N-trimethylsilyl amines for controlled ring-opening polymerization of amino acid N-carboxyanhydrides and facile end group functionalization of polypeptides

Article abstract of DOI:10.1021/ja803304x

We report a new strategy that uses N-trimethylsilyl (N-TMS) amine to mediate controlled ring-opening polymerization of amino acid N-carboxyanhydrides (NCAs). This polymerization proceeds via a unique, trimethylsilyl carbamate (TMS-CBM) propagating group t

Full text of DOI:10.1021/ja803304x

Published on Web 09/03/2008
N-Trimethylsilyl Amines for Controlled Ring-Opening Polymerization of Amino
Acid N-Carboxyanhydrides and Facile End Group Functionalization of
Polypeptides
Hua Lu and Jianjun Cheng*
Department of Materials Science and Engineering, UniVersity of Illinois at Urbana-Champaign,
Urbana, Illinois 61801
Received May 4, 2008; E-mail: jianjunc@uiuc.edu
Polypeptides have been extensively utilized in drug delivery,1
Scheme 1
tissue engineering,2 sensing,3 and catalysis.4 To prepare polypep-
tides for these applications, it is essential to control their molecular
weights (MWs)5-10 as well as their end groups8,11-13 during the
ring-opening polymerizations (ROPs) of amino acid N-carboxyan-
hydrides (NCAs).14 We recently reported hexamethyldisilazane
(HMDS)-mediated, controlled NCA polymerization.15 This polym-
erization proceeds via a unique, trimethylsilyl carbamate (TMS-
CBM) propagating group (Scheme 1a), which involves cleavage
of the Si-N bond of HMDS during the initiation step. The resulting
TMS-amine (red, Scheme 1a) opens the NCA ring at its CO-5
position to form a TMS-amide at the C-end while the TMS group
(blue, Scheme 1a) is attached to the N-end to form a TMS-CBM
(the propagating chain end). The propagation of polypeptide chains
proceeds through the transfer of the TMS group from the terminal
TMS-CBM to the incoming monomer and forms a new TMS-CBM
propagating chain end (Scheme 1a). We postulate that when a
N-TMS amine is used as the initiator, cleavage of its Si-N bond
will generate an amine and a TMS group (Scheme 1b) that will
subsequently form the corresponding amide at the C-end and a
TMS-CBM at the N-end after NCA ring opening (Scheme 1b).
Thus, chain propagation should proceed in the same manner as
HMDS-mediated polymerization (Scheme 1a). Because a large
variety of N-TMS amines are readily available, this method should
allow facile functionalization of the C-termini of polypeptides
(Scheme 1b).
and narrow MWDs via sequential addition of Lys- and Glu-NCA
(Table S1 and Figure S2).
We next compared the polypeptides obtained through 1-initiated
and 1-TMS-initiated Glu-NCA polymerizations. The PBLG result-
ing from 1-initiated polymerization at an M/I ratio of 20 had a
broad MWD (Mw/Mn ) 1.52) based on the MALDI-TOF MS
analysis (red, Figure 1c). In contrast, a similar polymerization
mediated by 1-TMS resulted in PBLG with a much narrower MWD
(Mw/Mn ) 1.17). The MALDI-TOF MS spectrum of the latter
polymer showed a Poisson distribution centered at the exact
To demonstrate this concept, N-TMS allylamine (1-TMS, 1 )
allylamine, Scheme 1c) was utilized as the initiator for the
polymerization of γ-benzyl-L-glutamate NCA (Glu-NCA) (Scheme
1b). As shown in Figure 1a, 1-TMS had remarkable control of Glu-
NCA polymerization and gave poly(γ-benzyl-L-glutamate) (PBLG)
with the expected MWs and narrow MWDs over a broad range of
monomer/initiator (M/I) ratios (M/I ) 20-300). The obtained Mn’s
of PBLG at an M/I ratio of 20 and 300 were 4.6 × 103 and 7.01
× 104 g/mol, respectively, both of which were nearly identical to
the expected Mn’s (4.4 × 103 and 6.57 × 104 g/mol, respectively).
All polymerizations were finished within 12-24 h at room
temperature under atmospheric pressure, in contrast to a few recently
reported controlled polymerizations that require either reduced
temperature10 or vacuum.9 The Mn’s of PBLG showed a linear
correlation with the conversions of Glu-NCA, which were in good
agreement with the expected Mn’s (Figure 1b). This experiment
demonstrated that the propagation of PBLG chains proceeded
through a living chain end. 1-TMS also showed remarkable control
of polymerizations of ε-Cbz-L-lysine NCA (Lys-NCA) and resulted
in poly(ε-Cbz-L-lysine) (PZLL) with the expected MWs and very
narrow MWDs (Figure S1). Block copolypeptides, such as PZLL-
b-PBLG, can also be readily prepared with the anticipated MWs
Figure 1. (a) MW and MWD of the PBLG prepared via 1-TMS-mediated
ROP of Glu-NCA at various M/I ratios. (b) MW and MWD of the PBLG
prepared via 1-TMS-mediated ROP of Glu-NCA at an M/I ratio of 100
relative to the conversion of Glu-NCA. (c) MALDI-TOF MS spectra of
the PBLG prepared via 1-TMS (blue)- and 1 (red)-mediated Glu-NCA
polymerization at an M/I ratio of 20. The number on the MS spectra denotes
the number of repeating units of PBLG (n). (d) GPC curve of the PBLG
prepared via 1-TMS (blue)- and 1 (red)-mediated Glu-NCA polymerization
at an M/I ratio of 50, respectively.
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10.1021/ja803304x CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Table 1. N-TMS Amines for Glu-NCA Polymerization (M/I ) 100)
Figure 2. ESI-MS of equal molar mixture of 1-TMS and Lys-NCA.
theoretical MW of the PBLG containing the anticipated allyl amide
end group (MW ) allylamine + (Glu)20) (blue, Figure 1c). This
MS distribution pattern differed dramatically from that of the former
PBLG (red, Figure 1c). The bimodal GPC curve of the former
PBLG (red, Figure 1d) compared to the monomodal curve of the
latter PBLG (blue, Figure 1d) further demonstrated the improved
control of polymerization through subtle modification of the amine
initiator.
via the “activated monomer mechanism” is eliminated since TMS-
CBM is unable to deprotonate the NH-3. Chain propagation thus
can only proceed through the ring opening at the CO-5 position of
the NCA, resembling ammonium-mediated NCA polymerization,
but occurring much faster.7
To confirm the formation of TMS-CBM during 1-TMS-mediated
NCA polymerization, we mixed equal molar Lys-NCA and 1-TMS
in DMSO-d6 and then analyzed the mixture using ESI-MS. As
expected, 1-Lys-TMS-CBM (iii, m/z 436.5) was successfully
identified as the dominating component in the mixture (Figure 2).
This experiment demonstrated cleavage of the Si-N bond of
1-TMS and subsequent NCA ring opening by 1 at CO-5 and
formation of TMS-CBM (Scheme 1b). When the MS experiment
was performed under anhydrous conditions, iii and its decomposed
derivatives ii and i were detected (Figure 2). However, when the
reaction mixture was exposed to air or when D2O was added, only
i was detected (data not shown). These observations were in
agreement with the expected moisture sensitivity of TMS-CBM.15
Formation of TMS-CBM during the initiation step was further
confirmed by 13C NMR (Figure S3).
In conclusion, N-TMS amines can initiate controlled NCA
polymerizations and allow facile functionalization at the C-termini
of polypeptides. Polymerizations initiated by N-TMS amines are
fast, give quantitative monomer conversion, and do not require
excessive monomer purification.9 This methodology is useful for
controlled synthesis of functional polypeptides, polypeptide mac-
romonomers, and polypeptide copolymers.
Acknowledgment. This work was supported by the National
Science Foundation No. CHE-0809420, the American Chemical
Society Petroleum Research Fund, and partially by the Siteman
Center for Cancer Nanotechnology Excellence (SCCNE)-Center for
Nanoscale Science and Technology (CNST, University of Illinois
at Urbana-Champaign). We thank Professors Martin Burke and
Jeffrey Moore for providing anhydrous solvents.
To evaluate whether this 1-TMS-mediated, controlled NCA
polymerization can be extended to other N-TMS amines, we
selected benzylamine (2), morpholine (3), propargylamine (4),
N-(aminoethylene)-5-norbornene-endo-2,3-dicarboximide (5), and
mPEG2000 amine (6) to represent primary (2) and secondary amines
(3), amines containing functional groups that can be used for further
reactions such as click chemistry (4)16 and ring-opening metathesis
polymerization (5),17 and terminal amines of polymers (6).12
N-TMS’s of 2-6 were prepared (see Supporting Information) and
used to initiate Glu-NCA polymerization. As expected, all initiators
gave excellent control of PBLG MWs. At an M/I ratio of 100, the
Mn’s of PBLG were 2.35 × 104, 2.18 × 104, 2.19 × 104, 2.38 ×
104, and 2.85 × 104 g/mol for PBLG derived from 2-TMS through
6-TMS mediated Glu-NCA polymerizations, respectively, which
were in nearly perfect agreement with the expected Mn’s (Table
1). Polymerizations of Glu- and Lys-NCAs with these initiators
over a broad range of M/I ratios all gave corresponding polypeptides
with controlled MWs and narrow MWDs (Table S2).
In amine-initiated NCA polymerization, an amine can function
in two ways. It can be a nucleophile that attacks the CO-5 of the
NCA by following the so-called “amine mechanism”. It can also
function as a base to deprotonate the NH-3 of NCA by following
the so-called “activated monomer mechanism”.18,19 These complex
and concurrent mechanisms make it very difficult to achieve
controlled NCA polymerization using amine initiators. When
N-TMS amine is used as an initiator, polypeptide chain transfer
Supporting Information Available: Experimental procedure, GPC
and NMR data. This materials is available free of charge via the Internet
at http://pubs.acs.org.
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