Grafting of Functional Monomers on Biomaterials - Biointerfaces: Where Material Meets Biology - page 324

Box 3: Examples of post polymerisation reactions for introduction of ionic
functional groups 54,58

Carboxylate Hydrolysis of tert-butyl acrylate (tBA) or tert-butyl methacrylate
(tBMA) using strong acid (e.g., 10 % HCl or CF 3 COOH).
Pyrolysis of tBA or tBMA at 190-200 1C.
Esterfication of 2-hydroxyethyl methacrylate (HEMA) with
succinic anhydride in the presence of pyridine.

d n 3 r 4 n g | 2

Amine CDI activation of HEMA followed by reaction with ethylene

diamine.
Amination of glycidyl methacrylate (GMA) by reaction with
excess ethylene diamine.

Sulfate Sulfation of GMA using sodium sulfite.

Sulfation of styrene (St).

the vapour phase, upon exposure to plasma, is converted into reactive spe-
cies as a result of fragmentation reactions. These species in turn recombine
to produce the plasma polymer on the substrate surface exposed to the
plasma. The polymers formed in the plasma do not necessarily have similar
chemical composition or topology to the polymers obtained by the con-
ventional polymerisation techniques discussed in the sections above. 1,4
Generally, plasma polymers are pin-hole free and highly cross-linked as il-
lustrated in Scheme 11.1D. They are therefore insoluble, chemically inert,
thermally stable and mechanically tough. They are not attached to the
polymer substrate via covalent bonds as the graft co-polymers described in
the sections above but they are rather coated onto the substrate. One of the
main advantages of this approach to surface grafting of functional mono-
mers is that the same outcome is achieved regardless of the substrate which
is in contrast to the previous methods discussed above where optimisation
of the grafting conditions is required for each polymer substrate. In plasma
polymerisation, once the conditions for producing the plasma polymer have
been optimised for a particular monomer, it can be applied to any sub-
strate. 1,4 However, the reaction conditions do need to be optimised for a
particular plasma reactor. Another advantage of this technique is that it is
solvent-free eliminating concerns of residual toxic solvents in the modified
substrate. However, due to the processes of fragmentation and recombin-
ation taking place in the plasma reactor loss of functional groups can be a
drawback. There are many parameters that affect the outcome of a plasma
polymerisation reaction including the plasma conditions (power, frequency,
pressure, monomer flow rate, exposure time, position of sample in plasma
chamber) and the monomer. It is generally found that low radio-frequency
power and/or pulsing result in better retainment of the functional groups;
however, too low a power will lead to low cross-linking density of the plasma

.