A ring to rule them all: A cyclic ketene acetal comonomer controls the nitroxide-mediated polymerization of methacrylates and confers tunable degradability

Article abstract of DOI:10.1039/c5cc04610f

2-Methylene-4-phenyl-1,3-dioxolane (MPDL) was successfully used as a controlling comonomer in NMP with oligo(ethylene glycol) methyl ether methacrylate (MeOEGMA) to prepare well-defined and degradable PEG-based P(MeOEGMA-co-MPDL) copolymers. The level of

Full text of DOI:10.1039/c5cc04610f

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A ring to rule them all: a cyclic ketene acetal
comonomer controls the nitroxide-mediated
polymerization of methacrylates and confers
tunable degradability†
Cite this:DOI: 10.1039/c5cc04610f
Received 4th June 2015,
Accepted 6th July 2015
Vianney Delplace,a Elise Guegain,a Simon Harrisson,b Didier Gigmes,c
´
DOI: 10.1039/c5cc04610f
Yohann Guillaneufc and Julien Nicolas*a
www.rsc.org/chemcomm
2-Methylene-4-phenyl-1,3-dioxolane (MPDL) was successfully used controlling comonomer with favorable kinetic parameters
as a controlling comonomer in NMP with oligo(ethylene glycol) (i.e., low K and low cross-propagation rate constant) such as
methyl ether methacrylate (MeOEGMA) to prepare well-defined and styrene (S),4 acrylonitrile (AN)5 or 9-(4-vinylbenzyl)-9H-carbazole.6
degradable PEG-based P(MeOEGMA-co-MPDL) copolymers. The level Most polymer chains were then shown to contain a unit of
of ester group incorporation is controlled, leading to reductions in the controlling comonomer in the terminal position, which
molecular weight of up to 95% on hydrolysis. Neither the polymer explained the high proportion of living chains.4b
nor its degradation products displayed cytoxicity. The method was
also successfully applied to methyl methacrylate.
The flexibility and robustness offered by RDRP have led to
a recent surge in the design of innovative and sophisticated
vinyl monomer-based materials intended to find applications
Reversible deactivation radical polymerization (RDRP) enables in different bio-related areas such as drug delivery or tissue
the synthesis of well-defined, complex and functional macro- engineering.7 However, because their carbon–carbon backbones
molecular architectures.1 Nitroxide-mediated polymerization resist degradation, these materials may cause prohibitive toxicity,
(NMP)1c is perhaps the simplest RDRP technique as it does which will hamper their translation to clinical settings and
not require addition of a metal catalyst or radical initiator, but eventually to the market. On this basis, innovative strategies to
relies on reversible thermal activation of a dormant alkoxyamine confer different levels of (bio)degradability to vinyl polymers have
end-functionality to form propagating radicals and a stable emerged. Radical ring-opening polymerization (rROP) of cyclic
nitroxide free-radical. Although early drawbacks of NMP (e.g., ketene acetals (CKAs) is one of the most efficient strategies
requirement for high temperatures, applicability to a limited for the incorporation of degradable groups in the polymer back-
number of monomers), have been substantially overcome by the bone, enabling its complete degradation.8 Recent years have seen
development of second-generation nitroxides such as SG1,2 the a resurgence of interest in CKAs, which may be explained by their
control of methacrylic esters is still a challenge. The high activation- ability to copolymerize with traditional vinyl monomers by both
deactivation equilibrium constant (K) of SG1 results in a high conventional free-radical polymerization and RDRP techniques.
concentration of propagating radicals, favoring the occurrence of The versatility of this approach has been illustrated by the
irreversible termination reactions.3 The polymerization stops at low synthesis of a variety of different copolymer structures,9 which
conversion and highly disperse polymers of uncontrolled molecular are intended for biomedical applications.
weight are obtained. Greatly improved control can be obtained
Herein, we report the discovery that 2-methylene-4-phenyl-
by adding a small amount (typically 2–9 mol%) of a suitable 1,3-dioxolane (MPDL), a scarcely studied CKA,10 both acts as a
controlling monomer for the NMP of methacrylic esters and
confers tunable degradability to the resulting copolymer, leading to
a
´
Institut Galien Paris-Sud, Universite Paris-Sud, UMR CNRS 8612,
non-cytotoxic degradation products (Scheme 1). This was applied to the
SG1-mediated polymerization of methyl methacrylate (MMA) and
oligo(ethylene glycol) methyl ether methacrylate (MeOEGMA); the latter
being a widely-used monomer for the preparation of non-degradable,
PEG-based polymers with potential biological applications.9g,11 The
choice of NMP is particularly relevant for the preparation of polymer
biomaterials as it has been shown that both SG1-terminated polymers
and the SG1 nitroxide itself displayed no cytotoxicity over three different
mammalian cell lines even at high doses.5b
´
´
ˆ
Faculte de Pharmacie, 5 rue Jean-Baptiste Clement, F-92296 Chatenay-Malabry
cedex, France. E-mail: julien.nicolas@u-psud.fr; Web: www.twitter.com/julnicolas
b
c
´
´
Universite de Toulouse, CNRS, Laboratoire des Interactions Moleculaires et
´
´
Reactivite Chimique et Photochimique UMR 5623, 118 route de Narbonne,
F-31062 Toulouse, France
´
Aix-Marseille Universite, CNRS, Institut de Chimie Radicalaire UMR 7273,
Avenue Escadrille Normandie-Niemen, F-13397 Marseille cedex 20, France
† Electronic supplementary information (ESI) available: Experimental details,
SEC chromatograms, additional kinetics, 1H and 31P NMR spectra, determination
of the reactivity ratios. See DOI: 10.1039/c5cc04610f
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Scheme 1 Synthesis, degradation and cytotoxicity of poly[(oligo(ethylene
glycol) methyl ether methacrylate)-co-(2-methylene-4-phenyl-1,3-dioxolane)]
(P(MeOEGMA-co-MPDL)) prepared by NMP.
MPDL is a 5-membered ring CKA which is easier to synthesize
than the more widely used CKAs, 2-methylene-1,3-dioxepane (MDO)12
and 5,6-benzo-2-methylene-1,3-dioxepane (BMDO).13 We recently
used MPDL to prepare degradable P(MeOEGMA-co-AN-co-MPDL)
terpolymers.9a,14 Analysis by 31P NMR spectroscopy highlighted
the competition between AN and MPDL to be the terminal
monomer unit, with most copolymer chains terminated by
MPDL-SG1 sequences.14 This set us thinking: could MPDL itself
act as a controlling comonomer for the NMP of methacrylic
esters and at the same time confer tunable degradability to the
resulting copolymers?
MPDL was obtained after a multigram-scale synthesis (see
ESI† and Fig. S1). The polymerization of MeOEGMA with a
variable amount of MPDL ( fMPDL,0 = 0–0.7) was initiated by the
BlocBuilder alkoxyamine at 90 1C in 50 wt% toluene without
additional SG1 (Fig. 1 and Table S1, expts 1–4, ESI†). As expected,
the homopolymerization of MeOEGMA (fMPDL,0 = 0) was initially
very fast but rapidly stopped, yielding polydisperse PMeOEGMA
Fig. 1 NMP of MeOEGMA and MPDL in toluene initiated by the Bloc-
Builder alkoxyamine at 90 1C, as a function of the initial amount of MPDL:
E, expt. 1 (fMPDL,0 = 0); m, expt. 2 (fMPDL,0 = 0.2); K, expt. 3 (fMPDL,0 = 0.4);
’, expt. 4 (fMPDL,0 = 0.7). (a) Ln[1/(1 À conv.)] vs. time (conv. = MeOEGMA
conversion). (b) Number-average molar mass, Mn, and dispersity, Mw/Mn,
vs. conversion. The full line represents the theoretical Mn (MeOEGMA) and
the dashed ones are guides for the eye only.
(Mn = 12500 g molÀ1, Ð = 1.72) with no control over the polymerization (Fig. 1b). The evolution of molecular weight
molecular weight. When 20 mol% of MPDL was added in the distribution with time further illustrated the controlled nature
comonomer feed, the situation significantly changed. Two of the copolymerizations (Fig. S2, ESI†). Two independent
kinetic regimes were observed. The polymerization was rapid batches of MPDL demonstrated very good reproducibility in
within the first 2 h before slowing dramatically, likely due to terms of kinetics and control, confirming the reliability of this
the insufficient amount of MPDL units inserted to prevent irrever- strategy (Fig. S3, ESI†). The copolymers were purified after 30 h
sible termination. Some evolution of the Mn with the conversion by precipitation in cold diethyl ether, giving Mn = 14700 g molÀ1
was however noticed, matching quite well the calculated ones, and Ð = 1.55 for expt. 2, Mn = 20 100 g molÀ1 and Ð = 1.44 for
with substantial improvement over the dispersities. Ð reached expt. 3, and Mn = 24500 g molÀ1 and Ð = 1.30 for expt. 4.
1.54 at 44% conversion for a 13300 g molÀ1 P(MeOEGMA-co-
1H NMR spectroscopy of the purified P(MeOEGMA-co-
MPDL) copolymer. At fMPDL,0 = 0.4, more rapid establishment of MPDL) copolymers (expts 2–4) showed all signals expected for
first order kinetics with a greater slope over 10 h were observed, a P(MeOEGMA-co-MPDL) structure (Fig. S4, ESI†). The aromatic
presumably due to earlier insertion of MPDL units. This amount protons of the phenyl group of MPDL (signal e) were clearly
of MPDL was perhaps not enough to prevent the occurrence of visible and confirmed that its level of incorporation in the
irreversible termination reactions in the long run, as seen by the copolymer could be adjusted by varying its initial concentration in
deviation from linearity at 24 h. Nevertheless a linear evolution of the comonomer feed. The preferential formation of the MPDL-SG1
Mn with monomer conversion between 7000 and 15 000 g molÀ1
,
terminal sequence was confirmed by 31P NMR spectroscopy as
and lower Ð values (i.e., 1.42–1.53) were observed. A further its spectrum is very similar to that of PS-SG1 (Table S1, expt. 5,
increase in the initial amount of MPDL (fMPDL,0 = 0.7) resulted ESI†), due to the styrene-like ring-opened structure of MPDL
in immediate establishment of first order kinetics up to high (Fig. S5, ESI†). This analogy in structures between the open
monomer conversion (B70%) and linear evolution of Mn over a radical forms of S and MPDL explains the ability of MPDL to act
wide range of values (B4400–27 000 g molÀ1). Dispersity was as a controlling comonomer. The copolymerization was also
also improved as it reached 1.31 at 39% monomer conversion. successfully performed with MMA, a representative methacrylate
In fact, the higher fMPDL,0, the lower the dispersities during the monomer (Table S1, 6, expt. ESI†). Under similar experimental
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conditions, a well-defined P(MMA-co-MPDL) copolymer (conv.
MMA (15 h) = 40%, Mn = 10 200 g molÀ1, Ð = 1.20) was prepared
(Fig. S6, ESI†). 31P NMR spectroscopy performed in the presence
of a known amount of diethyl phosphite as internal reference
gave a living chain fraction as high as 85% (Fig. S7, ESI†), which
is similar to those obtained using S or AN as a comonomer, thus
establishing the efficiency of MPDL as a controlling comonomer.
The molar fraction of MPDL in the copolymer, FMPDL, with
respect to MeOEGMA was determined by 1H NMR spectroscopy
to be 3.6, 11.3 and 24.8 mol% for expt. 2 (fMPDL,0 = 0.2), expt. 3
(fMPDL,0 = 0.4) and expt. 4 (fMPDL,0 = 0.7), respectively. As for the
vast majority of CKAs, unfavorable reactivity ratios impose high
concentrations of MPDL in the comonomer feed in order to
achieve substantial incorporation in the resulting copolymer.
The reactivity ratios were determined by fitting the integrated
form of the copolymer composition equation to the monomer
feed composition vs. conversion data using the visualization of
the sum of squares method,15 assuming non-negligible errors
in both variables. This procedure gave a point estimate of
6.95 for rMeOEGMA and 0 for rMPDL, with a 95% joint confidence
region spanning the range of 6.3–7.9 for rMeOEGMA and 0–0.038
for rMPDL (Fig. S8, ESI†). The values obtained are typical for
copolymerizations of methacrylate monomers with ring-opening
monomers such as MDO (rMMA = 34, rMDO = 0.057),16 1,1-dichloro-
2-vinylcyclopropane (rMMA = 11, rVCP = 0.07)17 and 7-methylene-2-
methyl-1,5-dithiacyclooctane (rMMA = 6.3, rMDTO = 0.52),18 and
reflect the lack of radical-stabilizing substituents on the double
bond of the CKA.
Fig. 2 Hydrolytic degradation in 5% KOH of P(MeOEGMA-co-MPDL) as
a function of the content in MPDL: E, expt. 1 (FMPDL,0 = 0); m, expt. 2
(FMPDL,0 = 0.036); K, expt. 3 (FMPDL,0 = 0.113); ’, expt. 4 (FMPDL,0 = 0.248).
(a) Evolution of the number-average molar mass, Mn, with time. Dashed
lines are guides for the eye only. (b) Evolution of the SEC chromatograms
at different time for expt. 4 during degradation.
To assess the degradability, hydrolytic degradation of the
copolymers was performed in accelerated conditions (i.e., 5%
KOH aqueous solution) and monitored over a period of 24 h
to estimate the kinetics of hydrolysis (Fig. 2a). Adjustable thus confirming the near-complete hydrolysis of the main–
incorporation of MPDL in the copolymer enabled fine tuning chain ester groups.
of the level of degradation, which typically reached its steady
Conferring PEG-based polymers with the ability to be degraded
state after 5 h. While the copolymer with the lowest amount of in a hydrolytic environment is a significant advance compared to
MPDL (expt. 2) led to modest degradation (B30% decrease in traditional linear or comb-like PEGs that are resistant to degrada-
Mn for FMPDL = 0.036), significant (even complete) degradation tion. However, neither the degradation products nor the polymer
was observed for greater amounts of MPDL. When FMPDL = 0.113 itself should be cytotoxic in an optimized product. This point
(expt. 3), the decrease in Mn was 81% whereas it reached 95% for is crucial as it will determine the ultimate biocompatibility of
FMPDL = 0.248 (expt. 4). The complete hydrolysis of the latter was the materials. The potential toxicity of different copolymers
confirmed by size exclusion chromatograms which shifted has been tested on two representative mammalian cell types:
toward lower molar masses (Fig. 2b). Note that the polymer murine fibroblasts (NIH/3T3) and murine macrophages (J774.A1).
without MPDL (expt. 1) was perfectly stable, thus ruling out Whereas NIH/3T3 cells are one of the most commonly used
reduction in molecular weight due to ester side chain hydrolysis. fibroblast cell lines, J774.A1 cells, which play a key role in
The theoretical decrease in Mn of these copolymers can be phagocytosis, were chosen to highlight possible toxicity of the
calculated from the average sequence length of PMeOEGMA copolymers after being engulfed by macrophages. The cytotoxi-
blocks in each copolymer. This can be directly estimated from city was evaluated in vitro by MTT assay at two copolymer
the copolymer composition, as due to the very low rMPDL, the concentrations (0.1 and 1 mg mLÀ1) (Fig. 3). Incubation of undeg-
probability of finding a sequence of two or more MPDL units is raded P(MeOEGMA-co-MPDL) copolymer (expt. 3, FMPDL,0 = 0.113)
negligible. From the MPDL content of the polymers it corre- with both cell lines resulted in 495% cell viability at 0.1 mg mLÀ1
.
sponds to, on average, 1 MPDL unit for every 26.8 (expt. 2), At 1 mg mLÀ1 cell viabilities were still high; 83% with NIH/3T3
7.8 (expt. 3) and 3.0 (expt. 4) MeOEGMA units. These lengths cells and 72% with J774.A1 cells. More importantly, MTT assays
correspond to Mn of 8210, 2530 and 1080 g molÀ1, respectively. revealed no cytotoxicity from the degradation products whatever
These values are in rather good agreement with those obtained the cell line and the copolymer concentration. Cell viabilities were
for the degraded polymers (12 600, 3500 and 800 g molÀ1
,
in the 97–99% range at 0.1 mg mLÀ1 and 86–92% at 1 mg mLÀ1
.
respectively), especially that with the highest amount of MPDL, These results indicate the absence of obvious toxicity from the
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Notes and references
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studied CKA, can both act as a controlling comonomer for the
SG1-mediated polymerization of methacrylic esters and confer
tunable degradability to the resulting copolymer. By adjusting
the comonomer feed, up to complete degradation was observed. No
significant toxicity was shown, either from P(MeOEGMA-co-MPDL)
or from its degradation products. Not only does this broaden
the range of suitable controlling comonomers for the NMP of
methacrylic esters, but it adds significant value to the copolymeri-
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The manuscript was written through contributions of all
authors. The authors thank the French National Research Agency
(ANR-11-JS08-0005) for the financial support of the PhD thesis of
VD and the French Ministry of Research for the financial support of
the PhD thesis of EG. Arkema is warmly acknowledged for kindly
providing the BlocBuilder MA alkoxyamine and the SG1 nitroxide.
CNRS is also acknowledged for financial support.
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