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the gel. One major drawback, however, is that so far, no
chiral, organic, solvent-based cross-linked gel is available.
The second, conceptually different method for partial
alignment is the use of organic-solvent-based LC phases;
these, however, often orient too strongly. So far, only lyo-
tropic LC phases made of dilute solutions of homo-polypep-
tides, namely poly-g-benzyl-l-glutamate (PBLG), poly-g-
ethyl-l-glutamate (PELG), and poly-e-carbobenzyloxy-l-
lysine (PCBLL) have proven to be suitable LC media for
the orientation of organic compounds.[32–48] The main advan-
tages are their ready availability (commercial suppliers) and
chemical inertness. Another advantage is the chirality of the
LC phases made of homo-polypeptides, which forms the
basis for the impressive work on enantiodiscrimination done
by the Courtieu and Lesot group.[38–48] Besides enantiodiscri-
mination, the chirality of the LC phase would, in principle,
allow the determination of the absolute configuration with-
out any derivatisation if the interaction of solute and liquid
crystal were known precisely. A first result in this field,
based on comparison of a chiral compound of unknown con-
figuration with chemically similar compounds of known con-
figuration, has been reported by Lesot et al.[49] The main dis-
advantage of the LC-based strategy of orienting compounds
is the necessity of a minimum concentration in order for the
LC phase to be formed, resulting in a minimum anisotropy,
which can only be dropped further by applying variable
angle sample spinning (VASS).[50,51]
PBLG was the first synthetic polymer to exhibit an LC
phase,[52] and due to its good solubility in many organic sol-
vents, it has become a standard when LC polymers are con-
sidered. In many organic solvents it adopts an a-helical
structure, making it also a standard when model compounds
for a-helices are required. PBLG is well investigated and
has widespread applications, but so far little is known about
its potential as an orienting medium for the measurement of
RDCs and the possibilities for influencing orientational
order and preferred orientation.[53] We[33] and others[34,41–43,54]
have applied PBLG LC phases for measuring RDCs. The
order induced in commercial PBLG, however, is often too
high, leading to complications in the interpretation of spec-
tra.
tion range in which the LC phase is stable widens with in-
creasing axial ratio L/D (in which L is the length of the rod
and D its corresponding diameter) and thus with increasing
molecular weight (MW) as D stays constant.[55,57,58] From the
results of DuPrꢃ et al. it can be anticipated that this is true
only when L is smaller than the persistence length (P) of
the polymer (L<P).[59] Persistence lengths reported for
PBLG, however, vary significantly in the literature (70–
150 nm). Starting from this point, we synthesised several
PBLGs with various chain lengths (axial ratios) and investi-
gated the orientational properties of isopinocampheol (IPC)
as the solute.
Results and Discussion
Synthesis of PBLGs of different MW: PBLG was synthes-
ised by ring-opening polymerisation of the corresponding N-
carboxyanhydride (NCA). Out of the three possible meth-
ods[60] we used the living polymerisation catalysed by transi-
tion metals (Deming protocol),[61–63] as it has several advan-
tages over the two conventional methods using initiation by
bases or nucleophiles. It allows controlled polymerisation of
both low- and high-MW polymers with narrow molecular
weight distributions (MWD) simply by choice of the ligand
attached to the metal centre or the solvent.
The monomer 3 was synthesised by adapted procedures
from Albert et al.[64] and Goodman et al.[65] as depicted in
Scheme 1. In the first step, l-glutamic acid 1 is reacted with
benzyl alcohol in the presence of two equivalents of tetra-
fluoroboric acid (HBF4) to exclusively give the g-ester 2.
The NCA 3 is obtained by reaction of 2 with phosgene in
dry THF.
Table 1 shows selected samples of PBLG covering a
broad range of MW, and hence axial ratios, of the resulting
a-helical rodlike polymers. In Table 1, the number averaged
MWs (Mn) and the molecular weight distributions (MWDs)
are reported, which were determined by gel permeation
chromatography (GPC) against polystyrene standard sam-
ples (for more details see Experimental Section). All entries
in Table 1 were synthesised by the Deming protocol except
sample 4b, which is a commercial sample (Sigma–Aldrich).
The rather broad MWDs are unusual, since the polymeri-
sation has living character. To date, we cannot exclude the
possibilities that the broad MWD might be either an arte-
fact (adsorption phenomenon onto the stationary phase)
from the gel permeation chromatography (GPC), since the
Since information about factors modulating the degree of
order and the preferred orientation introduced are sparse,
we have started a systematic investigation into the factors
influencing the orienting properties of PBLG, the two most
obvious being its molecular weight and the solvent used.
For the applicability of PBLG LC phases, the induced
degree of order, which can be
described by an order parame-
ter, should be scalable over a
wide range. It is known that
the order parameter of the
PBLG LC phase is closely re-
lated to the volume fraction of
PBLG.[55,56] Furthermore, for
rod-like molecules like PBLG
it is known that the concentra-
Scheme 1. Synthesis of monomer 4 by adopted procedures of Albert et al.[64] and Goodman et al.[65]
Chem. Eur. J. 2009, 15, 254 – 260
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