Synthesis of hybrid block copolymers via integrated ring-opening metathesis polymerization and polymerization of NCA

Article abstract of DOI:10.1039/c1cc13531g

Linear hybrid block copolymers with well controlled molecular weights and narrow polydispersities were synthesized via ring-opening metathesis polymerization (ROMP) followed by ring-opening polymerization of amino acid N-carboxyanhydrides.

Full text of DOI:10.1039/c1cc13531g

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COMMUNICATION
Synthesis of hybrid block copolymers via integrated ring-opening
metathesis polymerization and polymerization of NCAw
Yugang Bai,a Hua Lu,a Ettigounder Ponnusamyb and Jianjun Cheng*a
Received 14th June 2011, Accepted 11th August 2011
DOI: 10.1039/c1cc13531g
Linear hybrid block copolymers with well controlled molecular
weights and narrow polydispersities were synthesized via
ring-opening metathesis polymerization (ROMP) followed by
ring-opening polymerization of amino acid N-carboxyanhydrides.
amines and ROMP catalysts could also be utilized to prepare
linear hybrid block polymers by using an N-TMS amine func-
tionalized cis-alkene as CTA.37 Consequently, the N-TMS
terminal groups, which are amenable for NCA polymerization
yet compatible with olefin metathesis catalyst, can be transferred
to the polyolefin chain ends, making the synthesis straight-
forward with no need to protect the amine group (Scheme 1b).
CTA (2) was designed as a symmetrical, N,N0-bis(TMS)-
diamine functionalized cis-olefin (Scheme 2) instead of an
N-TMS amine functionalized terminal alkene because the
former usually leads to a higher degree of chain-end func-
tionalization than the latter.25,38–40 2 was synthesized in a
few steps from inexpensive, commercially available starting
materials, and fully characterized by 1H-NMR and 13C-NMR
(see ESIw). To test whether 2 can act as a CTA and give
complete end-capping of ROMP polymers, Grubbs catalyst 1
was mixed with 2 (5–6 equiv.) and stirred for 1 h to ensure
Hybrid copolymers, which contain segments with distinct
structures and properties, are useful materials in various
applications such as novel plastics,1,2 polymeric catalysis,3,4
smart materials,5–7 self-assembly,8–14 and bionanotechnology.15–21
They can be prepared through conjugation of hybrid polymeric
segments or polymerization with corresponding initiators. The
latter approach is particularly attractive as the hybrid poly-
mers with tuneable molecular weights (MWs) and narrow
polydispersities (PDIs) of each block may be readily achieved
via controlled polymerizations. In parallel to the studies of
utilizing AB-type dual initiator to facilitate two distinct,
controlled polymerizations from the initiators A and B of
the same centre,22 there is also significant interest in preparing
linear hybrid copolymers via controlled, sequential poly-
merizations, using a mono-initiator for the synthesis of the
first polymeric block followed by transformation of the active
chain end of the resulting first block using chain transfer
agents (CTAs) or terminating agents (TAs) to a macromole-
cular initiator with a desired terminal group for the synthesis
of a second polymer block.23–27 Here, we report the utilization
of the CTA approach to synthesize polypeptide-b-poly(oxa)-
norbornene hybrid copolymers, via controlled ring-opening
metathesis polymerization (ROMP) followed by ring-opening
polymerization of amino-acid N-carboxyanhydrides (NCAs).
We recently reported the integration of ring-opening poly-
merization (ROP) of NCAs22,28–30 and ROMP31–33 to prepare
polypeptide-grafted brush-like polymers in a one-pot fashion
(Scheme 1a).34 The strategy involves the ROMP of a norbornene
monomer containing N-trimethylsilyl (N-TMS) amine followed
by the controlled ROP of NCAs mediated by the pendent
N-TMS amine group, a method we developed recently.29,34–36
We reasoned that the excellent compatibility between N-TMS
complete carbene exchange, yielding
a N-TMS bearing
ruthenium catalyst (Step 1, Scheme 2). The colour gradually
turned brown from green, indicating the olefin exchange and
the formation of S3. The ROMP monomer M1 (30 equiv.)
was then added directly to the S3 solution and the polymeri-
zation proceeded rapidly (Step 2, Scheme 2). We expected that
an N-TMS amine terminated ROMP polymer (S4, Scheme 2)
with an active chain-end would be generated from S3. Since
the ROMP of M1 was several orders of magnitude faster than
the cross-metathesis of 2,20,24 the chain termination of the
ROMP polymer by excess 2 (Scheme 2, Step 3) would happen
a Department of Materials Science and Engineering, University of
Illinois at Urbana–Champaign, Urbana, IL, 61801, USA.
E-mail: jianjunc@illinois.edu; Tel: +1 217 244 3924
b Sigma-Aldrich, 3500 DeKalb Street, St. Louis, MO 63118, USA
w Electronic supplementary information (ESI) available: Experimental
details and characterizations of compounds. See DOI: 10.1039/
c1cc13531g
Scheme 1 (a) Synthesis of polypeptide-containing brush-like copoly-
mers using an N-TMS amine bearing ROMP monomer; (b) synthesis
of linear hybrid block copolymers via an N-TMS amine functionalized
chain-transfer agent.
c
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Table 1 Characterization of poly(oxa)norbornene polymers and
polypeptide-poly(oxa)norbornene-polypeptide triblock copolymers
Conv. of
monomer Mn (Mn*)c
(ROMP/
PP(%))b
(ꢀ 103
Polymer Monomer Initiator M/Ia
g molꢁ1
)
PDId
P1
P2
P3
P4
P5
M1
M1
M3
M2
M1 +
M4
M5
M5
M5
M6
M8
M5
M7
1 + 2 30 : 1
1 + 2 100 : 1 499/-
1 + 2 75 : 1 499/-
1 + 2 100 : 1 499/-
1 + 2 (100 + 499/-
25) : 1
499/-
7.5 (8.1) 1.02
25.8 (25.8) 1.02
19.9 (18.1) 1.02
19.1 (19.5) 1.05
33.3 (33.2) 1.06
P6
P7
P8
P9
P10
P11
P12
P1
P2
P2
P2
P3
P4
P5
100 : 1 499/499 28.5 (29.5) 1.12
100 : 1 499/499 50.2 (47.7) 1.03
200 : 1 499/499 68.1 (69.6) 1.03
100 : 1 499/499 49.5 (48.2) 1.03
100 : 1 499/499 47.8 (44.8) 1.02
100 : 1 499/499 47.6 (41.2) 1.13
300 : 1 499/496 113.6 (111.4) 1.07
a
b
M/I = monomer/initiator ratio. Conversion of monomer of the
ring-opening metathesis polymerization (ROMP)/the ring-opening
polymerization of NCA for the synthesis of polypeptide (PP).
c
Mn = molecular weight measured by GPC; Mn* = molecular
d
weight expected. Polydispersity index (Mw/Mn).
Scheme 2 Structures of the Grubbs-catalyst (1), CTA (2), ROMP
monomers (M1–M4) and NCA monomers (M5–M8) used in the
study, and the scheme of the synthesis of polypeptide-b-
poly(oxa)norbornene-b-polypeptide triblock polymers. Py = pyridine;
Bn = benzyl; Mes = 2,4,6-trimethylphenyl.
only after the monomer M1 was completely consumed, and
thus resulted in N-TMS amine groups on both ends of the
resulting ROMP polymer (S5, Scheme 2). S5 was isolated by
precipitation in dry hexane in a glove-box and analysed by
1H-NMR. The N-TMS amine group was successfully detected
with 499% end-capping efficiency (see ESIw). For the poly-
merization mediated at a monomer/initiator (M/I) ratio of
30 : 1, gel permeation chromatography (GPC) analysis showed
Fig. 1 Overlay of the GPC curves of the ROMP polymer (P2) and
the triblock copolymers (P7 and P8).
a monomodal peak with an Mn value of 7.5 ꢀ 103 g molꢁ1
,
which was close to the expected MW (8.1 ꢀ 103 g molꢁ1), and
a narrow PDI (1.02, P1, Table 1). Another polymerization
performed under the same condition at an M1/initiator ratio
of 100 : 1 yielded P2 with a MW identical to the expected value
and a narrow PDI of 1.02 (Table 1; Fig. 1).
polymerizations of M5, which yielded P7 and P8 at M/I ratios
of 100 : 1 and 200 : 1, respectively (Table 1), also showed
remarkable control over molecular weights and PDIs,
evidenced by the GPC analysis (Fig. 1). P7 and P8 had an
obtained MW of 50.2 and 68.1 ꢀ 103 g molꢁ1, respectively,
which was deviated only 5% and 2% from the expected values
(Table 1). Both P7 and P8 had very narrow PDIs (1.03).
We then tested the same strategy using several different
ROMP monomers (M2–M4 in Scheme 2) and NCA mono-
mers (M6–M8 in Scheme 2). Both the ROMP for the synthesis
of P3–P5 and subsequent ROP of NCAs proceeded smoothly
and gave a variety of well-defined polypeptide-b-poly(oxa)-
norbornene-b-polypeptide triblock copolymers (P9–P14)
using different combinations of ROMP and NCA monomers
at various M/I ratios (Table 1 and ESIw). The facile synthesis
of those block copolymers validated the generality of this
controlled, sequential polymerization strategy.
To study whether the N-TMS amine situated at the terminus
of the ROMP polymers is still reactive, we quenched P1
with di-tert-butyl dicarbonate ((Boc)2O). Complete trans-
functionalization from N-TMS amine to N-Boc was confirmed
1
by H-NMR (see ESIw). After confirming the activity of P1’s
N-TMS amine, we went on to synthesize a series of poly-
peptide-b-polynorbornene-b-polypeptide copolymers (S6,
Scheme 2) by treating S5 with various NCAs (M5–M8).
A triblock copolymer P6 (Table 1) was successfully obtained
by mixing NCA monomer M5 and P1 at 100 : 1 M/I ratio in
anhydrous DMF. The GPC analysis of the resulting mixture
showed a monomodal peak with an Mn value close to the
theoretical value and a narrow PDI of 1.12. The MW obtained
from the 1H-NMR spectrum of P6 agreed well with the
Mn obtained from GPC (see ESIw). Similarly, P2-initiated
Synthesis of polynorbornene-b-polypeptide diblock copoly-
mers were also attempted by following a slightly modified
strategy (Scheme 3).25 ROMP of M1 was first initiated by 1
c
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This journal is The Royal Society of Chemistry 2011