Table 2 Comparative surface polarities of starch, a typical polyalkane and
a typical starch–polyalkane material
Polyalkane+starch
ratio
Sample
l
max/nm
Sample polarity
Potato starch
Polyalkane
N/A
N/A
592
630
0.543
0.453
Starch–polyalkane
material
1.33+1
538
0.693
This indicates a surface polarity for the composites that is
greater than either of its component parts (Table 2). Scanning
electron microscopy may help to explain the origin of this effect
Fig. 4 Reaction profiles for the Knoevenagel condensation reaction of ethyl
cyanoacetate and cyclohexanone catalysed by aminopropylsilane on starch
(Fig. 3). Potato starch exists as pebble-like granules which vary
between 10 and 100 mm in size. The composite materials
however, are apparently made up of smaller particles ( < 5 mm)
bound to larger (polyalkane) particles (up to 200 mm). The
higher effective surface area of the starch may also result in a
higher available concentration of surface hydroxyl groups
leading to greater surface polarity. Despite the relatively high
surface polarity as measured by Reichardt’s dye, the materials
are hydrophobic and float on water unchanged for periods of
several weeks before water eventually permeates the structure
sufficiently for them to sink.
(
¶), aminopropylsilane on a typical starch–polyalkane material (Ω) and
aminopropylsilane on a mechanical mixture of starch and a typical
polyalkane (8).
differences are dramatic. While the ordinary starch based
material has almost no activity in the reaction and that based on
the starch–polyalkane mechanical mixture is low, the derivi-
tised new material is very active and comparable to their porous
6
silica analogues. Starch that has been treated with sodium
metal or sodium hydroxide solution but in the absence of the
halocarbon gives a material that has an unchanged FTIR
spectrum, thermal analysis and interaction with Reichard’s dye
compared to ordinary starch. It does however show a higher
capacity for reaction with the aminopropylsilane although the
resulting material is only as active in the model reaction as the
starch–polyalkane mechanical mixture.
We have extended the synthetic methodology to other a,w-
dibromoalkanes. 1,6-Dibromohexane and 1,2-dibromoethane
can be used in place of 1,10-dibromodecane with apparently
very similar reactions occurring. All of the materials are
resistant to water over periods of several weeks, all can be
derivatised to high levels with aminoalkylsilanes (0.3–0.6
mmol g ) and all of the resulting derivatised solids are active
solid base catalysts in the test reaction. The ability to increase
substantially the availability of the starch hydroxyl groups
enabling high degrees of derivatisation while rendering the
materials water-resistant would seem to be an attractive
combination of properties. The method of polymerising starch-
intercalated monomers to achieve this may well open the door
to a family of new and useful materials based on an inexpensive
renewable resource.
Fig. 3 Scanning electron microscopy (SEM) images of a typical starch–
polyalkane material (STPAM) before (left) and after (right) removal of
sodium colloid by water washing.
2
1
Perhaps the most dramatic effect of the enhanced surface
activity of the new materials is their capacity for chemical
modification with silanes. Stirring the materials with 3-(trime-
thoxy)aminopropylsilane in refluxing toluene for 24 h before
filtration followed by through washing with toluene and then
refluxing aqueous ethanol gives solid materials which were
dried at 110 °C for 24 h. The measured loadings of aminoalkyl
2
1
21
groups of up to 0.6 mmol g (1.4 mmol g based on the starch
present) are over 103 greater than the maximum loading that
We gratefully acknowledge the support of the Royal
Academy of Engineering and the Engineering and Physical
Sciences Research Council (EPSRC) for a Clean Technology
Fellowship to J. H. C. and ICI/EPSRC for a studentship to
J. E. H.
2
1
can be obtained with potato starch (0.05 mmol g ). It is
interesting to calculate that the level of derivatisation of potato
starch corresponds to < 1% of the total number of hydroxyl
groups that are present—an indication of the poor availability of
these groups. By increasing the availability of the groups in the
new materials the degree of functionalisation has increased to
ca. 10%.
Notes and references
The derivatisation values that can be achieved with the new
materials are comparable to those that can be obtained on a high
surface area porous silica gel.6 This comparison is further
reinforced by measuring the activity of the aminoalkyl-
derivitised starch–polyalkane materials in a typically base-
catalysed reaction. The reaction chosen was the Knoevenagel
reaction, a useful carbon–carbon bond forming reaction that
relies on the base-activation of a carbon acid. The rates of the
Knoevenagel reaction between ethyl cyanoacetate and cyclo-
hexanone in cyclohexane catalysed by a typical aminopropyl-
starch–polyalkane composite material, an aminopropyl-func-
tionalised mechanical mixture of starch and a polyalkane, and
aminopropyl-potato starch are compared in Fig. 4. The
1
2
J. Jane, Pure Appl. Chem., 1995, A32, 751.
Carbohydrates as Organic Raw Materials, ed. H. Van Bekkum,
Carbohydrate Research Foundation, VCH, New York, 1994; J. A.
Radley, Industrial uses of Starch and its Derivatives, Applied Science,
London, 1976.
R. B. Seymour and C. E. Carraher, Polymer Chemistry, an Introduction,
Marcel Dekker, 2nd edn.,1998; G. J. L. Griffin, Proc. Symp. Degradable
Plastics, Soc. Plastic. Indust., Washington, 1987.
R. E. Benifield, R. H. Cragg, R. G. Jones, S. A. McIntosh, A. C. Swain
and A. J. Wiseman, Z. Phys. D, 1993, 26(S), 18.
S. J. Tavener, J. H. Clark, G. W. Grey, P. A. Heath and D. J. Macquarrie,
Chem. Commun., 1997, 12, 1147.
3
4
5
6 D. J. Macquarrie, J. H. Clark, J. E. G. Mdoe and A. Priest, React.
Functional Polym., 1997, 35, 153.
336
Chem. Commun., 2001, 335–336