|
These new materials are obtained through molecular catalysis, and
require breaking the cyclic dimer of lactic acid �lactide� to obtain
polylactic acid (PLA). The lactide is a renewable natural resource
that occurs as a by-product of the fermentation of biomass with high
starch content, such as maize, wheat, or sugar beet. As in every
polymerisation process, a catalyst is required and in this case the
active compound must be a metal.
Consequently, this catalytic process has been studied with different
metals such as tin, yttrium, titanium, aluminium and other lanthanides.
However, since on some occasions residues of the catalyst can be
incorporated into the polymer, it is important to preserve the
biocompatibility and zero toxicity of the PLA by insuring that the
metallic catalyst used is biologically benign and does not have a
negative impact on tissue. These medical uses have favoured the use of
metals like magnesium, calcium or zinc, all of them common inside the
human body.
On a different front, PLAs are being investigated as a possible raw
material of many manufactured products, since they present similar and
in some cases better properties than traditional polymers that are
derived from the bioresistant poly (a-olefin), with the significant
added benefit of biodegradation.
While their production costs were considered too high in the past,
recent developments in the treatment and production combined with the
contrasting ecological hazard represented by petroleum derived
polymers have brought these types of biodegradable polymers to very
competitive positions.
One of the most recent and relevant examples that confirm this growing
expansion, is the joint endeavour by Cargill. Inc., and The Dow
Chemical Co., who have recently announced the mass production of many
tons of PLAs.
The scientific community shows a growing interest to find catalysts
that are capable of producing such biomaterials with well defined
microstructures, since this defines the mechanical properties, the
biodegradability, and the overall usability of the material.
With this in mind, the research group from the URJC, formed by Dr
Andr�s Garc�s and Carlos Alonso and coordinated by Dr Luis Fernando
S�nchez-Barba, is working in collaboration with the UCLM to develop
different families of catalysts based on magnesium and zinc and
stabilised by ligands like heteroscorpionate of they type �NNN�,
capable of polymerising the ε-caprolactone and the lactide in a
controlled manner. These are extremely active initiators with a
chemical formula of [M(R)(NNN)] (M = Mg, Zn) that achieve a
productivity of 21.000 Kg of poly-α-
caprolactone (PLC) produced per mol of Mg each hour at room
temperature.
Moreover, some of these initiators allow for a controlled growth of
the PLA�s microstructure. This is linked to the influence that the
heteroscorpionate exerts during the process of opening the cyclic
dimmer, which in turn grants control over the future specifications
and applications of the produced material such as a high molecular
mass, crystallinity as well as high fusion temperature (165�C), all of
it generating a great interest from industry.
|
