A versatile polypeptoid platform based on N-allyl glycine

Article abstract of DOI:10.1039/c2cc33881e

N-Allyl glycine N-carboxyanhydride (NCA) was synthesized and polymerized by ring opening polymerization under homo- or heterophase conditions to afford well-defined polypeptoids (M_W = 1.5-10.5 kg mol^-1, PDI = 1.1-1.4). Poly(N-allyl glycine) demonstrated stimuli-responsive behaviour in water and was readily modified via thiol-ene photochemistry under experimentally benign conditions. This journal is

Full text of DOI:10.1039/c2cc33881e

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A versatile polypeptoid platform based on N-allyl glycinew
Joshua W. Robinson and Helmut Schlaad*
Received 30th May 2012, Accepted 18th June 2012
DOI: 10.1039/c2cc33881e
N-Allyl glycine N-carboxyanhydride (NCA) was synthesized
and polymerized by ring opening polymerization under homo- or
heterophase conditions to afford well-defined polypeptoids
(MW = 1.5–10.5 kg molꢀ1, PDI = 1.1–1.4). Poly(N-allyl glycine)
demonstrated stimuli-responsive behaviour in water and was
readily modified via thiol–ene photochemistry under experimentally
benign conditions.
Recently, Luxenhofer et al. provided a detailed report of a
series of peptoid homo- and block copolymers.29 Due to our
interest in ‘‘smart’’ bioinspired, or biohybrid, polymers
(in particular glycopolymers), we rationalized that poly-
(N-allyl glycine), as a regioisomer of poly(a-allyl glycine)30
and a structural isomer of poly[2-(3-butenyl)-2-oxazoline],31
would be an interesting initial target.
A caveat of our approach includes the post modification of
the side chains as opposed to the synthesis of a complex
monomer prior to polymerization.32,33 Although post modifi-
cations often suffer from incomplete functionalization and
poor repeating unit selectivity, they greatly enable the user
to screen a number of hydrophobic, hydrophilic, and binding
moieties for increased solubility and receptor interaction from
one platform. The advancements in polymer click chemistry
(e.g. azide–alkyne [3+2], (hetero-) Diels–Alder [4+2] cyclo-
additions, thiol–ene/yne additions, etc.) have been shown to
overcome such problems and facilitate high conversions.34,35
The N-allyl glycine NCA monomer was synthesized starting
from allylamine (1) and a-bromo ethylacetate (2), as outlined
in Scheme 1 (see ESIw for experimental details). Steps i and ii
were adapted from a procedure disclosed by Liskamp et al.36
and step iii was modified from a protocol by Zhang et al.27
(Leuchs’ method) affording us the N-allyl glycine NCA
(3-allyloxazolidine-2,5-dione, 5) in high purity as a colourless
oil with an overall yield of 58% (step i - iii) (ESIw). This was
a straightforward three-step synthesis that required simple
purification techniques such as flash chromatography, liquid–
liquid extraction, and vacuum distillation, respectively (ESIw).
However, during our early distillation attempts for the crude
NCA 5, we also isolated 1,4-diallylpiperazine-2,5-dione (6)
with a yield of 31% (step iv). Although cyclic peptides and
peptoids have been previously reported,23 the heat-induced
formation of 6 is still unclear.
Pseudo-peptides and peptidomimetics,1 such as b-peptides,
peptoids, and also poly(N-acyl ethyleneimine)s, have been, and
are currently being, investigated for their stimuli-responsiveness,
self-assembly, and/or formation of hierarchical structures.2–4
Investigations are being driven forward by their respective
potential in biomedical applications, such as therapeutics
(i.e. drug delivery systems)5 and tissue engineering (i.e. extra-
cellular matrices).6 Intuitively, these polymers, constructed with
amides in the backbone and/or side chain, allow for an increase in
biocompatibility. Particularly, sequence and non-sequence specific
N-substituted glycine oligomers (aka a-peptoids) have been
demonstrated to resist enzymatic degradation7 while maintaining
prescribed biological activity.8 Furthermore, peptoids have been
shown to form stable polyproline type I helices9 and self-
assemble.10,11 Additional application based investigations have
included DNA drag tags,12 nanosheets,13 cell permeation
labels,14,15 and calcium carbonate mineralization.16
A straightforward and efficient submonomer solid-phase
synthetic technique was developed17,18 and improved19,20 for
these peptide regioisomers. More recently, a gram-scale
solution-phase synthesis was described, which closely resembles
the submonomer approach with repetitive rotovaporizations.21
A more frequently utilized solution phase synthesis that leads
to well-defined polypeptide materials includes the ring opening
polymerization (ROP) of a-amino acid N-carboxyanhydrides
(NCAs).22 Previously, these techniques were adapted by
Kricheldorf et al. for N-methyl glycine (sarcosine) during their
extensive investigations into the active monomer and zwitter-
ion mechanisms of N-unsubstituted NCAs.23–26 Zhang et al.
described the first bottom up approach of N-substituted NCAs
that were polymerized via an N-heterocyclic carbene (NHC).27,28
Polymerizations of 5 (Scheme 2, step (i)) were carried out at
concentrations of 1–2 wt% in CH2Cl2, N,N0-dimethylacetamide
Max Planck Institute of Colloids and Interfaces, Department of
Colloid Chemistry, Research Campus Golm, 14424 Potsdam,
Germany. E-mail: schlaad@mpikg.mpg.de; Fax: +49 331 5679502;
Tel: +49 331 5679514
w Electronic supplementary information (ESI) available: Experimental
procedures, 1H/13C NMR, FT-IR, GC-MS, SEC, DSC/TGA, and
turbidity data. See DOI: 10.1039/c2cc33881e
Scheme 1 Synthesis of N-allyl glycine NCA. (i) NEt3, THF, 0 1C;
(ii) (a) Boc2O, dioxane, (b) NaOH, MeOH; (iii) PCl3, CH2Cl2, 0 1C;
(iv) >100 1C.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 7835–7837 7835

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Scheme 2 Synthesis and functionalization of poly(N-allyl glycine).
(i) (a) BnNH2, r.t., (b) Ac2O; (ii) R-SH, DMPA, hn, r.t.
(DMA), or benzonitrile (PhCN) in sealable flasks (where reduced
pressure techniques were employed)37 and initiated by a 1 M
solution of benzyl amine (BnNH2) in N-methyl-2-pyrrolidone
(NMP; targeted number of repeat units, n: [5]0/[BnNH2]0 =
20, 100) at room temperature.38 Reactions were terminated
with acetic anhydride when either consumption of the mono-
mer was determined or no further progress was observed by
GC-MS. The molecular characteristics of the isolated polymer
samples 7a–c are summarized in Table 1. In general, the
monomer was consumed over 2–5 days for oligomers (n o 25)
and 3–4 weeks for polymers (n B 50) in CH2Cl2. Further analysis
indicated that the monomer was not fully incorporated into the
polymer chain ([5]0/[BnNH2]0 = 100) suggesting that degradation
occurred in CH2Cl2 (data omitted). In contrast, the monomer was
found to be stable over the duration for which the polymerizations
ran in DMA or PhCN. As expected from earlier reports of
Luxenhofer et al.,29 the sample with the highest molecular weight
Fig. 1 MALDI-ToF mass spectra (matrix: DCTB, Na+) of (a)
poly(N-allyl glycine) 7d and (b) derived glucosylated polyglycine 8B.
stagnant heptanes, and (iii) directly into the monomer oil. The
reactions were terminated 12 hours later (before full conversion
of NCA was reached, GC-MS) with the addition of acetic
anhydride following the aforementioned approach. The iso-
lated polymers 7e–g had similar molecular weights (n B 100,
%
1H NMR) and PDIs o 1.4 (SEC) (Table 1). Procedure (i)
appears to be the best suited to produce high molecular weight
polymers with a low PDI (B1.1; 7e).
Poly(N-allyl glycine)s exhibit glass transition temperatures (Tg)
in the range of 50–60 1C (DSC, Table 1) and are thermally stable
up to 200–300 1C (thermogravimetric analysis, TGA) (ESIw). In
general, the polymers were soluble in organic solvents such as
MeCN, PhCN, DMF, DMA, NMP, methanol, ethanol, THF,
CH2Cl2, and CHCl3 (ESIw). Furthermore, low molecular weight
(n B 80) was obtained in PhCN (DMA: n B 34). Due to these
%
%
results, a polymerization of 5 on a larger scale (5 g) was conducted
in PhCN (10 wt%) targeting a polymer with 20 repeating units.
After 4 days, the monomer was no longer detectable by GC-MS,
the propagating end group was deactivated with acetic anhydride
(Ac2O), and the polymer (7d) was isolated by precipitation
in petroleum ethers and centrifugation. Gravimetric yield was
samples (n o 20) are soluble in water (B 1% w/w) and show
%
lower critical solution (LCST) behaviour. The cloud point
temperature (Tcp) of 7d was 32 1C at 0.1 wt% in water (ESIw).
For conceptual purposes, thiol–ene additions (Scheme 2,
quantitative and 1H-NMR end group analysis indicated n B 18.
%
In addition, this sample’s chemical structure was exemplarily
confirmed by MALDI-ToF MS (Fig. 1a: mass of repeat unit,
Dm = 97.2 Da ([C5H7NO]+ = 97.1 g molꢀ1); residual mass,
step ii) were carried out on poly(N-allyl glycine)s 7d (n B 18)
%
and 7g (n B 107) with 1-thioglycerol (A) (- samples 8A and
%
8A0, respectively, Table 2). Additionally, 7d was post modified
with 1-thio-b-D-glucose tetraacetate (HS-GlcAc4, B - 8B)
and 1-thio-b-D-glucose (HS-Glc, C - 8C and 8C0). In general,
polymers were dissolved at 10% w/w in DMF, or 1% w/w
in H2O (solubility restrictions), with the respective thiol
([SH]0/[CQC]0] B 1.2) and stirred for 10 minutes prior to
the addition of a photo initiator (2,2-dimethoxy-2-phenyl-
acetophenone (DMPA) in DMF; 2-hydroxy-40-(2-hyroxy-
ethoxy)-2-methylpropiophenone (HEMP) in H2O; o 5 mol%),
where the combined mixture was then irradiated with UV light
(Hg medium pressure lamp) for 12 or 24 hours, respectively.
Modest to quantitative degrees of side arm modifications
were readily achieved (Table 2), as suggested by 1H NMR
analysis of the products in DMF-d7 (ESIw). Exemplarily, the
r.m. = 149.1 (BnNH/Ac, C H NO); n B 20).
%
9
11
We further attempted ROP of the NCA under heterophase
conditions. A mixture of 5 (0.6 mL) and non-solvent heptanes
(30 mL; C7H16),39 previously degassed with argon, was initiated
with a 1 M solution of BnNH2/NMP ([5]0/[BnNH2]0 = 400) by
injecting into (i) a rapid stirring solution, (ii) the layer of
Table 1 Molecular characteristics of poly(N-allyl glycine)s obtained
under different reaction conditions
e
g
[5]0/
Reaction Yield
Mn
Tg
Sample Solvent [BnNH2]0 time (d) (%) nd (kg molꢀ1) PDIf (1C)
7a
7b
7c
7d
7e
7f
CH2Cl2 20
DMA 100
PhCN 100
PhCN 20
4
72 14 1.5
35 34 3.4
59 80 7.9
>99 18 1.9
1.16 —
1.23 50
1.10 54
1.07 55
1.10 51
1.33 60
1.39 60
15
28
4
0.5
0.5
0.5
Table 2 Thiol–ene modification of poly(N-allyl glycine)s
a
C7H16 400
C7H16 400
n/a
98 9.7
b
Reaction
Initiator Solvent time (d) ma
n/a 105 10.3
n/a 107 10.5
c
C7H16 400
Sample Precursor Thiol
7g
a
Heterogeneous phase with initiator added to the high stirring
b
solution. Heterogeneous phase with initiator added to the stagnant
8A
8A0
8B
8C
8C0
7d
7g
7d
7d
7d
HS-glycerol DMPA DMF 0.5
HS-glycerol DMPA DMF
HS-GlcAc4 DMPA DMF 0.5
>0.99
1
0.94
>0.99
>0.99
0.57
c
heptanes. Heterogeneous phase with initiator added directly into the
monomer layer. Average number of repeat units (1H NMR). Number-
d
e
HS-Glc
HS-Glc
HEMP H2O
None H2O
1
1
average molecular weight (1H NMR). f Apparent polydispersity index
Degree of modification (1H NMR) (ESI).
g
(SEC). Glass transition temperature (DSC) (ESI).
a
c
7836 Chem. Commun., 2012, 48, 7835–7837
This journal is The Royal Society of Chemistry 2012

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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 7835–7837 7837