Page 57 - Biodegradable Polyesters

2.2 Radical Ring-Opening Polymerization (RROP) of Cyclic Ketene Acetals 35

An ion-containing polymer with low mol% of ionic groups along the polymer
backbone chains or as pendant groups (defined as ionomers) shows clustering of
ionic units. This is in contrast to the ionic–ionic repulsions in polyelectrolytes.
The clustered ionic groups act as fillers and provide altered physical properties,
such as enhanced mechanical properties, high melt viscosity, increased thermal
stability, and altered degradability characteristics to the polymer. Bringing ionic
units onto the biodegradable polyester backbone/side chain could be challenging
by conventional synthetic routes of making polyesters. There were only few
examples of biodegradable polyester-based ionomers known [47, 48]. In one of
the approaches, polyester-based random ionomers were made via conventional
two-step polycondensation using adipic/succinic acid and 1,4-butanediol in
the presence of dimethyl 5-sulfoisophthlate [49, 50]. Radical ring-opening
copolymerization of CKAs with vinyl monomers having charged units or with
monomers capable of undergoing polymer-analogous reactions for the formation
of charged structures would provide another option of making biodegradable
polyester-based ionomers. Radical terpolymerization of MDO with MMA and
N,N-dimethylaminoethyl methacrylate (DMAEMA) followed by quaternization
of DMAEMA with ethyl bromide provided PCL-based degradable ionomers [51].
The ionomers showed strong ionic interaction with the formation of aggregates
with a diameter around 30 nm as proved by small-angle X-ray scattering (SAXS)
analysis and the transmission electron microscope (TEM). The aggregates acted
as fillers and provided improved modulus 390–570 MPa and elongation at break
200–250% depending on the amount of ionic groups. Ionomers containing
40 mol% of ester group and 20 mol% of ionic group showed compostability as
shown in the Figure 2.2.
Further biodegradable polycations are of interest as nonviral gene transfection
agents for gene therapy, which is used for the potential treatment of genetic
and inherited diseases. Novel degradable gene transfection agents were made
by copolymerization of BMDO and DMAEMA with an additional poly(ethylene
oxide) (PEO) block for increasing the water solubility and biocompatibility
of polymers. PEO blocks were introduced either by using PEO-azo-initiator
[52, 53] or clicking PEO block using alkyne–azide chemistry [54]. The polymers
were enzymatically degradable (Figure 2.3) and biocompatible showing signifi-
cantly less toxicity with an MTT assay using L929 cell lines and promising DNA
transfection efficiency compared with the gold standard poly(ethyleneimine).
For many biomedical applications and smart surfaces, thermoresponsive
polymers have been researched. A thermoresponsive polymer shows a sharp
change in properties such as hydrophilicity/hydrophobicity upon small change
of temperature. The outcome is the temperature-controlled phase separa-
tion of thermoresponsive polymers in water and organic solvents showing
either lower critical solution temperature (LCST) [55] or upper critical solu-
tion temperature (UCST) [56]. Poly(N-isopropylacrylamide) (PNIPAAm) and
poly(oligo(ethyleneglycol) methacrylates) are examples of thermoresponsive
polymers showing LCST. Both of these polymers have C–C backbone and hence
are not biodegradable. It is desirable to have thermoresponsive biodegradable