Radical Cascade-Triggered Controlled Ring-Opening Polymerization of Macrocyclic Monomers

Article abstract of DOI:10.1021/jacs.8b05365

A strategy for the controlled radical ring-opening polymerization of macrocyclic monomers is reported. Key to this approach is an allyl alkylsulfone-based ring-opening trigger that can undergo a radical cascade reaction to extrude sulfur dioxide and gener

Full text of DOI:10.1021/jacs.8b05365

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Communication
Radical Cascade-Triggered Controlled Ring-
Opening Polymerization of Macrocyclic Monomers
Hanchu Huang, Bohan Sun, Yingzi Huang, and Jia Niu
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 19 Jun 2018
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Radical Cascade-Triggered Controlled Ring-Opening Polymeriza-
tion of Macrocyclic Monomers
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Hanchu Huang, Bohan Sun, Yingzi Huang, and Jia Niu*
Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02467, United States
Supporting Information Placeholder
Scheme 1. Development of Radical Ring-Opening
Polymerization
ABSTRACT: A strategy for the controlled radical ringꢀopening
polymerization of macrocyclic monomers is reported. Key to this
approach is an allyl alkylsulfoneꢀbased ringꢀopening trigger that
can undergo a radical cascade reaction to extrude sulfur dioxide
and generate an alkyl radical for controlled chain propagation. A
systematic study correlating reaction conditions and polymer moꢀ
lecular weight and molecular weight distribution allowed excelꢀ
lent control over polymerization. The versatility of this radical
cascadeꢀtriggered ringꢀopening polymerization (RCTꢀROP) apꢀ
proach was further demonstrated through the first radical block
copolymerization of macrocyclic monomers, and the incorporaꢀ
tion of degradable functionality into the polymer backbone.
Over the recent decades, controlled radical polymerization has
emerged as a powerful technique for the preparation of functional
polymers with wellꢀdefined architectures.1 Despite the successful
incorporation of diverse functional pendant groups, traditional
controlled radical polymerization techniques have been limited to
simple vinyl (acrylic, methacrylic, and styrenic) monomers. Ringꢀ
opening polymerization (ROP) is a powerful approach capable of
incorporating functional groups into the polymer backbone,2 but
the majority of ROP reactions that occur through a radical mechaꢀ
nism to date still require highly strained cyclic monomers
(Scheme 1A),3 limiting their utility in generating synthetic polyꢀ
mers with extended mainꢀchain structural motifs.4
resulted from ring opening cannot be reversibly deactivatꢀ
ed/controlled, resulting in low reactivity and poor control over
polymerization (Scheme 1B).4 Inspired by the seminal work of
QuicletꢀSire and Zard, which suggested that the extrusion of SO2
from alkylsulfonyl radicals can readily occur if the resulting alkyl
radical is stabilized,13 a radical cascadeꢀtriggered ringꢀopening
polymerization (RCTꢀROP) approach was designed to provide the
driving force for ring opening and extrude gaseous SO2 to generꢀ
ate an alkyl radical (Scheme 1C). Central to this design is an allyl
alkylsulfoneꢀbased ring opening trigger, which can undergo a
radical cascade process consisting of the βꢀelimination of alkylꢀ
sulfone and a following rapid αꢀscission to extrude SO2, resulting
in a secondary alkyl radical stabilized by an adjacent carbonyl
group.14 This “stable” secondary alkyl radical is structurally simiꢀ
lar to the propagating radical of polyacrylates, thus enabling conꢀ
trolled chain growth via its reversible deactivation.
We sought to develop a general strategy for the controlled radiꢀ
cal ROP of macrocyclic monomers with low ring strain (size ≥
12ꢀmembered ring5). The major challenge for this vision is that
the ringꢀopening products are energetically disfavored for the
ROP of macrocyclic monomers.1d, 6 To overcome this challenge,
we envision that a novel “ringꢀopening trigger” based on the radiꢀ
cal cascade reaction, in which substrates undergo sequential radiꢀ
cal processes to form or cleave chemical bonds,7 could be devised
to provide the driving force for the ROP of macrocyclic monoꢀ
mers.8
Due to their synthetic accessibility and propensity for the hoꢀ
molytic cleavage of the C—S bond under mild conditions, allylic
sulfides and allyl sulfones have become ubiquitous radical proꢀ
genitors in radical coupling and polymerization reactions.9 Cho et
al. first reported the radical ROP of αꢀvinyl cyclic sulfones, but
this early example still relies on the relief of ring strain as the
primary driving force.10 Rizzardo11 and Hawker12 developed a
series of (macro)cyclic allylic sulfide monomers that can undergo
radical ROP independent of the ring strain. However, in all of
these previous examples, the propagating thiyl or sulfonyl radical
To test this design, a concise route was first devised to syntheꢀ
size a “triggerꢀtesting” compound 1 (Scheme 2A). Benzaldehyde
was first coupled with methyl acrylate via the MoritaꢀBaylisꢀ
Hillman reaction.15 Upon protecting the hydroxyl group with aceꢀ
tic anhydride, this intermediate was reacted with ethyl 2ꢀ
mercaptopropanoate to afford a thioether. Final oxidation of the
thioether with metaꢀchloroperoxybenzoic acid (mCPBA) gave the
compound 1 (Scheme S1). The αꢀphenyl substitution of the allyl
sulfone was introduced to not only accelerate the βꢀelimination of
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sulfonyl radical by forming a conjugated structure, but also steriꢀ
and other trithiocarbonates (Table S2) were evaluated by their
ability to achieve control over polymerization. Among all CTAs
screened, CTA2 achieved better control over polymerization than
others, indicated by the high Mn and the low Đ of the resulting
polymer (Table 1, Entry 3). Next, polymerization in various solꢀ
vents, including DMF, dimethyl sulfoxide (DMSO), tetrahydrofuꢀ
ran (THF), dioxane, chlorobenzene, and toluene was investigated
(Table S3), and toluene was found to yield polymers with the
highest Mn among all solvents screened (Table 1, Entry 5). Invesꢀ
tigation on the effect of monomer concentration on the polymeriꢀ
zation found that increasing monomer concentration did not lead
to changes over the Mn and Đ, but reaction became sluggish at
monomer concentrations lower than 0.1 M (Table S4). Consistent
with controlled polymerization, Mn was found to be proportional
to the ratio of monomer/CTA (Table 1, Entry 6). Further imꢀ
provement of polymerization control was achieved by decreasing
the amount of AIBN (Table S5). Lastly, increasing the reaction
temperature to 100 °C was found to improve the polymerization
rate while still maintaining good control (Table S6). As such, the
1
2
3
4
5
6
7
8
cally inhibit the crosslinking and backbiting at the resulted double
bond. Next, the ability of 1 to undergo the radical cascade process
was investigated. The reaction in the presence of azobisisobuꢀ
tyronitrile (AIBN) in N,Nꢀdimethylformamide (DMF) at 60 °C
afforded the coupling product 2 in 75% isolated yield over 10
hours (Scheme 2A). Notably, the alkylsulfonyl radical byproduct
that failed to extrude SO2 was not detected throughout the reacꢀ
tion, suggesting that the alkylsulfonyl radical was shortꢀlived and
could rapidly undergo αꢀscission to form the alkyl radical.
9
Encouraged by this result, we then developed an efficient and
scalable synthesis of the macrocyclic monomers containing the
ringꢀopening trigger. Similar to the synthesis of 1, the coupling
product of benzaldehyde, tertꢀbutyl acrylate, and 2ꢀ
mercaptopropionic acid was deprotected by hydrochloric acid to
yield diacid 3, which was then cyclized in one step by reacting
with diols in a highly diluted solution (Scheme S2). In order to
investigate the RCTꢀROP of macrocyclic monomers of different
ring sizes, 1,4ꢀbutanediol, diethylene glycol, 1,6ꢀhexanediol, and
1,8ꢀoctanediol were chosen to couple with 3 to generate the correꢀ
sponding macrocyclic thioethers, which were further oxidized by
mCPBA to give macrocyclic monomers 4, 5, 6, and 7, respectiveꢀ
ly, in 4ꢀ11% overall yield (Scheme 2B). The macrocyclic strucꢀ
ture of the monomer 4 was confirmed by Xꢀray crystallography.
Notably, this short route employed inexpensive, commercially
available reagents, and up to multiꢀgram quantities of monomers
were readily obtained.
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optimal condition of the polymerization at
a
monoꢀ
mer/CTA/initiator ratio of 100:1:0.3 in toluene at 100 °C resulted
in 62% monomer conversion in two hours (Mn = 12.1 kg/mol, Đ =
1.15, Table 1, Entry 7). Higher monomer/CTA ratio of 300:1
under the same condition was found to increase polymer molecuꢀ
lar weight and Đ (Mn = 21.6 kg/mol, Đ = 1.32, Table 1, Entry 8).
The chain growth nature of the polymerization and the excellent
control are further confirmed by the linear increase of the molecuꢀ
lar weights with respect to the monomer conversion and low Đ
(Figure 1A). Notably, the linear relationship between
ln([M]0/[M]t) versus reaction time indicates a firstꢀorder kinetics
consistent with controlled polymerization, where [M]0 is the iniꢀ
tial monomer concentration and [M]t is the monomer concentraꢀ
tion at a given time (t) (Figure S7).17
Scheme 2. Synthesis of Trigger-Testing Compound 1 and
the Macrocyclic Monomers 4-7 (numbers in the structure
indicate ring size)
A. Model reaction of the trigger-testing compound
OMe
O
S
AIBN
O
O
2
OMe
Ph
Table 1. RCT-ROP of Macrocyclic Monomer 4
DMF, 60 o
C
O
EtO
OEt
- SO2
75%
O
Ph
O
1
O
O
AIBN
- SO2
O
O
S
S
S
O
R
R’
O
S
R’
O
+
O
R
S
S
S
O
O
B. Synthesis of macrocyclic monomers
O
Ph
4
Ph
chain transfer agents
n
P-4
S
OH
1) Oxalyl chloride
chain transfer agents
HOOC
mCPBA
O
O
O
O
O
O
O
Ph
S
3
S
S
2)
NC
S
OH
HO
OH
O
O
S
S
HOOC
S
S
C2H5
O
C2H5
C2H5
Ph
Ph
S
O
S
S
CTA1
CTA2
CTA3
O
Converꢀ
sionb
16
O
14
O
c
[M]/[CTA]/[initiator]a
CTA
Solvent
Mn (SEC)
Đc
13
12
O
O
O
O
O
O
O
Entry
O
O
O
O
O
O
O
S
S
S
S
O
O
O
O
O
O
O
O
Ph
Ph
Ph
4
Ph
1
2
50/0/1
50/1/1
ꢀ
DMF
DMF
86%
42%
50%
31%
76%
76%
62%
42%
9800
4300
1.70
1.17
1.20
1.32
1.21
1.29
1.15
1.32
X-ray of 4
7
5
6
CTA1
CTA2
CTA3
CTA2
CTA2
CTA2
CTA2
3
50/1/1
DMF
4900
Next, the RCTꢀROP of 4 under different reaction conditions
was examined (see supplementary information for details), with
the representative results summarized in Table 1. Consistent with
the triggerꢀtesting reaction, free radical polymerization of monoꢀ
mer 4 in the presence of the initiator AIBN in DMF at 65 °C unꢀ
der nitrogen atmosphere successfully yielded a polymer with a
number average molecular weight (Mn) by sizeꢀexclusion chromaꢀ
tography (SEC) of 9.8 kg/mol and dispersity (Đ, defined as the
ratio of weight average molecular weight to Mn) of 1.70 (Table 1,
Entry 1). With this promising result, we then focused on achievꢀ
ing control over the polymerization. Noting that secondary alkyl
radical formed after the radical cascade reaction resembled the
structure of the propagating radical of polyacrylates, we hypotheꢀ
sized that existing controlled radical polymerization techniques,
such as the reversible additionꢀfragmentation chain transfer
(RAFT) polymerization mediated by chain transfer agents
(CTAs),16 can be applied to reversibly deactivate/control the chain
propagation of this alkyl radical. CTA1ꢀ3 (Table 1, Entries 2ꢀ4)
4
50/1/1
DMF
3100
5
50/1/1
Toluene
Toluene
Toluene
Toluene
7500
6
100/1/1
100/1/0.3
300/1/0.3
13100
12100
21600
7d
8 e
a
Experimental conditions: monomer concentration [M] = 0.2 M, 65 °C under nitrogen for 15 h,
b
1
c
unless otherwise noted. Monomer conversion was determined by H NMR spectroscopy.
Molecular weight and dispersity (Đ) were determined by SEC analysis calibrated to polystyrene
d
e
standards. 100 °C, 2 h. 100 °C, 3 h.
The controlled synthesis of polymers with desired mainꢀchain
structural motifs was further confirmed by NMR and mass specꢀ
trometry analyses of P-4, the polymerization product of 4. In orꢀ
der to increase the molar fraction of the chain ends with respect to
the polymer backbone to facilitate the quantitative chainꢀend
analysis using 1HꢀNMR spectroscopy, polymerization with
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Figure 1. Controlled synthesis of homopolymers and block copolymers by RCT-ROP. (A) Plot of Mn (red) and Đ (blue) versus monꢀ
omer conversion. (B) MALDI analysis of P-4-3k. SEC analysis of block copolymers: (C) P-5ꢀbꢀP-4 and (D) P-6ꢀbꢀP-4.
reduced monomer/CTA ratio (20/1) and limited monomer conꢀ
version (49 %) was conducted, affording P-4-3k with low moꢀ
lecular weight measured by SEC (Mn(SEC) = 3.3 kg/mol). The
these are the first examples of controlled radical block copolyꢀ
merization of macrocyclic monomers. These results illustrate
the potential to incorporate diverse mainꢀchain structural motifs
by RCTꢀROP into synthetic polymers, allowing for material
properties ranging from biodegradability to selfꢀassembly to
biomimicry.
1
molecular weight of P-4-3k determined by the HꢀNMR peak
integration ratio of polymer backbone to the chainꢀend group is
3.4 kg/mol, consistent with the theoretical value based on monꢀ
omer conversion Mn(theo) = 3.1 kg/mol and Mn(SEC), indicating a
quantitative preservation of the polymer chainꢀend groups (Fig-
ure S8). Subsequent analysis of P-4-3k by matrixꢀassisted laser
desorption/ionization timeꢀofꢀflight (MALDIꢀTOF) mass specꢀ
trometry further confirmed intact chainꢀend groups of the indiꢀ
vidual oligomers of P-4-3k (Figure 1B and Figure S9).18 The
spacing between these discrete oligomers was consistent with
the expected mass of the repeating unit (288 g/mol), unambiguꢀ
ously validating the RCTꢀROP process. The high chainꢀend
fidelity highlights the capability of RCTꢀROP to generate wellꢀ
defined polymer architectures, such as block copolymers.
To demonstrate the utility of the RCTꢀROP technique in preꢀ
paring block copolymers, chain extension experiments were
studied in detail. First, RCTꢀROP of a 13ꢀmembered macrocyꢀ
clic monomer 5 yielded a macroinitiator P-5 (Mn = 7.5 kg/mol,
Đ = 1.14), to which monomer 4 was polymerized. A clear shift
to higher molecular weight was observed, yielding a diblock
copolymer P-5ꢀbꢀP-4 (Mn = 10.8 kg/mol, Đ = 1.21, Figure 1C).
Next, the ability of our approach to incorporate macrocyclic
monomers with increased ring sizes was investigated. Encouragꢀ
ingly, RCTꢀROP of macrocyclic monomer 6 (14ꢀmembered
ring) and 7 (16ꢀmembered ring) successfully yielded the conꢀ
trolled polymers P-6 (Mn = 7.2 kg/mol, Đ = 1.11) and P-7 (Mn =
8.3 kg/mol, Đ = 1.09), respectively. P-6 was chosen to be furꢀ
ther extended by monomer 4 to yield P-6ꢀbꢀP-4 (Mn = 12.8
kg/mol, Đ = 1.26, Figure 1D). To the best of our knowledge,
Figure 2. Degradation of Copolymer P10.
Despite their wideꢀspread utilities, acrylic polymers are exꢀ
tremely resistant to chemical and biological degradation proꢀ
cesses, causing serious environmental consequences.19 We enviꢀ
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(h) Ouchi, M.; Sawamoto, M. Macromolecules 2017, 50, 2603−2614.
sion that RCTꢀROP of macrocyclic monomers consisting of
hydrolytically degradable ester linkages offers a promising soluꢀ
tion to this challenge. In order to test this concept, the degradaꢀ
tion reactivity of the homopolymer P-4 was first investigated. P-
4 was treated with sodium methoxide using the conditions deꢀ
veloped by Hawker and coworkers,12 and the degradation prodꢀ
ucts 8 and 9 from the cleavage of the ester linkages were unamꢀ
biguously confirmed by NMR (Scheme S9). Next, the degradaꢀ
tion of P10, a copolymer of methyl acrylate (88 mol%) and
monomer 4 (12 mol%) (Mn = 10.2 kg/mol, Đ = 1.17, Figure
S12) was investigated (see Supplementary Information for the
synthesis and compositional analysis of the copolymer). SEC
analysis of the degradation of P10 exhibited a dramatic molecuꢀ
lar weight reduction after only 10 seconds, with the reaction
reaching completion after 10 minutes (Figure 2). These degraꢀ
dation experiments highlight the utility of RCTꢀROP to fabriꢀ
cate synthetic polymers with functional mainꢀchain structures
such as novel (bio)degradable materials.
(2) (a) Handbook of RingꢀOpening Polymerization. WileyꢀVCH Verlag
GmbH & Co.: Weinheim, Germany, 2009. (b) Strandman, S.; Gautrot, J.
E.; Zhu, X. X. Polym. Chem. 2011, 2, 791ꢀ799.
(3) There are cases in which the driving force of ROP comes not only
from the relief of the ring strain but also from the formation of a carbonꢀ
oxygen double bond or aromatization. For selected examples, see (a)
Bailey, W. J.; Ni, Z.; Wu, S. R. Macromolecules 1982, 15, 711ꢀ714. (b)
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Opening Polymerization: Molecular Designs, Polymerization
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In summary, macrocyclic monomers containing an allyl alꢀ
kylsulfone motif that can undergo radical cascadeꢀtriggered
ringꢀopening polymerization have been developed for the conꢀ
trolled synthesis of polymers with extended mainꢀchain strucꢀ
tures. Excellent control over polymerization and high chainꢀend
fidelity allows for the preparation of polymeric systems with
wellꢀdefined architectures, exemplified by the first radical block
copolymerization of macrocyclic monomers, and the incorporaꢀ
tion of degradable structural elements in polymer backbone.
Future work will investigate the generality of the radical casꢀ
cadeꢀtriggered transformations in polymer chemistry. The appliꢀ
cation of this approach to the synthesis of polymers with diverse
mainꢀchain structural motifs with tailored functions will be anꢀ
other focus of our future exploration.
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(14) Without stabilization of the resulting alkyl radical, the equilibrium
generally favors the alkylsulfonyl radical. For the reverse process to
incorporate SO2 in radical polymerization, see (a) Jiang, Y.; Fréchet, J.
M. J. Macromolecules 1991, 24, 3528ꢀ3532. (b) Possanza Casey, C. M.;
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(17) The plot deviates from the linear trend at higher conversion due to
reduced polymerization rates.
ASSOCIATED CONTENT
Supporting Information
Complete synthetic experiments, optimization tables, experiꢀ
mental methods, and additional experimental data. This material
is available free of charge via the Internet at http://pubs.acs.org
AUTHOR INFORMATION
Corresponding Author
jia.niu@bc.edu
Notes
The authors declare the following competing financial interꢀ
est(s): A provisional patent based on this work has been filed
(U.S. no. 62/684,810).
(18) Pasch, H.; Schrepp, W. MALDIꢀTOF Mass Spectrometry of
Synthetic Polymers; SpringerꢀVerlag: Berlin, 2003.
ACKNOWLEDGMENTS
(19) (a) Amass, W.; Amass, A.; Tighe, B. Polym. Int. 1999, 47, 89ꢀ144.
(b) Delplace, V.; Nicolas, J. Nat. Chem. 2015, 7, 771ꢀ784. (c) Hillmyer,
M. A.; Tolman, W. B. Acc. Chem. Res. 2014, 47, 2390ꢀ2396. For
theoretical study about degradation behaviors, see (d) Gigmes, D.; Van
Steenberge, P. H.; Siri, D.; D'hooge, D. R.; Guillaneuf, Y.; Lefay, C.
Macromol. Rapid Commun. 2018, 1800193.
We thank Marek Domin, Bo Li, Will Gutekunst, and Jeff Byers
for characterization assistance and helpful discussions. The
research is supported by a startup fund from Boston College to
J.N.
REFERENCES AND NOTES
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