Biodegradable - Bioresorbable Polymers
I
Linear aliphatic polyesters and copolymers:
polylactide (PLA), polycaprolactone (PCL)
II
Poly(ester-ether): polydioxanone (PDX)
III
Poly(amino acid)s: poly(L-glutamate), poly(Llysine), poly(L-leucine)
1
I – Linear aliphatic polyesters and copolymers: polylactide (PLA),
polycaprolactone (PCL)
Polymerization of lactides and ε-caprolactone
Polylactides (PLAs) are synthesized from
lactides (Table 1), which are obtained from
lactic acid. These polymers are interesting
materials in the field of surgery and
ROP of lactides and ε-caprolactone has been
performed either in a non-living or living
fashion using various catalysts.
pharmaceutics as biodegradable sutures,
artificial skins and implantable carriers for
drug delivery. Apart from their applications
as specialty materials, polylactides also have
potential applications as commodity materials
as for instance in packaging.
Polylactides are obtained from their
corresponding monomers by ring opening
Non-living ROP
The non-living ROP of lactides in presence of
catalysts based on tin, zinc, magnesium and
titanium has been extensively studied since
the 70's. Tin(II) octanoate, zinc lactate and
zinc powder are the most commonly used in
polymers
for
biomedical
applications.
However polymerization in the presence of
polymerization (ROP), with a large range of
molar masses. Lactides (cyclic diester of
lactic acid), which exist as four different
isomers (Table 1), lead to structurally and
morphologically distinct polymers. However,
those which have received most attention are
poly(L-lactide)-(L-PLA) and poly(D,L-lactide)(D,L-PLA).
L-PLA is a highly crystalline
these initiators does not exhibit a living
behaviour and leads to polymers with broad
molar mass distributions (I ≥ 2) indicating the
presence of termination and transfer
reactions. The anionic polymerization of
lactides with ROK, ROLi, BuLi and Bu2Mg also
give rise to non-living ROP. It has been
studied to a much lesser extent as compared
isotactic polymer (Tg=55-60°C, Tm =180°C) and
is preferred in applications where high
mechanical strength and toughness are
required such as sutures, staples and
orthopedic devices.
D,L-PLA is generally
amorphous (Tg=45-50°C) and is usually
considered in applications such as controlled
drug release.
to metal compounds. However most of the
polymerizations do not exhibit living
characteristics.
Polycaprolactone (PCL) synthesized from εcaprolactone (Table 1), is also a very
attractive polyester due to a valuable set of
properties such as high permeability,
biodegradability and capacity to be blended
with various commercial polymers or
biopolymers.
Aluminium Schiff's base complexes have also
proved to be effective initiators for the
controlled polymerization of various lactones
and lactides. More recently, the alkoxides of
lanthanides have been used for this type of
polymerization.
Narrow molecular weight
distributions are obtained and the molar
masses are predictable.
Living ROP
Metal alkoxides, and more particularly
aluminium alkoxides, give rise to controlled
or living ROP of lactones and lactides.
2
Table 1
Structure of lactides and ε-caprolactone
Monomer
Structure
H3C
D-lactide
O
O
H
H
O
O
H
CH 3
O
L-lactide
O
H3C
CH 3
O
Racemic
lactide
H
O
H
H3C
O
O
lactide
O
H
H3C
H
CH 3
Meso-
O
O
O
O
O
H
H3C
O
CH 3
O
H
CH 3
O
O
H
O
O
ε-caprolactone
3
Our contribution to ROP of lactides and ε-caprolactone
I. Controlled ROP of Lactides using Schiff’s base complexes
I-1 Synthesis of new Schiff’s base Aluminium
complexes – HAPENAlOR
CH3
Al OCH3 + O
We has developed a new family of aluminium
Schiff’s base complexes – HAPENAlOR (Fig. 1)
which give rise to controlled ROP of lactides
and ε-caprolactone at ambient temperature
with minimization of side –reactions.
C
N
N
O
O
CH3
CH3
O
O
Al O
O
O
Al
O
O
insertion
C
Al
O
O
O
Al O
X
O
O coordination
O
R
Y
R
Al O
O
H o
C C O
CH3
O
H o
C C
CH3
OCH3
X
Scheme 1. Coordination-insertion mechanism
OR'
X = H; R = CH3; Y = (CH2)2; R' = OCH3
HAPENAlOCH3
X = H; R = CH3; Y = (CH2)2; R' = OCH(CH3)2
HAPENAlOiPr
Fig. 1. HAPEN-type Al alkoxides
I-2 Controlled Polymerization at ambient
temperature
The
polymerization
proceeds
via
a
coordination-insertion mechanism (Scheme 1)
at ambient temperature.
The rate of
propagation was found to be faster compared
Table 2. Apparent
polymerizations
Initiator
HAPENAlOMe
SALENAlOMe
Al(OiPr)3
HAPENAlOMe
SALENAlOMe
rate
constants
kapp*103 (h-1)
385
72
138
15
2
to other known Schiff’s base complexes at
ambient temperature and to other aluminium
complexes at 70°C (Table 2).
4
of
The livingness of the polymerization was
studied both in toluene at 70°C and
dichloromethane at ambient temperature.
The
polymers
exhibited
narrow
and
(Table 3). This is most probably due to a
greater polarization of the Al-O bond due to
an increase in bond length.
monomodal distributions. The linear plots of
molar masses against conversion (Fig. 2), the
good correlation between calculated and
experimental molar masses even at high
conversion,
narrow
molecular
weight
distribution in the range 1.1 to 1.2 coupled
with linear plots of ln[M]0/[M]t versus time,
demonstrated the living character of the
Table 3. Apparent
polymerizations
polymerization.
1HNMR
Mn
Solvent
toluene
DCM
constants
of
kapp*103 (h-1)
385
607
138
15
27
It was also shown that replacement of the
aliphatic diamine used to synthesize HAPEN
and SALEN initiators by an aromatic diamine
lead to a decrease in rate (Table 4).
10000
[M]/[I]=25
[M]/[I]=50
[M]/[I]=75
8000
Initiator
HAPENAlOMe
HAPENAlOiPr
Al(OiPr)3
HAPENAlOMe
HAPENAlOiPr
rate
6000
4000
Table 4. Apparent rate constants
polymerizations in toluene at 70°C
2000
of
0
0
20
40
60
conversion (%)
80
100
Fig. 2. Variation of molar mass with
conversion
in
DCM
(25°C),
using
HAPENAlOMe, [M] = 1 M
I-3
Polymerization
using
structurally
modified Schiff’s base complexes
Modifications of the structural parameters of
the initiator do not only affect the rate of
polymerization but they also have an
influence
on
the
occurrence
of
transesterification
reactions,
on
the
microstructure of the obtained polymers and
on their thermal properties. The replacement
of a methoxide ligand by an isopropoxide
ligand lead to an increase in rate without
significant deviation from living character
Initiator
HAPENAlOiPr
5-ClSALENAlOiPr
5-ClSALPHENAlOiPr
kapp * 103 (h-1)
607
121
20
We have also carried out an in-depth study of
ε-CL polymerization using the Al-Schiff’s base
Linear
initiator, namely HAPENAlOiPr.
variations of ln[M]o/[M]t v/s time were
obtained. However, the presence of induction
periods was detected for the polymerizations
of ε-CL initiated by the Al-Schiff’s base
complex.
After the induction periods,
polymerization was first order in monomer,
indicating a constant number of active
centres, a sign of living characteristics (Fig.
3).
5
Fig. 3. Kinetics of polymerization showing an induction period
I-4 Physical characteristics of polylactides
synthesized with Al-Schiff’s base complexes
I-4-1 Predominantly isotactic polymers
Predominantly isotactic polymers were
obtained with HAPENAlOR family of initiators
(Table
5).
This
confirmed
that
polymerization
followed
a
Markovian
statistics. In contrast, Al(OiPr)3 gave rise to
polymers showing a slight tendency towards
isotacticity. The enhanced isotacticity of the
polymers confirmed the stereospecificity of
the Al-Schiff’s base complexes.
the
Table 5. Theoretical and experimental proportions of the different n-ads
Initiator
Predominantly isotactic
Disyndiotactic
Atactic
HAPENAlOMe
HAPENAlOiPr
5-ClSALENAlOiPr
5-ClSALOPHENAlOiPr
Con.(%)
94
96
95
92
iii-iis-sii-sis-ssi
75
50
50
84
85
87
80
Tetrads
isi
25
50
12.5
16
11
13
18
sss
0
0
12.5
0
3
1
iss
0
0
12.5
0
1
1
6
I-4-2 Predominantly linear Polylactides chains
HAPENAlOR initiators gave predominantly
linear polylactide chains up to high
percentage conversion as shown by MALDITOF-MS.
Only even-membered chains and narrow
polydispersity indices were obtained as
I-4-3 Semi-crystalline PDLAs
Poly(D,L-lactide)s
are
known
to
be
amorphous, exhibiting a Tg in the range 4550°C. A remarkable feature of poly(D,Llactide)s synthesized using HAPENAlOR
complexes was their semi-crystallinity such
that they exhibited a melting transition
(Table 6).
The existence of a melting
indicated by the spectra. These results are in
contrast with the previously reported SALEN
initiator, which showed side-reactions even at
very low conversions (29%).
transition is explained by the formation of
sufficiently long stereosequences of D and L
units.
These stereoblocks gave rise to
stereocomplexes between the chains.
Table 6. Thermal properties of PDLAs
Initiator
Conversion (%)
Toluene, 70°C
96
DCM, 25°C
85
90
HAPENAlOiPr
HAPENAlOMe
HAPENAlOiPr
II.
Polymerization
of
lactides
The use of LDA (Fig. 4) as an anionic initiator
for the ROP of lactones has been reported in
using
Tm (°C)
138
159
144
Lithium
II-1 Establishment
polymerization
Diisopropylamide
of
mechanism
of
2001 for the first time by our group.
We have established the mechanism of
polymerization using model reactions.
The initiation and propagation mechanisms
(Scheme 2) may thus be summarized as
follows:
ƒ the first step is a deprotonation at the αcarbon atom giving rise to an enolate (1);
ƒ the enolate reacts with another lactide
molecule with acyl-bond cleavage and
subsequent formation of an alkoxide (2),
and
ƒ the alkoxide is most probably the active
Fig. 4. Structure of LDA mono THF
species in the propagation (3).
7
O
O
O
CH3
O
H
H
CH3
O
(1)
CH3
O
H
O
H
O
H3C
H3C
O
O
H3C
Li
O
NH(CH(CH3)2)2
Li N
(2)
O Li
(2)
O
CH3
O
H
O
H
H3C
(3)
O
(3)
O
O
CH3
CH3
O
O
O
H
H
O
O
O
HO
H3C
n H3C
O
O
HO
O
O
O
n
O
Scheme 2. Initiation and propagation of D,L-lactide polymerization using LDA.monoTHF
II-2 Polymerization characteristics
In dioxane at temperatures ≤ 70°C and in
toluene at 70°C, the polymerization was
found to be quite fast and polymer yields
were quantitative irrespective of the nature
of the solvent used (Table 7).
Table 7. Apparent
polymerizations
Temp (°C)
70
25
rate
Solvent
toluene
dioxane
constants
of
kapp (min-1)
0.26
0.39
toluene and in dioxane. kapp is larger in
dioxane than in toluene, although a lower
temperature is used, attirbuted to a lesser
extent of aggregation in dioxane.
II-3 Microstructure of polymers
Polymers obtained in dioxane exhibited a
highly syndiotactic structure (Table 8). A
comparison of polymers prepared in dioxane
and toluene showed that in the latter solvent,
significantly lower values of p2 were obtained
due to intramolecular transesterification
reactions and racemization.
An induction period was observed at the
beginning of the polymerization both in
8
Table 8. Methine tetrad intensities of poly(D,L-lactide)s synthesized using LDA.monoTHF
p1
p2
Li
66.0
0.32
0.68
2.94
38.0
62.0
0.24
0.76
2.63
100
36.0
64.0
0.28
0.72
2.78
32
42.0
58.0
0.16
0.84
2.38
93
38.0
62.0
0.24
0.76
2.60
70
40.0
60.0
0.20
0.80
2.50
Solvent
Temp.
(°C)
Conv.
(%)
Toluene
70
62
34.0
Dioxane
0
42
-10
25
-10
Tetrad intensities (%)
isi
sis,iis,sii,iii
II–4 Study of occurrence of side-reactions
13
C NMR spectra of the
Using MALDI-TOF-MS, we were able to discern
polymers without the forbidden tetrads did
not allow one to conclude on the incidence or
not of side-reactions. We have proved that a
better insight was obtained by further
characterization by MALDI-TOF-MS.
These
two techniques were not in agreement in all
13
C NMR showed absence of
cases.
transesterification reactions but the MALDI-
between the intra and intermolecular sidereactions. Absence of cycles and presence of
odd-membered chains in the MALDI-TOF
spectra indicated the occurrence of mainly
intermolecular transesterification.
Another interesting feature was the detection
of side-reactions by only MALDI-TOF-MS and
not 13C NMR at very low conversions. MALDI-
TOF spectrum of the polymer indicated the
presence of cyclic species. The presence of
cycles was an indication of the occurrence of
intramolecular transesterification reactions.
TOF spectrum at low conversions (Fig. 5)
indicated the presence of linear chains (even
and odd-membered) as well as cycles.
We have shown that
Mpeak
Mcala
Assignment
5068
Cyclic [OCH(CH3)CO]70
5067.2
5086.1
H-[OCH(CH3)CO]70-OH….Na+
5085.2
5102.1
H-[OCH(CH3)CO]70-OH….K+
5101.3
5140
Cyclic [OCH(CH3)CO]71
5139.26
5158.1
H-[OCH(CH3)CO]71-OH….Na+
5174
H-[OCH(CH3)CO]71-OH….K
+
5157.26
5173.36
Fig. 5. Part of MALDI-TOF-MS spectrum of PLA
9
Publications
1. Cameron PA, Jhurry D, Gibson VC, White AJP, Williams DJ, Williams S
Controlled polymerization of lactides at ambient temperature using [5-Cl-Salen]AlOMe
Macromolecular Rapid Communications (1999) 2, 616
2. Bhaw-Luximon A, Jhurry D, Spassky N
Controlled polymerisation of DL-lactide using a Schiff’s base Al-alkoxide initiator
derived from 2-hydroxyacetophenone
Polymer Bulletin (2000) 44, 31-38
3. Jhurry D, Bhaw-Luximon A, Spassky N
Synthesis of polylactides by new Al-Schiff’s base complexes
Macromolecular Symposia (2001) 175, 67-76
4. Bhaw-Luximon A, Jhurry D, Motala-Timol S, Lochee Y
Polymerization of ε-caprolactone and its copolymerisation with γ-butyrolactone using
metal complexes
Macromolecular Symposia (2006) 231(1), 60-68
5. Motala-Timol S, Jhurry D, Bhaw-Luximon A
Kinetic Study of the Al-Schiff’s Base Initiated Polymerization of ε-caprolactone and
synthesis of graft copolymers
Macromolecular Symposia (2006) 231(1), 69-80
6. Bhaw-Luximon A, Jhurry D, Spassky N, Belleney J, Pensec S
Anionic polymerisation of DL-lactide initiated by Lithium diisopropylamide
Polymer (2001) 42(24), 9651-9656
10
II – Poly(ester-ether): polydioxanone (PDX)
Poly(p-dioxanone)
poly(1,4-dioxan-2-one),
We have to date investigated the synthesis of
(PDX) is a thermoplastic, biocompatible, and
1,4-dioxan-2-one (DX) and its (co)polymerization
biodegradable
special
using different metal complexes as initiators.
characteristics. The ester bonds are responsible
The syntheses and detailed characterization of
for the hydrolytic degradation, while the ether
1,4-dioxan-2-one and its analogues have been
bonds confer good flexibility and high degree of
successfully achieved.
softness. Polydioxanone is mainly used in the
Ring-opening polymerization of 1,4-dioxan-2-one
medical sector as sutures, generally extruded
was performed at various temperatures using
into monofilament fibers. The polymer should be
initiators
processed at the lowest possible temperature in
octanoate/n-butyl alcohol (Fig. 6), Aluminium
order to avoid its spontaneous depolymerization
tris-isopropoxide and Aluminium Schiff’s base
back to the monomer. The sutures prepared with
complex, (HAPENAlOiPr) and rates constants of
this
polymerization
material
or
material
typically
with
lose
some
half
of
their
such
as
tin(II)
octanoate,
compared.
tin(II)
Experimental
mechanical strength in about three weeks and
conditions to achieve high molar masses have
complete degradation takes place in about six
been established. Hydroxy-terminated poly(3-
months. Polydioxanone is now being used for
MeDX) obtained after hydrolysis of the Al-O bond
mitral and tricuspid heart valve repair in the
has been fully characterized by NMR (Fig. 7),
pediatric
MALDI,
population.
The
polydioxanone
SEC
and
DSC.
It
was
found
that
annuloplasty rings allows remodeling, flexibility,
polymerization must be carried out at low
and preservation of the growth of the native
temperature to limit depolymerization of the
annulus. There is still need for improvement of
growing polymer chain. Synthesis of random
existing
copolymers by the non-sequential polymerization
PDX
or
copolymer
thereof
for
applications as medical implants or prostheses.
Our contribution
of 1,4-dioxan-2-one with other lactones was
investigated
using
various
initiators.
Best
copolymerization results were obtained with
This study is carried out in joint collaboration
Sn(Oct)2 at 80ºC and with Al(OiPr)3 at 60ºC.
with Prof A Kalangos (cardiac surgeon, Geneva
Thermal properties of the copolymers have also
University Hospital) and is still under progress.
been determined.
Fig. 6. Plot of ln([M]o-[M]e/[M]t-[M]e) v/s time for bulk polymerization of DX at 100oC,
[Sn(Oct)2] = 250, 500, 1000, and 2000 ppm w.r.t. monomer, co-initiator nBuOH, M/I = 30
11
O
O
O
A
O
O
O
n
B
Fig. 7. 1H NMR (CDCl3) spectrum of purified (A) 1,4-dioxan-2-one (DX) (B) poly(1,4-dioxan-2one)
12
III – Poly(amino acid)s
Synthesis via ROP of N-carboxyanhydrides (NCAs)
Polypeptides are another class of polymers
that have recently become very attractive as
high-performance
materials.
Structural
proteins (silks, collagen and elastin) are being
isolated and studied since their excellent
physical properties, biocompatibility and
One of the main limitations of NCA
polymerization, using conventional initiators,
is the occurrence of chain-breaking transfer
and termination reactions.
These sidereactions restrict control over molar masses
and lead to broad molar mass distributions.
biodegradability make them well suited for
biomedical applications (sutures, artificial
tissues and implants).
Aluminium
compounds,
namely
trialkylaluminiums, have also been shown to
be active initiators in NCA polymerization.
But
their
initiation
and
propagation
mechanisms remain unresolved and only
limited advantages were seen for use of these
compounds over conventional initiators, i.e,
amines.
The
best
route
to
high-molar-mass
polypeptides is the ROP of α-amino acid-Ncarboxyanhydrides
(NCAs)
(Scheme 3),
typically using nucleophiles or bases. NCAs
are obtained by phosgenation of amino acids.
The mechanisms of polymerization in
presence of the various initiators used are
quite complex and have been the subject of
debate for quite long, as thoroughly discussed
in several review articles and books.
n CO2
R
H
O
N
O
ROP
initiator
O
H
R
N
H
O
n
More recently, the use of organometallic
compounds as a new class of initiators for
polymerizing NCAs has been reported. Some
of these compounds, in particular the
organonickel complexes, were found to
eliminate considerably termination and
transfer reactions and displayed all the
characteristics of a living chain-growth
process.
This type of initiation was
successfully used in the preparation of block
copolymers.
Scheme 3. ROP of NCAs
13
Our contribution to the synthesis of poly(amino acid)s: poly(Lglutamate), poly(L-lysine), poly(L-leucine)
I-1 Use of novel initiators
We were the first group to report the use of
Al-Schiff's base complexes and LDA.monoTHF
as effective initiators for polymerization of γmethylglutamate NCA.
The Schiff’s base
complexes were then used on other NCAs
namely: L-lysine and L-leucine.
I-2 Determination of mechanism of
polymerization using 1H NMR and MALDITOF-MS
The mechanism of polymerization with AlSchiff's base complexes was determined using
a model reaction, NCAGluOMe was reacted
with HAPENAlOiPr in a 1:1 mole ratio. 1H NMR
spectrum (Fig. 8) showed the presence of
aromatic and isopropoxy protons. Moreover,
the integration of the peaks appeared to be
concordant with the proposed structure.
MALDI-TOF-MS of the polypeptide indicated
the presence of OMe and OiPr as end-group
(Fig. 9). The molar mass of the polymer was
lower than expected in agreement with a fast
initiation and a slow propagation which result
in the formation of oligopeptides.
In the light of the model reaction and MALDITOF-MS analysis, the mechanism depicted in
Scheme 4 was proposed for the NCAGluOMe
polymerization with aluminium Schiff's base
complexes. It consisted of the following
steps:
1) coordination of the Al-Schiff's base
complex on the nitrogen atom of the
NCA and subsequent formation of
ROH;
2) ROH attacks the NCA at C(5)=O;
3) ring-opening of the NCA with loss of
carbon
dioxide,
the
resulting
intermediate corresponds to an
insertion of the NHCH(R)CO into the
Al-OiPr bond;
4) propagation proceeds via nucleophilic
attack of another NCA by the nitrogen
containing the Al-moiety;
5) after loss of carbon dioxide and
proton transfer, an amino group is
generated, and
6) propagation then proceeds as with
primary amines via attack of the NH2
end-group
on
incoming
NCA
molecules, and finally one chain-end
is an isopropoxy-ester or methoxyester while the other is an NH2 group
after precipitation in water as
confirmed by MALDI-TOF-MS and NMR.
14
b
c
CH2CH 2COOCH 3
HN
b
H3C
C
N
c O
N
C
CH3
a
b
CD2Cl2
Al
O
CH3
OCH
c CH3
a
O
b
c
aromatic
Fig. 8. 1H NMR spectrum (CD2Cl2) of reaction product (NCAGluOMe + HAPENAlOiPr)
Fig. 9. Expansion of MALDI-TOF-MS spectrum of a poly(NCA) synthesized using HAPENAlOMe
(conversion = 100%, Mn = 2700, I = 1.14)
15
Al
R
H
N
Al-OR
O
R
N
fast
+
O
O
ROH
O
O
O
CO2
fast step
Initiation
R
Al
N
H
OR
proton transfer
Al
O
N
O
O
O
Propagation
H
R
OR
O
N
H
R
R
O
O
CO2
R
H
N
O
OR
H2N
N
OR
NH
O-
R
Al
R
proton transfer
O
O
Al
n NCA
R
H
N
H2N
O
R
O
OR
n
R
N
H
O
R
H2N
precipitation in
H2O
O
H
N
R
OR
N
O
n
R
Al
O
Scheme 4. Mechanism of NCA polymerization initiated with Al-Schiff's base complexes
I-3 Synthesis of
Copolypeptides
Random
and
Block
We have synthesized random and block
copolypolyeptides
derived
from
γmethylglutamate
and
leucine
NCarboxyanhydrides using Al-Schiff’s base
complexes and allylamine as initiators. The
calculation of the statistical average block
lengths of the random copolymers reveals the
presence of longer methylglutamate units in
the copolymers (Table 9).
The determination of the reactivity ratios
indicated a slightly higher reactivity of γ-
methylglutamateNCA
as
compared
to
leucineNCA (Table 10).
Block copolypeptides containing glutamate
and leucine units were obtained by sequential
polymerization of the two NCAs using AlSchiff’s base complexes or allylamine in
dioxane as solvent (Scheme 5). Based on 13C
NMR spectra of copolymers exhibiting two
signals corresponding to peptide linkages, we
confirmed the block structure and concluded
that the copolymerization proceeds by attack
of an amino group present on a glutamate
chain end onto a LeuNCA (Fig. 10).
16
Table 9. Average block lengths of γ-Meglutamate( L
(monomer feed: 50/50; [M]/[I] = 50)
Initiator
HAPENAlOiPr
HAPENAlOMe
Primary amine
Tertiary amine
Glu)
and Leucine( L
L Glu
L Leu
2.86
3.12
2.84
4.83
2.43
2.53
2.53
4.56
Leu)
residues
Table 10. Reactivity ratios determined by Finemann-Ross and Kelen-Tudos methods
γ-MeGluNCA (r1)
2.55 ± 0.1
2.56 ± 0.2
Method
Finemann-Ross
Kelen-Tudos
LeuNCA (r2)
2.22 ± 0.09
2.22 ± 0.09
R' = CH2CH=CH2
CO2
R'NH2
O
O
O
O
Nucleophilic
attack
R'
HN
+
N
H2
O
H
N
O
-
N H2
R'
fast H+
transfer
O
N
H
CH2CH2COOCH3
CH2CH2COOCH3
CH2CH2COOCH3
MeGluNCA
n MeGluNCA
LeuNCA
CH2CH2COOCH3
O
H
N
O
O
m LeuNCA
H2N
NH2
R'
O
n+1
N
H
m+1
CH2CH(CH3)2
O
CH2CH2COOCH3
H
N
R'
n+1
O
HN
polyMeGlu
CH2CH(CH3)2
polyMeGlu-b-polyLeu
Scheme 5. Sequential copolymerization pathway
17
a COOCH3
O
cC
Leu-Leu
H3C
CH2
Glu-Glu
c
OCH3
O
b
C
CH
CH2
CH
CH3
O
a
C
N
H
n
CH2
CH
N
H
m
b
Fig. 10. 13C NMR spectrum showing the carbonyl region (TFA-d) of a block copolymer
obtained with initiator HAPENAlOiPr
Viscometry analysis further showed that molar masses of the copolypeptides obtained with AlSchiff’s base were quite close to those derived from allylamine, supporting the proposed
mechanism of copolymerization.
Publications
1. Bhaw-Luximon A, Jhurry D, Belleney J, Goury V
Polymerization of γ-methylglutamate N-carboxyanhydride using Al-Schiff’s base
complexes as initiators
Macromolecules (2003) 36(4), 977-982
2. Goury V, Jhurry D, Bhaw-Luximon A, Belleney J, Novak B M
Synthesis and Characterization of Random and Block Copolypeptides Derived from Methylglutamate and Leucine N- Carboxyanhydrides
Biomacromolecules (2005) 6(4), 1987-1991
18
I-4 Synthesis and characterization of sugar-based polypeptides
consisted in first preparing the polypeptide
via ring-opening polymerization of Ncarboxyanhydrides using either an AlSchiff’s base metal complex or a primary
amine. The free amino side groups of the
polylysine units are then reacted with
gluconolactone, thus leading to a main
polypeptide backbone and linear sugar
moieties as pendant groups (Scheme 6).
Sugar moieties have also been attached to
statistical and block copolypeptides bearing
hydrophilic and hydrophobic units such as Llysine and L-leucine respectively. We have
thus anchored linear sugar moieties
containing free hydroxyl groups via covalent
linkage onto homopolypeptides as well as
onto statistical (Fig. 11) and block
copolypeptides (Fig. 12).
Researchers have been interested in
polypeptides to which sugar molecules are
attached as they can be used as model
antigens and as drug delivery systems. For
instance, polylysine-based carbohydrates
are reported to be suitable non-viral vectors
for selective gene delivery.
For their
biological applications, the synthesis of
well-defined sugar-based polypeptides is of
paramount importance. A first method to
access such sugar-based polypeptides
consists in polymerizing protected sugarsubstituted NCAs followed by deprotection
of the sugar moieties. Another approach for
preparing sugar-based polypeptides consists
of attaching the sugar moieties directly to
the polypeptides.
We have synthesized
poly(L-lysine) carrying linear gluconoyl
moieties as side groups. The strategy
O
O
H
N
C
H
N
OH
CH
O
n
CH2
CH
triethylamine
+
OH
O
OH
(CH2)3
DMF, 50oC, 24hrs
OH
CH2
n
H2C
NH
δ-gluconolactone
CH2
CH2
NH3Br
O
HO
HO
CH
OH
CH
OH
CH2
OH
CH
CH
Scheme 6. Synthesis of poly(N-gluconamidolysine)
19
Fig. 11. 1H NMR (TFA/CDCl3) spectrum of poly(gluconoyl-L-lysine-co-L-leucine)
Fig. 12. 1H NMR (TFA/CDCl3) of poly(gluconoyl-L-Lysine-b-L-leucine)
Publication
Goury V, Jhurry D, Bhaw-Luximon A
Synthesis of sugar-based polypeptides, Designed Monomers and Polymers, in press (2008)
20