Grafting of Functional Monomers on Biomaterials - Biointerfaces: Where Material Meets Biology - page 322

for the attachment of an ATRP initiator (commonly 2-bromoisobutyrate
bromide, BIBB) or a chain transfer agent (CTA) for ATRP and RAFT poly-
merisation, respectively. The polymerisation reaction can subsequently take
place from these surface-bound molecules. In some polymer substrates,
functional groups exist as part of the structure enabling direct attachment of
an initiating molecule to the polymer substrate. Cellulose is an example of
such a polymer for which a comprehensive review by Roy et al. on modifi-
cation by polymer grafting serves as an excellent reference. 56 In other polymer
substrates, however, an initial step of introducing functional groups onto the
surface is required before CRP can take place. This is the case for most of the
polymer substrates covered in this chapter (both biostable and biodegrad-
able) and many methods exist which allow introduction of functional groups
as illustrated in Table 11.3. The factors which affect the grafting outcome
when using CRP are similar to those affecting chain length in solution-based
CRP and include not only monomer concentration but also concentration of
other reagents. The density of the grafted chains will of course be dependent
on the density of the functional group and subsequently the density of the
attached initiating molecule. This can be controlled to some extent by using a
mixture of initiating molecules and blocking agents attached to the surface. 57
In addition, for RAFT polymerisation using the R-group approach it has been
found that the presence of a CTA in the grafting solution (and not only
attached to the substrate surface) has a significant effect on the grafting
outcome as evident in recent reviews. 54,56
In addition to the use of monomers already carrying functional groups, it
is common to carry out post polymerisation reactions 54,58 to introduce some
of the functional moieties which are the focus of this chapter. Box 3 gives
examples of such post polymerisation reactions. This approach is often used
for CRP due to the low tolerance of CRP to certain functional groups and
examples of grafting monomers suitable for post polymerisation reactions
are thus included in Table 11.3. It should be noted, however, that while such
approaches are useful for chemically stable polymer substrates they may not
be able to be transferred to hydrolytically labile substrates such as poly-
esters. Furthermore, the approach is not applicable if the surface-bound
initiating molecule contain, for example, ester bonds as some of the grafted
chains may be lost during the post polymerisation reaction. As an alternative
to initiating the polymerisation reaction from an initiating molecule-func-
tionalised surface, radiation or plasma mediated CRP from a surface can be
used without the requirement for functional groups to be present on the
substrate's surface. Most work in this area has used monomers which re-
quire post polymerisation reactions to introduce the functional groups of
interest as illustrated in Table 11.3.

d n 3 r 4 n g | 2

.

11.2.4 Plasma Polymerisation
Plasma polymerisation, also known as plasma deposition, is a means to
create a thin film (termed a plasma polymer) on a substrate. A monomer in