor-
mation is expected60 to slow down with increasing polymer length, since it takes longer
on average for Tet-1 to slide to the end of a longer polymer chain. Second, the complex
1
association constants (Ka) determined (Figure 5) by the H NMR titration experiments
demonstrated that the longer NH2-POA polymer chains bind stronger to Tet-1 than
theshorteronesbind. Interestingly,forenzymes,itiswell-known61 thatifanenzymebinds
too strongly (i.e., too ‘‘tight’’) to its substrate (but not as strongly to the transition state),
the enzymatic reaction will slow down with increasing substrate-binding strength.
Here, we now observe a similar phenomenon (Figure 9C) for a fully synthetic system. Spe-
cifically, we discovered (vide infra) that the shorter NH2-POA polymers (which bind the
weakest to Tet-1 catalyst) nevertheless react the fastest with Tet-1. Overall, this section
provided evidence that Tet-1 is able to thread-over and partially unfold/stretch out the
NH2-POA polymers in CD2Cl2 (which is an ideal solvent for glyme-catalyzed aminolysis),
laying the foundation to use Tet-1 as a size-selective organocatalyst as described below.
Organocatalytic Polymer Functionalization with Tet-1
Triglyme functional groups act62 as organocatalysts to accelerate aminolysis reac-
tions. Since Tet-1 possesses 12 peripheral triglyme chains and binds to amine-func-
tionalized NH2-POA polymers, Tet-1 can engage in size-selective catalytic (Figure 6)
functionalization of these polymeric substrates. We evaluated the catalytic perfor-
mance of Tet-1 by monitoring the kinetics of the aminolysis reaction with 1H NMR
in the presence of an internal standard (1,2,4,5-tetrabromobenzene [TBB]). Five re-
action mixtures were prepared in CD2Cl2 as the solvent. Each of the reaction mix-
tures contained (1) an excess of 4-nitrophenyl-3,5-dinitrobenzoate (NDB) active
ester, (2) NH2-POA polymer as the substrate, and (3) one of the following cata-
lysts/controls: (a) Tet-1, (b) Edge-model (a model for an edge of Tet-1), (c) triglyme
(Triglyme) + the simple control hydrazone N0-benzylidene-2,4-bis(hexyloxy)benzo-
hydrazide (Control) in equimolar amounts, (d) triglyme by itself, and (e) no additives.
We selected NDB as the active ester, since it attaches dinitrobenzoate groups onto
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Figure 6. Evidence of Tet-1’s Catalytic Activity for Functionalizing NH2-POA Polymers
The graph shows plots of the inverse total amine concentration ([amine]ꢀ1) versus time. The fact that these plots are linear demonstrates that the
reactions are all second order in the total amine concentration. The kinetic experiments were carried out in 600 mL of CD2Cl2 at 298 K using an initial
amine concentration of 1.8 mM, with an excess (5.1 mM) of the active ester 4-nitrophenyl-3,5-dinitrobenzoate (NDB) in the presence of one of the
following catalysts: 0.28 mM Tet-1 (green circles); 1.7 mM Edge-model (red triangles); 3.4 mM Triglyme + Control (gray squares); no catalytic additives
(blue triangles). See Table 1 for the rate constants (kobs) obtained from these rate plots.
the polymers, which are readily observed in the aromatic region of the spectrum in
1H DOSY NMR. This fact enabled us to determine (vide infra) the size selectivity of
the polymer functionalization reactions.
We followed the progress of the reactions by integrating (see Figures S13–S26) the
distinct 1H NMR resonance at ꢂ6.9 ppm, which corresponds to p-nitrophenol
formed upon aminolysis of the NDB active ester. From the p-nitrophenol
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Table 1. Observed Rate Constants (kobs) for the NH2-POA Polymer Acylation Reactions
a
Organocatalyst
kobs
mb
Tet-1 (0.28 mM)c
(8.3 G 0.9) 3 108
(7.8 G 0.6) 3 103
(3.6 G 0.3) 3 103
(3.8 G 0.3) 3 103
M
M
M
M
ꢀ3 h–1
ꢀ2 h–1
ꢀ2 h–1
ꢀ2 h–1
3.0 G 0.2
2.0 G 0.1
2.0 G 0.1
1.9 G 0.1
1.5 G 0.1
Edge-model (1.7 mM)c
Control + Triglyme (3.4 mM each)c
Triglyme (3.4 mM)d
No organocatalystc
(7.9 G 0.9) 3 100 M–1 h–1
aObserved rate constants (defined in Equations 1–3). Data are represented as mean G SEM.
bAverage number of acylated amines per polymer chain after 48 h of reaction time. Data are represented
as mean G SEM.
cSee Figure 6 for the corresponding rate plots with Tet-1, Edge-model, and Control + Triglyme.
dSee Figure S27D for the corresponding rate plot with only Triglyme as the catalyst.
concentration and the initial amine concentration ([amine]0 = 1.8 mM), the concen-
trations of the remaining amino groups ([amine]) on the NH2-POA polymers were
calculated for each time point. Our kinetic data show that plots of [amine]–1 versus
reaction time (Figure 6) are linear for all the samples and therefore the reactions
are all second order in the amine concentration. The amino groups on the polymers
are therefore not just acting as the nucleophiles for aminolysis but also as base cat-
alysts required to deprotonate62 the tetrahedral amine-adducts. Furthermore, we
found by changing the catalyst concentrations (see Figures S27B and S27C for the
corresponding rate plots) that the reactions are approximately first order in the cata-
lyst concentration for the simple Triglyme + Control as well as for the Edge-model
catalysts. This result is in line with the prior literature for glyme-catalyzed aminolysis
reactions.62 However, with Tet-1 as the catalyst, we discovered (Figure S27A) that
the reaction is not just second order in the amine concentration but also second or-
der in the Tet-1 concentration. Given this information, we can63,64 write the rate laws
for the PSPM reactions with cat = Edge-model or Triglyme as
v½amineꢃ
2
= kobs½catꢃ0½amineꢃ :
(Equation 1)
vt
On the other hand, with cat = Tet-1, the rate law can be written as
v½amineꢃ
2
2
= kobs½catꢃ0½amineꢃ :
(Equation 2)
vt
Finally, for the control reaction without any catalyst added, we define the rate con-
stant kobs of the reaction in an analogous manner:
v½amineꢃ
2
= kobs½amineꢃ :
(Equation 3)
vt
kobs in Equations 1, 2, and 3 are observed rate constant (defined in Equation S3),
[amine] is the total concentration of amino groups on all the polymer chains, and
[cat]0 is the initial concentration of the organocatalyst. The rate constants kobs
(Table 1) and slopes obtained from the second-order rate plots (Figure 6) demon-
strate that acylation of the amines on the polymers with Tet-1 as the catalyst is signif-
icantly faster than with any of the other control reactions. Thus, Tet-1 is a more effec-
tive catalyst than either simple Triglyme or Edge-model, both of which lack the large
cavity of the hydrazone cage.
Size Selectivity of the Organocatalytic Polymer Functionalization Process
We have been able to demonstrate that the Tet-1 catalyst can distinguish between
different chain lengths of the NH2-POA substrates directly in the complex mixture of
all the polymer chains and partially reverse the intrinsic size selectivity for the
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O2N
NO2
O
O
1. Triglyme (3.4 mM)
1. Tet-1 (0.28 mM), NDB
HO
+ Control (3.4 mM), NDB
O
N
O
2. Equalize with
Triglyme (3.4 mM)
+ Control (3.4 mM)
H2
HO
2. Equalize with
Tet-1 (0.28 mM)
HO
O
OH
HN
O
NH2
HO
O
O
H2N
H2N
HO
NH2
HO
N
NH2
HO
H2
O
O
O
O
OH
HO
NH2
HO
H2N
O
H2N
HO
O2N
O
NO2
NH2
NH2
Long-NH2-POA
NO2
HO
HO
O
O
O2N
NO2
O2N
O
HO
H2N
NH
O
Long-NH2-POA
NH2
Acylated-Long-NH2-POA
OH
O
NH
O
OH
OH
O
O
NH
OH
NH2
O
OH
NH2
NH2
O
Short-NH2-POA
OH
Acylated-Short-NH2-POA
Short-NH2-POA
O
Longer chains more likely to form
H-bonded aggregates (more amines/chain)
Solvodynamic Diameter = (4.9 0.5) nm
Longer chains acylated (less aggregation)
Solvodynamic Diameter = (2.8 0.2) nm
Shorter chains acylated (more aggregation)
Solvodynamic Diameter = (4.5 0.6) nm
Figure 7. Solvodynamic Diameters of the NH2-POA Polymers Provide Evidence for Size Selectivity in the Polymer Mixture
The NH2-POA polymers contain (Scheme 2) both NH2 and COOH functional groups. In the nonpolar CD2Cl2 solvent used for the acylation reactions,
these functionalities form strong hydrogen bonds that are required (Figure 5C) for Tet-1 to bind to the polymeric substrates. In addition, the hydrogen-
bonding capabilities of the polymers also induce some aggregation of the polymers. Overall, longer polymer chains have a statistically higher chance to
form aggregated structures, since they contain a larger number of NH2 and COOH functional groups on average. When the NH2 groups on the polymers
are acylated with NDB, some of the [–NH2.HOOC–]-hydrogen bonds disappear, which reduces aggregation of the polymers. Now, if longer polymers
get acylated the most, the observed reduction in aggregation will be stronger than that if shorter polymer chains get acylated the most. From the
measured solvodynamic diameters, we observe a smaller reduction in average size (from 4.9 to 4.5 nm) for the reaction executed with Tet-1 as the
catalyst than for the control reaction (4.9 to 2.8 nm). These data therefore indicate that shorter polymer chains got acylated the fastest with Tet-1 as the
catalyst, while longer polymer chains got acylated preferentially in the control reaction. Data are represented as mean G SEM. Tet-1 (0.28 mM) was
added to measure the solvodynamic diameter of the dissolved NH2-POA starting material to obtain a more accurate size-comparison with the product
samples, which also contained Tet-1 in the same concentration.
catalytic polymer functionalization process. Our evidence for the size selectivity in-
cludes the following:
(1) A distinct difference (Figure 7) in the average solvodynamic diameter for the
polymers functionalized with Tet-1 compared with the polymers functional-
ized with the control catalysts.
(2) 1H DOSY NMR spectra (Figure 8), which confirm directly that—when the reac-
tion is executed with the real polymer mixture—the functionalized polymers
display a smaller solvodynamic diameter than the unfunctionalized ones
when Tet-1 is used as the catalyst.
(3) Explicit measurements (Figure 9) of all the catalytic rate constants, kobs (Table
2), for the short and long polymer chains with all catalysts (Tet-1, Edge-model,
and Triglyme).
Evidence of Size Selectivity Based on Solvodynamic Diameters
To investigate the size selectivity in the presence of all polymer chains, we first ran
PSPM reactions with different catalysts and NDB as the acylation reagent to the
same conversion (24%). For the first catalytic system, Tet-1 (0.28 mM) was used as
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Figure 8. Additional Evidence for Size Selectivity in the Acylation Reactions of the Polymer
Mixture Based on Relative Diffusion Constants
(A and B) A direct comparison of the relative diffusion constants for the functionalized polymers
with the average diffusion constants of all the polymer chains (both functionalized and
unfunctionalized). (A) With Tet-1 as the catalyst. (B) Control reaction with Triglyme + Control as the
catalysts. The average diffusion constant of the functionalized polymers was measured via the
diffusion constant of the distinct 1H NMR resonance at 8.82 ppm, which corresponds to the ortho
protons (resonance X) of the 3,5-dinitrobenzamide units attached to the functionalized polymers.
The average diffusion constant of all polymer chains was determined from the diffusion constant of
the broad 1H NMR resonances between 0.8 to 1.4 ppm, which correspond to the aliphatic protons
of the polymer chains. All reactions were run to 24% conversion and all 1H DOSY NMR spectra were
obtained for reaction mixtures ‘‘equalized’’ in the following manner: (A) for the functionalization
reaction catalyzed by Tet-1 (0.28 mM), we added Triglyme (3.4 mM) and Control (3.4 mM) just
before recording the 1H DOSY NMR spectrum; (B) for the control reaction catalyzed by Triglyme
(3.4 mM) + Control (3.4 mM), we added Tet-1 (0.28 mM), again just before recording the 1H DOSY
NMR spectrum.
the catalyst, while a combination of Triglyme + Control (3.4 mM each) was used as
the control. To be able to directly compare the different reaction systems, we equal-
ized them before analysis with the following protocol: (1) for the functionalization re-
action catalyzed by Tet-1 (0.28 mM), we added Triglyme (3.4 mM) + Control
(3.4 mM) just before analysis, and for the control reaction catalyzed by Triglyme
(3.4 mM equivalents) + Control (3.4 mM), we added Tet-1 (0.28 mM), again just
before analysis. All measurements were executed with volumetric additions from
identical stock solutions. In this manner we ensured that, besides the differently
functionalized polymers, all other components and amounts thereof were exactly
the same for all the samples in the end. As a result, different properties of the equal-
ized reaction mixtures are expected to directly correlate with differences in the func-
tionalization patterns of the polydisperse NH2-POA substrates.
1H DOSY NMR spectra of all reaction mixtures were recorded immediately after
equalization. From these spectra, the average solvodynamic diameters of the
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Figure 9. Rate Plots Obtained for Isolated Short and Long Polymer Samples Confirm that Tet-1 Is
a More Active Catalyst for Shorter Polymer Chains
(A and B) Plots of the inverse total amine concentration ([amine]–1) versus time for Short-NH2-POA
(A) and Long-NH2-POA (B). The kinetic experiments were carried out in 600 mL of CD2Cl2 at 298 K
using an initial amine concentration of 1.2 mM for Short-NH2-POA and 0.5 mM for Long-NH2-POA
with an excess (5.1 mM) of the active ester 4-nitrophenyl-3,5-dinitrobenzoate (NDB) in the presence
of one of the following catalysts: 0.28 mM Tet-1 (green circles); 1.7 mM Edge-model (red triangles);
3.4 mM Triglyme + Control (gray squares). See Table 2 for the rate constants (kobs) obtained from
these rate plots.
(C) Proposed model (schematic) explaining why the Tet-1 catalyst reacts faster with the shorter
polymer chains, while it binds stronger to the substrates (but not to the transition states) with longer
polymer chains. DGz (Long) = Activation Gibbs free energy with Long-NH2-POA; DGz (Short) =
Activation Gibbs free energy with Short-NH2-POA; DGb (Long) = Binding Gibbs free energy with
Long-NH2-POA; DGb (Short) = Binding Gibbs free energy with Short-NH2-POA.
polymers were determined. Significant differences in the solvodynamic diameters of
the polymers were observed between the control reaction and the Tet-1-catalyzed
variant. These differences indicate (Figure 7) a change in size selectivity for polymer
functionalization with Tet-1: with the control reaction, the longer polymer chains get
acylated the fastest, which results in a significant reduction of the number-averaged
solvodynamic diameter of the polymer (from 4.9 to 2.8 nm) upon acylation of the
amines. This reduction in polymer size can be explained with less aggregation occur-
ring after acylation (since fewer strong [–NH2.HOOC-]-hydrogen bonds are present
after acylation) and the fact that longer polymers have a higher tendency to aggre-
gate because they contain more NH2 and COOH groups on average than shorter
ones. A much smaller reduction in the solvodynamic polymer diameter upon
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Table 2. Observed Rate Constants (kobs) for the Acylation Reactions of the Individual Short- and
Long-NH2-POA Polymers
Organocatalyst
kobsa,b with Short-NH2-POA
kobsa,c with Long-NH2-POA
Tet-1 (0.28 mM)
(2.9 G 0.4) 3 109
(6.8 G 0.6) 3 104
(3.4 G 0.2) 3 104
M
M
M
ꢀ3 h–1
ꢀ2 hꢀ1
ꢀ2 hꢀ1
(1.9 G 0.3) 3 109
(4.6 G 0.6) 3 104
(4.3 G 0.4) 3 104
M
M
M
ꢀ3 h–1
ꢀ2 hꢀ1
ꢀ2 hꢀ1
Edge-model (1.7 mM)
Control + Triglyme (3.4 mM each)
aObserved rate constants (defined in Equations 1 and 2). Data are represented as mean G SEM.
bSee Figure 9A for the corresponding rate plots with Short-NH2-POA.
cSee Figure 9B for the corresponding rate plots with Long-NH2-POA.
acylation (from 4.9 to 4.5 nm) is observed for functionalization with Tet-1 as the cata-
lyst. This finding indicates that, with Tet-1 as the catalyst, shorter polymer chains
(which have a weaker tendency to aggregate) get acylated the fastest.
Comparing the Diffusion Constants of the Functionalized to the Unfunctionalized
Polymers Provides Direct Evidence for Size Selectivity in the Polydisperse Mixture
To obtain additional evidence for the size selectivity in the polymer mixture, we
compared (Figure 8) the average size of the functionalized polymer chains with the
average size of all the polymer chains (both functionalized and unfunctionalized) with
1H DOSY NMR spectroscopy. We acylated with NDB—which attaches 3,5-dinitrobenza-
mide units with distinct aromatic 1H NMR resonances to the polymers. Thus, we were
able to determine the average diffusion constant of the functionalized polymers from
the 1H DOSY NMR resonances of the 3,5-dinitrobenzamide units. The results obtained
clearly show (Figure 8) that the aromatic resonance X corresponding to the acylated
NH2-POA substrates was lined up with the shorter polymer chains of the sample when
Tet-1 was used as the catalyst. In contrast, when Triglyme + Control were used as the
catalysts, the medium/longer polymer chains reacted faster in the PSPM reaction.
Measurements of the Relative Rate Constants for the Acylation of Short and Long
Polymers Separately Confirm Tet-1’s Selectivity for Shorter Polymers
We managed to isolate (see Figure 5; Experimental Procedures) some of the shortest
(Short-NH2-POA; Mw = 2.6 kDa) and longest (Long-NH2-POA; Mw = 9.9 kDa) polymer
chains contained withinthe original NH2-POA sample. Thus, we wereable to measure (Fig-
ure 9) the acylation rate constants (Table 2) of the short and long polymer samples sepa-
rately, analogous to how we determined (Figure 6) the rate constants (kobs) with the mixed
NH2-POA sample.
The results shown in Figure 9 tell us the following:
(1) The short polymers (Short-NH2-POA; kobs = 2.9 G 0.4 3 109 M–3 h–1) indeed
react faster with the Tet-1 catalyst than the longer ones (Long-NH2-POA;
k
obs = 1.9 G 0.3 3 109 M–3 h–1), which confirms the size selectivity determined
directly (see Figures 7 and 8) in the polymer mixture.
(2) We see that the longer chains intrinsically react faster than the shorter ones, if
we take the reactivity with the small Triglyme catalyst as a measure for the
intrinsic reactivity of the polymer chains. This finding once again agrees with
the conclusions from the 1H DOSY NMR measurements (Figures 7 and 8).
We hypothesize that the longer polymer chains intrinsically react faster than
the shorter ones, since they contain more amino groups per polymer chain
on average, which are needed (Scheme 3) for intramolecular base catalysis.
(3) Overall, the rate constants determined for the short and long polymer chains
by themselves are faster with all the catalysts than with the mixed polymer
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Scheme 3. Proposed Catalytic Cycle for Tet-1-Catalyzed Acylation of NH2-POA Polymers
The tetrahedral cage catalyst Tet-1 (1) helps to unfold (see Figures 3, 4, and 10 for evidence) the polymeric substrates, which frees up the amino groups
on the polymers for acylation and base catalysis. In addition, Tet-1 (2) utilizes its 12 peripheral triglyme units to stabilize the tetrahedral aminolysis
intermediates, while (3) an internal amino group (freed up by a second equivalent of Tet-1) functions as the base to deprotonate the intermediate. The
fact that the amines also act as a base in the mechanism results in the observed second-order rate dependence on the amine concentration. The two
Tet-1 cages bound to the proposed rate-determining transition state lead to the observed (see Figure S27A) second-order rate dependence on the Tet-
1 concentration.
sample. This finding tells us that the mixture does not behave exactly like its
individual components, which is expected for a complex system with many in-
teracting parts. For example, we know that the longer polymer chains bind
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(Figure 5B) stronger to the Tet-1 catalyst than the shorter ones. Nevertheless,
the shorter chains react faster with the catalyst, as weaker binding to the sub-
strate helps to further lower61 (Figure 9C) the activation Gibbs free energy of
the reaction. Thus, from the measured binding and rate constants, we can
conclude that the longer polymers must inhibit the reaction with the shorter
polymers, which is likely why the rate constant for the shorter polymers alone
with Tet-1 (kobs = 2.9 G 0.4 3 109 M–3 h–1) is indeed larger than the rate con-
stant of the mixture (kobs = 8.3 G 0.8 3 108 M–3 h–1) with Tet-1.
(4) The relative rate-plot slopes obtained with the three different catalysts also
provide (see Figures 3 and 4 for additional evidence) direct evidence for poly-
mer unfolding by Tet-1 during acylation catalysis as detailed in Scheme 3 and
the Discussion.
Overall, we conclude from this section (based on the evidence shown in Figures 7, 8,
and 9) that Triglyme catalyzes the reaction with the long polymer chains faster than
with the short ones. In turn, with Tet-1 as the catalyst, the shorter polymer chains react
faster than the longer ones, which demonstrates that the Tet-1 catalyst is able to switch
the intrinsic reactivity of acylation toward selective functionalization of the shorter
chains via size-selective acceleration of the reaction with the shorter chains.
DISCUSSION
Mechanism of the Size-Selective Catalytic Polymer Functionalization
The control reactions executed with simple Triglyme and Edge-model demonstrate
that not only the triglyme functions of Tet-1 but also the cavity of the tetrahedron are
important for catalysis. For instance, the slope (= kobs[cat]0) of the rate plot with the
NH2-POA substrate increased (Figure 6) from 7.9 G 0.9 Mꢀ1 hꢀ1 (R2 = 0.96) without
any additives to 13.0 G 0.9 Mꢀ1 hꢀ1 (R2 = 0.97) with simple Triglyme (3.4 mM) as the
catalyst, demonstrating glyme-catalysis62 in our system. Next, with Edge-model
(1.7 mM) as the catalyst, the slope of the rate plot—13.3 G 0.7 Mꢀ1 hꢀ1 (R2
=
0.98)—turned out to be almost identical to the value obtained with Triglyme. How-
ever, we then observed a very significant jump in the rate-plot slope (= kobs[cat]02) to
64.9 G 3.0 Mꢀ1 hꢀ1 (R2 = 0.99) by utilizing the whole tetrahedron (Tet-1, 0.28 mM) as
the catalyst. These data are consistent with a mechanism (Scheme 3), where both the
glyme functionalities and the voluminous cavity of the molecular tetrahedron Tet-1
actively participate in the size-selective polymer functionalization mechanism.
Furthermore, from the measured second-order rate dependence on the Tet-1 con-
centration, we can conclude that two tetrahedra must be interacting65 with the poly-
meric substrate in the rate-determining transition state of the reaction. Thus, based
on (1) our kinetic data and (2) the fact that the solvodynamic diameter of the poly-
mers increases very significantly upon binding to Tet-1 (see Figures 3, 4, and 10
for corresponding experimental and computational evidence), we propose the
mechanism shown in Scheme 3 for the observed catalytic polymer functionalization.
In the proposed mechanism, the hydrazone functionalities hydrogen bond (Fig-
ure 5C) with functional groups on the polymers and thereby help unfold (see Figure 4
for an MD simulation of this process) the polymer chains to render the amines more
reactive: the first Tet-1 molecule involved in the transition state frees up the amine
nucleophile, while the second Tet-1 frees up the amine base. At the same time,
the triglyme functional groups of Tet-1 engage in catalysis by stabilizing the tetrahe-
dral intermediates arising during aminolysis. Finally, we propose that another amino
group on the polymer acts as a base (freed up by the second equivalent of Tet-1
bound to the transition state) to deprotonate and break apart the tetrahedral
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O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
Triglyme
O
O
O
O
O
Small and flexible enough to reach
amines inside the folded
/aggregated polymers
O
O
OH
HO
O
O
O
O
O
O
NH2
HN
O
N
N
NH
O
H2N
O
O
O
Edge-model
O
HO
Too bulky for efficient catalysis
O
O
O
OH
O
O
O
O
H2N
NH2
O
O
O
H2N
O
O
O
HO
HO
O
O
O
O
O
NH2
O
OH
O
O
Tet-1 — Reacts faster than Edge-model
with Long-NH2-POA since it can unfold / break apart the polymers
Long-NH2-POA in the abscence of Tet-1
Figure 10. Tet-1’s Ability to Unfold/Break Apart the Polymer Chains Explains the Relative Acylation Rates Observed with Long-NH2-POA with the
Three Different Catalysts
Triglyme (3.4 mM) acts (Figure 9B) as a more effective catalyst than Edge-model (1.7 mM) for the Long-NH2-POA substrates, while for the Short-
NH2-POA substrates both of these catalytic systems perform (Figure 9A) at approximately equal rates. This reactivity trend is explained by polymer
folding/aggregation (Figure 7), which occurs preferentially for the longer polymers that contain more amino groups per chain on average. Remarkably,
with the even larger Tet-1 as the catalyst, the acylation reaction speeds up again for the long polymers. This finding indicates that the large cavity of
Tet-1 can unfold/break apart (see Figures 3 and 4 for additional evidence) the polymeric substrates to free up the amines on the polymers.
intermediate, which is clearly shown by the observed (Figures 6, 9A, and 9B) second-
order rate dependence on the amine concentration.
The Role of Tet-1’s Cavity during Catalysis
Figure 9 provides additional evidence that polymer unfolding is taking place driven
by the cavity of the Tet-1 catalyst. Specifically, we observed (Figure 9B) that an edge
of Tet-1 (Edge-model) leads to an overall slower acylation rate than the much smaller
Triglyme catalyst only for the longer polymeric substrates (Long-NH2-POA). At the
same time these two control catalytic systems performed (Figure 9A) at approxi-
mately identical rates with the shorter polymer chains (Short-NH2-POA). This
difference in catalytic activity is explained (Figure 10) by polymer folding and the for-
mation of aggregates (Figure 7). Since the longer polymers contain, on average,
more amino groups per chain than the shorter ones, polymer folding and aggregate
formation occur to a larger extent with the longer chains. Thus, the bulky Edge-
model performs worse as a catalyst for the longer polymers chains only, since the
amino groups occluded inside of the folded/aggregated long polymers cannot be
approached (Figure 10) effectively by the Edge-model catalyst.
While, these results are in line with what has been observed66,67 previously for non-cat-
alytic reactions—for which aggregation has been shown to reduce reaction rates—we
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have now been able to demonstrate that folding/aggregation-dependent reactivity can
be directly influenced by changing the nature of an organocatalyst. Remarkably, with the
evenbulkierTet-1asthecatalyst, thereactionspeedsupagain(Figure9B)withthelonger
polymer chains (Long-NH2-POA) as the substrates. This result is a clear indication that
Tet-1 is able to unfold/break apart the polymeric substrates. It further highlights the
importance of Tet-1’s large cavity, which is lacking in Edge-model and seems to be
requiredforefficientunfoldingandcatalyticfunctionalizationofthepolymericsubstrates.
Origin of the Observed Size Selectivity
It is likely that a nanosized catalyst like Tet-1 can distinguish polymeric substrates of
different sizes due to complex, large section contacts, resembling interactions be-
tween biological macromolecules. Such non-local supramolecular recognition is
difficult to achieve with small-molecule catalysts like triglyme, since a small-molecule
catalyst interacts with reactive groups on the polymers (e.g., amines in our case)
mostly in a local fashion. Although a small-molecule catalyst can also form multiva-
lent contacts with its polymeric substrates, it will, in most instances, simply not be
able to reach far away sections of a polymeric substrate to enable size-selective func-
tionalization of such large substrates.
As suggested by MD simulations (Figure 4), Tet-1 interacts with the polymeric substrates
in a non-local fashion. Specifically, other amines and carboxylic acids—far away from a
reactive amino group on the same polymer—can still influence the interactions between
thepolymericsubstrateandTet-1throughhydrogen-bondingnetworks(seeFigure5Cfor
an example). Therefore, we expect the Tet-1 catalyst to distinguish different, non-local as-
pects of a polymer’s structure, including (1) polymer length, (2) the number and types of
functional groups present on the polymer, and (3) the positioning of these functional
groups. In this work, we now provide evidence (see Figures 7, 8, and 9) for the effect of
polymer length on the catalytic rates with Tet-1 and with small-molecule catalysts. From
the kinetic evidence (i.e., the observed second-order dependence of the reaction rates
on the amine concentration), it is clear that longer polymer chains intrinsically react faster
withasmall-moleculecatalystsuchastriglyme. However,byswitchingtoTet-1 asthecata-
lyst, this intrinsic reactivity can be partially reversed. Overall, by providing initial evidence
for size selectivity with polymeric substrates, we lay the foundation to further investigate
the complex factors governing polymerization catalysis for selective post-synthetic poly-
mer modification (PSPM) reactions. Some key factors we identified thus far are as follows:
(1) The thermodynamic stability of the [polymer@Tet-1] complex. As has been es-
tablished for classical enzyme kinetics, where the binding strength of the sub-
strate with the enzyme has to be61 ‘‘just right’’ and neither ‘‘too tight’’ nor ‘‘too
loose’’ for efficient catalysis, we find (Figure 5B) that weaker binding of the
substrate can indeed enhance the catalytic activity of our PSPM catalyst.
(2) The availability of a large cavity in the catalyst, which is able to unfold the poly-
meric substrates. Notably, a simple edge of the tetrahedron (Edge-model)
does not perform nearly as efficiently as the full tetrahedron.
(3) A reactive complexation geometry, in which the polymers are not only
partially unfolded (and threaded through the cavity of the catalyst) but also
placed in close proximity to the catalytic triglyme units on the tetrahedron.
(4) The ability of the catalyst to free up at least two amino groups on a polymer,
the first one acing as the nucleophilic substrate for acylation and the second
one as the amine base needed for efficient deprotonation of the tetrahedral
acylation intermediates.
(5) Aggregation (Figure 7) of the polymer chains. While prior work has shown that
aggregation reduces the reaction rates for non-catalytic reactions,66,67 we
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discovered that for our system aggregation and folding effects are strongly
catalyst dependent. For example, folding/aggregation of the longer polymer
chains significantly reduces (Figure 9B) the reactivity of the long chains with
the Edge-model catalyst. At the same time, aggregation effects are not
strong enough (Figure 9B) to slow down the Tet-1-catalyzed reaction with
the long polymers in comparison with the small triglyme catalyst. Rather,
our results shown in Figure 9 clearly indicate that the observed size
selectivity is caused by selective acceleration of Tet-1’s reaction with the short
polymers. Overall, our findings (especially the relative rates shown in Figure 9)
clearly demonstrate (Figure 10) that aggregation/folding effects can
mostly be overcome by threading polymers through the cavity of a porous
catalytic tetrahedron, which leads to partial unfolding (Figure 4) of the poly-
mer chains.
Conclusions
In conclusion, we demonstrated the concept of post-synthetic selective polymer modi-
fication operating in complex mixtures of polymeric substrates. Our reaction proceeds
with size selectivity in the presence of a hydrazone-linked tetrahedron with wide open-
ings as the catalyst, in sharp contrast to the results observed with small-molecule cata-
lysts. This conclusion is supported by (1) distinct differences in the overall solvodynamic
radii of the polymeric products, (2) by a significant alteration in the relative diffusion con-
stants for the functionalized polymers compared with the unfunctionalized ones, and (3)
by rate constants measured separately for the shorter and longer polymer chains. Our
findings extend the scope of catalyst-controlled size selectivity to large substrates for
post-synthetic polymer functionalization reactions, applied to polydisperse polymer
mixtures. Our size-selective, catalytic approach represents a promising avenue to create
next-generation polymers.
EXPERIMENTAL PROCEDURES
Resource Availability
Lead Contact
Further information and requests for resources and reagents should be directed
to and will be fulfilled by the Lead Contact, Severin T. Schneebeli (severin.
schneebeli@uvm.edu).
Materials Availability
Tet-1 generated in this study will be made available on request, but we may require a
payment and/or a completed materials transfer agreement if there is potential for
commercial application.
Data and Code Availability
Raw data for (1) compound characterization (1H and 13C (1H) NMR, 1H-1H NOESY
NMR, 1H DOSY NMR, HR-ESI-MS, FTIR, and dynamic light scattering [DLS, Zetasizer
Nano ZSP, Malvern Panalytical]), (2) the 1H NMR titrations, and (3) the 1H NMR-based
kinetic measurements are available freely from the Mendeley Data repository at
http://dx.doi.org/10.17632/vn92mr6z69.1. The MD trajectory of NH2-POA is avail-
able freely from the Mendeley Data repository at http://dx.doi.org/10.17632/
k4nb4z3ff5.1. The MD trajectory of the [NH2-POA@Tet-1] complex is available freely
from the Mendeley Data repository at http://dx.doi.org/10.17632/69x74bkdb5.1.
General Synthetic Procedures
Chemical synthesis was performed as detailed below and in the Supplemental
Experimental Procedures. See Figures S30–S64 for the 1H and 13C (1H) NMR spectra
1488 Chem 6, 1469–1494, June 11, 2020

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of all products and key synthetic intermediates, Figure S65 for the FTIR spectrum of
NH2-POA, and Figure S66 for the DLS characterization data of the polymers.
Synthesis of Tet-1
Syn-7 (0.274 g, 0.208 mmol) was dissolved in 350 mL of anhydrous CH2Cl2, and the
solution was degassed with argon. Next, terephthalaldehyde (41.9 g, 0.312 mmol)
and high-performance liquid chromatography (HPLC) grade TFA (24 mL,
0.313 mmol) were added consecutively, and the reaction mixture was stirred at
room temperature under an argon atmosphere. After 48 h, the acid was neutralized
with a saturated aqueous sodium bicarbonate solution (10 mL), the organic
layer was separated, and the aqueous phase extracted with additional CH2Cl2
(30 mL). Finally, the combined organic extracts were washed with brine, dried
over anhydrous Na2SO4, filtered, and concentrated under reduced pressure. The
crude product was purified by size exclusion chromatography over polystyrene
beads (200–400 mesh) with CH2Cl2 as the eluent to afford Tet-1 as an orange-
yellow solid.
Yield and Characterization Data for Tet-1
Yield: 0.242 g (80%); 1H NMR (500 MHz, CD2Cl2): d 11.08 (s, 12H), 8.26 (s, 12H), 7.99
(s, 12H), 7.84 (s, 24H), 6.64 (s, 12H), 4.33 (t, J = 6.6 Hz, 24H), 3.89 (s, 36H), 3.63–3.44
(m, 168H), 3.29 (s, 36H), 2.16–2.08 (m, 24H), 1.88 (m, 24H), 1.67 (s, 36H); 13C (1H)
NMR (125 MHz, CD2Cl2): d 162.0, 161.2, 157.9, 146.6, 136.5, 135.7, 135.5, 135.1,
128.3, 124.6, 113.7, 96.4, 72.4, 71.1, 71.0, 71.0, 71.0, 70.9, 70.8, 70.1, 59.1, 56.3,
26.9, 26.9, 19.0; HRMS (ESI+) m/z: [M + 5H]5+ calcd 1,170.9964, found 1,170.9982.
Synthesis of NH2-POA
Poly(isobutylene-alt-maleic anhydride) (Mw = 6 kDa, 0.500 g, 0.083 mmol of polymer,
ꢂ3.237 mmol of anhydride units) was dissolved in anhydrous N,N-dimethylforma-
mide (DMF, 1.0 mL) while heating at 85ꢁC. In a different vial, 1-aminooctane
(0.378 g, 2.925 mmol) was dissolved in anhydrous DMF (1.0 mL) at the same temper-
ature. Next, the 1-aminooctane solution was added to the polymer solution and the
reaction mixture was stirred at 89ꢁC for 24 h. The temperature was then increased to
125ꢁC for 2 h under a stream of dry N2 to remove the water formed in the condensa-
tion reaction. Finally, the temperature was increased further to 170ꢁC and the reac-
tion mixture stirred for another hour at that temperature under a stream of dry N2 to
obtain an orange oil. Next, the crude sample (0.740 g, 0.0706 mmol) was dissolved in
anhydrous DMF (2.0 mL), followed by addition of excess 1,6-diaminohexane (75 mg,
0.64 mmol) and the reaction mixture was heated at 71ꢁC for 45 h. The temperature
was increased to 95ꢁC and the solvent was evaporated over dry N2 for 2 h. Finally, the
crude reaction mixture was purified by size exclusion chromatography over polysty-
rene beads (200–400 mesh) with CH2Cl2 as the eluent to afford NH2-POA as a viscous
yellow oil. Based on elemental analysis (vide infra), the percentages (defined in
Scheme 2) of repeating units a, b, and c were (with 2.2% crystal water): a = 57.8%,
b = 17.8%, and c = 22.2%.
Yield and Characterization Data for NH2-POA
Yield: 0.773 g (73%); 1H NMR (500 MHz, CD2Cl2): d 3.70–2.10 (broad m, ꢂ102H), 1.80–
0.60 (broad m, ꢂ490H); IR (film): n = 3,316 (broad, amide N-H stretching and carboxylic
acid O-H stretching), 2,928, 2,859, 1,694 (carbonyl C=O stretching); 1H DOSY NMR
(500 MHz, CDCl3, polystyrene standard, see Figure S5 for the calibration curve): Mw
=
6.3 kDa; DLS (CH2Cl2): polydispersity index (PDI) = 1.5; Anal. calcd (mass %) for
(C16H29NO3)12.1 (C22H43N3O2)3.7 (C24H46N2O2)4.7 ∙ 0.5 H2O: C, 67.77; H, 10.84; N,
6.39. Found: C, 67.80; H, 11.22; N, 5.99.
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Isolation of Short- and Long-NH2-POA
Analytical samples of the shortest (Short-NH2-POA, Mw [DOSY] = 2.6 kDa) and
longest (Long-NH2-POA, Mw [DOSY] = 9.9 kDa) polymer chains in the NH2-POA
sample were obtained by running a second size exclusion column of NH2-POA
(0.272 g) over polystyrene beads (200–400 mesh) with CH2Cl2 as the eluent. Collec-
tion of the compound eluting first (at ꢂ43 min) resulted in Long-NH2-POA, while
collection of the fractions eluting last (at ꢂ80 min) resulted in Short-NH2-POA. Based
on elemental analysis, the percentages (defined in Scheme 2) of repeating units a, b,
and c were a = 50.9%, b = 12.0%, and c = 34.7% (with 2.4% crystal water) for Short-
NH2-POA and a = 48.7%, b = 5.0%, and c = 43.6% (with 2.7% crystal water) for Long-
NH2-POA. The molecular weights of the different polymer samples were determined
(see Figures S4–S6; Tables S1 and S2) with DLS as detailed in the Supplemental
Experimental Procedures.
Yield and Characterization Data for Short-NH2-POA
Yield: 0.046 g (41%, based on the total amount of NH2-POA added to the second
size-exclusion column); 1H NMR (500 MHz, CD2Cl2): d 3.70–2.30 (broad m, ꢂ46H),
2.20–0.50 (broad m, ꢂ220H); 1H DOSY NMR (500 MHz, CDCl3, polystyrene stan-
dard, see Figure S5 for the calibration curve): Mw = 2.6 kDa; DLS (CH2Cl2): PDI =
1.3; Anal. calcd (mass %) for (C16H29NO3)4.6 (C22H43N3O2)1.1 (C24H46N2O2)3.1 ∙ 0.2
H2O: C, 68.14; H, 10.96; N, 6.30. Found: C, 68.13; H, 11.09; N, 6.33.
Yield and Characterization Data for Long-NH2-POA
Yield: 0.103 g (25%, based on the total amount of NH2-POA added to the second
size-exclusion column); 1H NMR (500 MHz, CD2Cl2): d 3.60–2.20 (broad m,
ꢂ162H), 1.70–0.70 (broad m, ꢂ854H); 1H DOSY NMR (500 MHz, CDCl3, polystyrene
standard, see Figure S5 for the calibration curve): Mw = 9.9 kDa; DLS (CH2Cl2): PDI =
1.5; Anal. calcd (mass %) for (C16H29NO3)16.1 (C22H43N3O2)1.6 (C24H46N2O2)14.4 ∙ 0.9
H2O: C, 68.34; H, 11.01; N, 6.05. Found: C, 68.36; H, 11.34; N, 5.77.
Sample Preparation and Measurements of Rate Constants
For all kinetic measurements, stock solutions of all reagents in CD2Cl2 were pre-
pared in advance and used promptly. The stock solutions were stored at ꢀ10ꢁC in
sealed vials under an argon atmosphere. All volumetric measurements were per-
formed with Rainin positive displacement (MR-10, -100, -1,000) micropipettes,
which are optimized for organic solvents with low vapor pressures like CD2Cl2.
The concentrations of reagents in the stock solutions were calibrated by
1H NMR integration and comparison of the integrals with the integral of
1,2,4,5-tetrabromobenzene (TBB) as the internal standard. For each time-depen-
dent 1H NMR experiment, the active ester (NDB, 5.1 mM) was added last to the
reaction mixtures and the addition time of NDB is reported as the start time of the
experiments. The progress of the aminolysis reactions was monitored by inte-
grating (Figures S13–S26) the 1H NMR resonance at ꢂ6.9 ppm, which corre-
sponds to p-nitrophenol formed (Scheme 3) upon aminolysis of the NDB active
ester. Absolute concentrations of the p-nitrophenol in all the samples were ob-
tained by comparing the integrations of the p-nitrophenol resonances to the in-
tegrations of the internal TBB standard. Finally, the concentrations of the remain-
ing amino groups ([amine]) on the NH2-POA polymers were calculated for each
time point by subtracting the amount of p-nitrophenol formed from the initial
amine concentrations ([amine]0). With the 1.8 mg of polymer sample used for
each kinetic experiment, the initial amine concentrations were, approximately:
1.8 mM for NH2-POA, 1.2 mM for Short-NH2-POA, and 0.5 mM for Long-NH2-
POA.
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1H DOSY NMR Analysis of Polymer Acylation Experiments
For the 1H DOSY NMR analysis of the polymer acylation reactions, two reaction
mixtures were prepared in separate NMR tubes, and reaction progress was moni-
tored by following the 1H NMR integrations of the p-nitrophenol resonances at
ꢂ6.9 ppm in both samples. Both reactions were then equalized as described in detail
in Figure 7 (caption) at 24% conversion. In order to minimize further reaction prog-
ress while recording the 1H DOSY NMR spectra, the 1H DOSY NMR spectra were re-
corded with 30 increments in 20–25 min. The diffusion delay D, was set to 60 ms, with
the diffusion gradient length d at 3 ms, which required 32 scans for each gradient-
strength measurement.
MD Simulations
In order to verify our proposed model for the observed kinetic behavior of Tet-1, we
employed 800-ns MD simulations with the OPLS3e68 force field in explicit CH2Cl2
solvent. See Video S1 for the trajectories. The MD simulations (1) help to explain
the outcome of the 1H DOSY NMR spectra shown in Figure 3. Specifically, we see
in the simulations that a typical NH2-POA polymer unfolds upon binding to Tet-1,
and it increases (Figure 4) in size during that process. (2) The MD simulations also
shed light on the alteration of the supramolecular interactions present within the
polymer upon binding to Tet-1. As shown in the supplemental information (Fig-
ure S29), the number of intramolecular hydrogen bonds folding up the polymer de-
creases by binding to Tet-1 as these intra-polymer hydrogen bonds get replaced
with hydrogen bonds formed between Tet-1 and the polymer.
Model Preparation
All models were constructed using the Maestro program (2018-2 release). Tet-1 and
[NH2-POA@Tet-1] systems were simulated in periodic simulation boxes of ꢂ121 3
3
3
˚
˚
121 3 121 A and ꢂ67 3 67 3 67 A , respectively, with CH2Cl2 molecules as the sol-
vent. Each construct went through minimization, equilibration, and 800-ns MD pro-
duction stages. At least two replicas with differing random seeds were run for all
simulations.
Simulation Setup and Analysis
Each model was simulated in the NPT ensemble (300 K, 1 atm, Martyna-Tuckerman-
Klein coupling scheme). All simulations were performed in the Maestro-Desmond
program (GPU version 5.4) with a time step of 2 fs. The Ewald technique was used
for the electrostatic calculations. The Van der Waals and short-range electrostatics
˚
were cut off at 9 A. Hydrogen atoms were constrained using the SHAKE algorithm.
MD trajectories were analyzed using in-house python scripts and the Schrodinger
¨
(2018-2 release) API.
SUPPLEMENTAL INFORMATION
Supplemental Information can be found online at https://doi.org/10.1016/j.chempr.
2020.05.011.
ACKNOWLEDGMENTS
We thank B.O. Rourke for high-resolution mass spectrometry. The synthesis of Tet-1
and execution of the initial kinetic experiments was supported by the National Sci-
ence Foundation (NSF, grant CHE-1609137 awarded to S.T.S.), while the kinetic ex-
periments with the isolated short and long polymers were supported by an NSF
CAREER award (grant CHE-1848444 awarded to S.T.S.). Partial support for the
computational work was also provided by the ACS Petroleum Research Fund (grant
Chem 6, 1469–1494, June 11, 2020 1491

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58219-DNI6), National Institutes of Health (grant R01-GM129431 awarded to J.L.),
and by the National Institutes of Health (grants S10-OD018126 and P30-
GM118228 supporting the UVM mass spectrometry facilities). The graphics process-
ing units used for the modeling were provided by NVIDIA via the GPU grant
program.
AUTHOR CONTRIBUTIONS
S.T.S. and J.L. conceived and guided the project, obtained funding, discussed the
experimental and computational results, and wrote the paper together with M.S.
M.S., K.T.M., D.R.M., N.D., S.C.R., J.P.C., and K.E.M. performed the experiments
and analyzed the experimental data. K.T.M. performed the MD simulations and
analyzed the computational results. M.I. assisted M.S. with the 1H DOSY NMR
experiments. Y.S. and A.R.B. assisted M.S. with the DLS measurements. All authors
discussed the results and revised the paper.
DECLARATION OF INTERESTS
The authors declare no competing interests. N.D. is currently a graduate student at
Yale University (New Haven, CT). K.E.M. is currently employed at the University of
North Carolina, Asheville (Asheville, NC). S.C.R. is currently a postdoctoral
researcher at the University of Southern Mississippi (Hattiesburg, MS). J.P.C. is
currently employed as a Scientist at Integrity Industrial Ink Jet Integration, LLC
(West Lebanon, NH). However, N.D., K.E.M., S.C.R., and J.P.C. performed the
work for this paper at the University of Vermont (Burlington, VT). Some of the authors
have also filed a provisional patent application related to this work.
Received: August 9, 2019
Revised: October 1, 2019
Accepted: May 14, 2020
Published: June 11, 2020
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