STIMULI-SENSITIVE PARTICLES FOR DRUG DELIVERY - Biologically-Responsive Hybrid Biomaterials - page 165

2.1. Reversible addition-fragmentation chain transfer (RAFT) technique
RAFT polymerization requires the presence of a reagent that reacts by reversible
addition-fragmentation chain transfer mechanism (Figure 6). The effectiveness of
these reagents depends on their transfer constant, which is determined by the
nature of the X, Z, and R groups. Certain thiocarbonylthio compounds where X
is sulfur, R is a free radical leaving group that is capable of reinitiating
polymerization, and Z is a group that modifies the activity of the transfer
constant proved to be the most effective reagents in RAFT polymerization. [43]
Initiation of RAFT polymerization starts with cleavage of the initiator (e.g.
azobisisobutyronitrile; AIBN) to yield reactive free radicals (I·) that react with
the monomer (M) to form a growing polymer chain (P n ·). Subsequently, the
growing chain adds to the RAFT agent followed by fragmentation and release of
a free radical leaving group (R·), which re-initiates the polymerization reaction.
The ratio of monomer to RAFT reagent determines the molecular weight of the
resulting polymer and its distribution. Given the fact that this reaction involves
the formation of free radical intermediates, radical-radical termination reactions
Fig. 6. Schematic drawing showing the mechanism of reversible addition-fragmentation chain
transfer (RAFT) polymerization.