Grafting of Functional Monomers on Biomaterials - Biointerfaces: Where Material Meets Biology - page 318

The phosphate-containing monomers MAEP and MOEP contain a large
percentage of the corresponding diene in the commercial monomers
(25%) and as a result highly branched or cross-linked graft co-polymers
form in most solvent systems. 27 However, a linear polymer topology of
the grafted chains can be achieved in a two-phase mixed solvent sys-
tem. 28 An additional complexity of using these phosphate-containing
monomers in graft polymerisation reactions is the instability of the
ester bonds during polymerisation (the extent of ester hydrolysis is
dependent on the solvent used) and detailed analysis of the graft co-
polymers (as well as soluble polymers produced by RAFT mediated
polymerisation 29 ) revealed that the MOEP graft co-polymer is best de-
scribed as MOEP-co-HEMA while the MAEP graft co-polymers are MAEP-
co-AA. 8-10,28,30

d n 3 r 4 n g | 2

11.2.2 Peroxy/hydroxyl Grafting
The peroxy/hydroxyl grafting method is a two-step method. In the first step
reactive species are formed upon exposure of the polymer substrate to ra-
diation and in the second step the monomer is grafted onto the activated
surface. In the first step, hydroperoxides or diperoxides species are intro-
duced onto the polymer substrate as shown in Scheme 11.1B. As above for
the simultaneous radiation method, abstraction of a hydrogen (or other
element) from the polymer backbone must take place before such reactive
oxygen species can be introduced. This can be achieved in a number of ways
as listed below and the first four methods will be considered in detail:

.

Exposure of the polymer substrate to high energy radiation (gamma ray
or electron beam) in vacuum or in the presence of an inert gas followed
by exposure to air
Exposure of the polymer substrate to high energy radiation (gamma ray
or electron beam) in the presence of oxygen or air
Plasma treatment in an inter atmosphere followed exposure to air
Plasma treatment in an oxygen, water or air atmosphere
Hydrogen peroxide or ozone treatment (sometimes combined with UV
radiation) 23

The peroxy/hydroxyl method is by some authors referred to as the 'pre-
irradiation method'. However, this is confusing as that term (together with
'post-irradiation grafting') refers to a process where there is no exposure to
an oxidising atmosphere at any point in the surface modification process.
Indeed, the first radiation step involves exposure of the polymer substrate to
radiation in vacuum or in the presence of an inert gas and in the second step
the polymer substrate is exposed to a monomer either in the gas phase or in
solution. The degassed monomer solution must be transferred to the poly-
mer substrate through an air-tight seal in order to avoid exposure to oxygen.
Thepre-irradiationmethodisnotcommonlyusedinbiomaterialsscienceand