Chemistry Reference
In-Depth Information
The macromonomer transfer agent employed for this polymerisation contained a
terminal CH
2
=C(CO
2
R)-CH
2
- moiety attached to a methacrylate polymer.
Even though
block copolymers were successfully synthesised by this technique, it only worked for
methacrylic monomers. To produce living polymerisation conditions for monomers
other than methacrylic ones, changes had to be made with respect to the transfer
agent. One common feature shared by both the allylic sulfide and the macromonomer
transfer agent in the previous examples is the unsaturated carbon-carbon double bond.
In 1998 the CSIRO group replaced the alkene functional group with a dithioester
[101]. The primary difference between the alkene transfer agent and the dithioester
transfer agent is the rate constant of transfer reactions. The transfer constant for
the dithioester compound is significantly greater than that of the alkene. This large
transfer constant allows for the rapid equilibrium reaction between the active and
dormant polymer states, as shown in
Figure 4.8
. By replacing the alkene functional
group with a dithioester, CRP was achieved for a multitude of monomers. The term
reversible addition-fragmentation transfer (RAFT) was first reported in the literature
in the seminal paper by Rizzardo and co-workers in 1998 [101]. The term RAFT
applies specifically to a transfer agent that contains the dithioester functional group
provided in
Figure 4.8
.
RAFT involves a reversible addition-fragmentation chain transfer between an active
and a dormant species and is performed by adding a suitable dithioester transfer
agent, which has the appropriate Z and R groups selected to provide an effective
transfer process. Adjusting the Z and R groups of the transfer agent seen in
Figure
4.8
has led to the production of myriad RAFT agents. The choice of Z and R groups
will have an effect on the chain transfer constant for a particular RAFT agent. The
purpose of the Z group is to stabilise the radical intermediate. Attaching a Z group
that stabilises the radical intermediate will increase the rate transfer constant. There
is a drawback, however, if the radical intermediate is too stable. The RAFT agent has
to produce a radical that is reactive enough to initiate polymerisation. If the radical
intermediate is too stable, it will not fragment to produce a radical initiator, thereby
terminating the reaction.
The selection of the R group is also important in the polymerisation. R should be
a good homolytically leaving radical group that should also efficiently reinitiate
polymerisation. There is a trade-off between the R group as a good leaving group
and a radical initiator: the better the leaving group, the worse the radical initiator.
The S-R bond strength influences the R group aptitude as a good leaving group and a
radical initiator. The most encountered R groups are ones that are sterically hindered
and contain an electron withdrawing substituent. Both factors help to stabilise the
radical formed by the homolytic cleavage of the S-R bond. By stabilising the radical,
the formation of the said radical is rapid.
