Biomedical Engineering Reference
In-Depth Information
polymerization may differ [23]. In addition, rare-earth metal complexes have been
shown to work as effective catalysts leading to high molecular weight polylactones
with low polydispersity [24, 25]. An effi cient cationic ring-opening polymerization
of lactones has been developed using scandium trifl uoromethanesulfonate as cata-
lyst. Poly(
-caprolactone) with narrow polydispersity and a molecular weight in the
order of 10
4
Da was produced in quantitative yield after 33 h at room temperature
in toluene. Only 0.16 mol% of catalyst was required. Similar results were obtained
for poly(
ε
- valerolactone). Notably, the reaction was relatively tolerant to the pres-
ence of moisture and other contaminants [26].
δ
1.2.2
Metal - Free Synthetic Processes
The use of low molecular weight organic molecules to catalyze ring-opening
polymerization is a rapidly expanding fi eld [27]. Organocatalytic routes toward
polyesters typically involve a nucleophilic polymerization mechanism; problems
with side reactions are minimal and products with high molecular weights and
very narrow polydispersities can be formed. In addition, organocatalysts may be
signifi cantly more abundant and less toxic than metal containing catalysts. These
factors point toward future large-scale synthesis applications. Many examples are
based around traditional acyl substitution catalysts such as phosphines [28], and
pyridine-derivatived nucleophiles [29]. In more recent developments,
N
- heterocyclic
carbenes have shown great promise, allowing the synthesis of polylactides with
molecular weights up to 10
4
Da [30]. Supramolecular catalysts are known, which
can stabilize transition-states in a noncovalent fashion and thus can exert a great
effect on reaction rate and mechanism. To this end, thiourea- containing supramo-
lecular catalysts have shown excellent promise [31].
The enzyme -catalyzed synthesis of polyesters is another technique that is being
developed as a very ecologically friendly process with several benefi ts over conven-
tional chemical polymerization [32]. Enzymatic reactions are often extremely regio-
and stereospecifi c, so unwanted side reactions can be largely eliminated.
Lipase-catalyzed ring-opening polymerization has been applied to many substrates
including a wide range of lactones and lactides. As an example, poly(d,l - lactide)
with molecular weights up to 10
5
Da could be synthesized in bulk, but recovery
yields were relatively low [33]. Among the problems associated with enzymatic
polymerization are the high cost of enzymes, long reaction times, and relatively
low molecular weight products. These challenges are to be met before enzymes
can be used for industrial scale synthesis.
1.2.3
Polyanhydrides
Polyanhydrides are an important class of biodegradable polymers which are closely
related to the polyesters [34, 35]. Monomers used are commonly hydrophobic long
chain fatty-acid-derived diacids or aromatic group containing diacids such as
shown in Scheme 1.4a. Polyanhydrides are treated in detail in Chapter 3. They are
