Polyester - grafted polysaccharide compositions by Ring Opening Polymerization
D. Rutot, I. Ydens, Ph. Degée, Ph. Dubois
Laboratory of Polymeric and Composite Materials
University of Mons-Hainaut, Place du Parc, 1 B-7000 Mons BELGIUM
Introduction
Due to the growing concern over the environmental impact of waste disposal in increasing expensive landfills, biodegradable materials raise up more and more the interest of researchers. In this study, we combine an aliphatic polyester,
poly(ε-caprolactone)(PCL), known for its biodegradability, with a polysaccharide (natural biodegradable polymer) such as either granular starch or dextran in order to prepare, respectively, biodegradable starch-PCL composites and dextrang-PCL grafted copolymers which display surfactant properties. Biodegradable corn starch/PCL composites can be useful in replacement of conventional plastic materials in field like packaging which is a great purveyor of waste. Polymer
blends and composites using starch are usually obtained by conventional melt-processing which provides the starch-based composites with very poor mechanical properties due to thermal decomposition of starch, strong water absorption and
lack of interfacial adhesion. So we study the chemical surface treatment of starch granules so as « in situ » ROP of ε-caprolactone (ε-CL) or δ-valerolactone(δ-VL) monomer is promoted directly from the starch phase and results in covalent
grafting of the polyester chains onto starch with a good interfacial adhesion between the two components. On the other hand, dextran-g-PCL copolymers have been synthetized by a three-step procedure that involves the ROP of ε-CL
promoted from free remaining hydroxyls all along the partially protected (silylated) dextran. These graft copolymers form totally biodegradable, amphiphilic and non-ionic brush-like graft copolymers which can be used as surfactants. These
amphiphilic copolymers may also contribute to overcome some current problems with implanted hydrophobic biomaterials or drug delivery devices. The presence of polysaccharide shell covalently linked to the hydrophobic polyester core
of delivery system should create a hydrophilic interface able to minimizing undesirable protein adsorption, thrombus formation and polyphasic drug release profiles.
Starch/PCL composites
Dextran-g-PCL copolymers
Synthesis
Polymerizations are initiated by the modified/activated hydroxyl groups of amylose and amylopectin constituents of
starch (Plsch - OH). This modification is carried out by reaction of the OH groups with triethylaluminum. The
effective fixation of the initiator has been evidenced by X-ray Photoelectron Spectroscopy in which aluminum
characteristic peaks are detected. It is worth pointing out that drying of starch constitutes a key-step of the process.
After activation, the polymerization is performed in bulk or in a 10wt% toluene suspension (scheme 1).
Plsch OH + AlEt3
Plsch O
Plsch O p AlEt3-p
1. ε-CL or δ-VL
2. H2O
O
p
A three-step procedure is described in order to control the synthesis of biodegradable poly(ε-caprolactone)-graftedpolysaccharide copolymers. In a first step, a water soluble polysaccharide, i.e., dextran has been reacted with
silylating agents in order to increase its solubility in organic solvents and to limit the content of hydroxyl functions by
which the grafting reaction will take place. In a second step, the ring opening polymerization of ε-caprolactone has
been initiated by the free remaining hydroxyl groups of the partially silylated dextran, by using either aluminum
isopropoxide (Al(OiPr)3), triethylaluminum (Al(Et)3), or tin bis(2-ethylhexanoate) (Sn(Oct)2) as catalysts. Deactivation
of the catalyst and removal of the protecting trimethylsilyl groups can be carried out simultaneously by adding a few
drops of an aqueous acidic solution (scheme 2).
AlEt3-p + p C2H6
CH2
CH2
O
Plsch O C (CH2)m O H
n
OH
where m = 4 or 5
OH
O
OR
OR
n
n
Where R=Si(CH3 )3 or H
Dried
dextran
Dextran
dried
Scheme 1
SD
OR
DMSO/THF (6 :4), NEt3 (0.2 éq.)
O
OH
Table 1 reports some of the results obtained under different experimental conditions.
CH2
After ε-CL polymerization, selective extraction in toluene was performed onto the as-recovered composite to
determine the monomer conversion and the grafting efficiency. The insoluble part contains PCL grafted onto starch
and the soluble one, non grafted polyester chains. The grafting efficiency is defined as follows :
Grafting efficiency =
O
HMDS (4éq.) , 80°C, 90 h.
Toluene (10%wt/v.)
O
THF (10% wt/v)
OR
OH
OH
OR
O
CH2
OR
ε-caprolactone, MXn
Where MXn = AlEt3,Al(OiPr)3
and Sn(Oct) 2
O
OR
OH
% grafted polyester
SD-g-PCL
OH
OR
O
O
C O
Where R=Si(CH3)3
% grafted polyester + % non grafted polyester
(CH2)5
O
Table 1 : Synthesis of PCL or PVL/starch (50/50 wt%)
n
H
CH2
O
Monomer
Al content
(wt%)
0.33
ε-CL
a)
ε-CL
0.36
ε-CL
0.10
0.55
δ-VL
a) On 10 wt% suspension in toluene
Temperature
(°C)
65
65
90
65
Reaction time
(min)
95
300
3
30
Monomer
conversion (%)
54
6
99
68
OH
THF, H3O+
Grafting
efficiency (%)
52
33
90
56
O
OH
CH2
OH
D-g-PCL
O
OH
O
OH
O
n
C O
(CH2)5
O
H
n
Scheme 2
Characterization
Presence of PCL grafts all along the polysaccharide backbone has been evidenced by IR and DSC.
Presence of PCL and PVL chains has been evidenced by IR, DSC (determination of the Tm and Tg) and also by SIMS
in which typical fragments, including monomeric molecular ones, are detected.
The amphiphilic copolymers are also characterized by the following techniques :
The composites are also characterized by the following techniques :
Nuclear Magnetic Resonance and Size Exclusion Chromatography
Laser Scattering Granulometry
Grafting efficiency has been attested by fractionation experiments as well as by SEC (Fig.3). The obtained grafted
copolymers are monodisperse and show a quite narrow polydispersity.
In granulometry analysis, we can verify the fixation of aliphatic polyesters onto the starch surface by checking the
increase of the coated particle diameter. We can see in Fig. 1 the progressive increase of the mean diameter with the
quantity of grafted polyester chains.
In the 1H-NMR spectrum, resonance signals of the PCL and polysaccharide sequences have been identified (Fig.4).
S.D. (µ m)
Starch-g-PCL ;
Mean diameter
wt%PCL
(µ m)
0
13.45
6.57
n
7
23.12
19.85
14
23.42
18.31
29
27.70
20.03
Fig. 1 : Evolution of the mean diameter of the coated particles with increasing amounts of
grafted PCL
y
Fig. 3: SEC of SD-g-PCL as obtained by ε-CL ROP from SD (DS
= 2.8) added with Al(Et)3.
Scanning Electron Microscopy
z
b b
c
d
x
Si(CH )
33
Fig. 4 : 1H-NMR spectrum of SD-g-PCL (in CDCl3 at r. temperature)
as obtained by ε-CL ROP from SD (DS = 2.8) added with AlEt3 (t =72h)
Surface activity
SEM images of the composites attest for the very good interfacial adhesion between starch and aliphatic polyesters,
after (Fig. 2 c) and before (Fig. 2 d) selective extraction experiment in toluene. In Fig. 2 b, we can also verify the total
lack of adhesion between the two components in a simple starch/PCL melt blend (50/50 wt/wt).
a)
a
Preliminary dynamic surface tension experiments have shown the potential of those amphiphilic graft copolymers
as surfactants, at least, for a graft copolymer of FPCL = 0.2 and a DP = 6.6 (see Fig.5).
Water
b)
Dextran
Copolymer (15wt.%PCL)
75
c)
γ (mN/m)
70
d)
65
60
55
50
0
c) insoluble part of starch-g-PCL; d) starch-g-PCL
250
500
750
1000
1250
1500
Temps (sec.)
Fig. 2 SEM images of a) untreated starch; b) melt blend of starch and PCL (50/50 wt/wt) ;
Future works
Kinetics of ε-CL/δ-VL ROP initiated by starch surface aluminum alkoxide groups will be investigated and
compared to data available for ROP promoted by soluble aluminum alkoxides. Sequential copolymerization will
be carried out as well. Thermal, mechanical and morphological properties of the related starch-g-polyester
composites will be studied and compared to their counterparts obtained by melt blending of untreated starch and
preformed polyester.
Fig.5: The dependence of surface tension versus time for various solutions as determined by the pendant-drop method.
Kinetics of ε-CL ring opening polymerization initiated by aluminum alkoxide groups pendent all along dextran
backbones will be investigated. Synthesis, surface activity, thermal and mechanical properties of different
polysaccharide-g-polyester will be also studied, including PCL and polylactides grafts as well as various
polysaccharide backbones.