# Polymer@gold Nanoparticles Prepared via RAFT Polymerization for Opto-Biodetection
## 3 RAFT Polymerization  
### General Concepts  
Reversible-deactivation radical polymerization (RDRP), historically known as living or controlled radical polymerization (CRP) [62], allows the synthesis of block copolymers and complex polymer structures with narrow and controlled molecular weights [63; 64; 65; 66; 67]. The most known and widely used RDRP technologies are: nitroxide mediated polymerization (NMP), atom transfer radical polymerization (ATRP) and reversible addition fragmentation chain transfer (RAFT) polymerization. All of these controlled polymerization mechanisms involve a polymerization mediator which in the case of RAFT polymerization helps to control the equilibrium between the dormant and active species during the propagation step [63; 64]. Amongst these mechanisms, RAFT polymerization has shown to be the most powerful mechanism due to its versatility associated with mild reaction conditions, no use of transition metals, the possibility of using several types of monomers, including monomers with functional groups (e.g., OH, NR\({}_{2}\), CO\({}_{2}\)H, SO\({}_{3}\)H, CONR\({}_{2}\)) and like others, allows well-controlled polymer architectures. Moreover, hydrophilic and amphiphilic polymers can be readily synthesized, which is important for bioapplications [64; 65; 66; 67; 68]. Additionally, RAFT polymerization has been successfully used for controlled functionalization of planar surfaces as well as micro- and nano-particles which is of major interest for a wide range of applications [1; 68]. The control over the composition and architecture of RAFT polymers depends on the kinetics of the reaction and the elimination/minimization of radical-radical terminations which is due to the use of a chain transfer agent (CTA), also known as RAFT agent. The CTA, with the structure shown in Figure 5, confers a \"living\" character to the polymerization since it mediates the polymerization via a reversible chain-transfer process and its efficiency depends on the nature of the groups X, Z and R (Figure 5a). Although CTAs where X = CH\({}_{2}\) (Figure 5c) have been used, the most efficient CTAs are thiocarbonylthio compounds (Figure 5b) [65; 66; 69]. Figure 4: Schematic representation of surface stabilization and or functionalization of gold nanoparticles (Au NPs), followed by surface (bio)functionalization. In turn, the Z and R groups are crucial for the efficiency of the addition-fragmentation reactions. The Z group controls the reactivity of the C=S double bond, influencing the rate of radical addition and fragmentation. The R group is the radical leaving group which also has to be able to reinitiate the polymerization for chain transfer. For an ideal RAFT agent, the dormant species should have a reactive C=S double bond, the intermediate radical should fragment rapidly and give no side reaction and lastly radicals (R\(\bullet\)) should efficiently re-initiate polymerization [1; 65; 68; 69; 70; 71]. The most commonly used CTAs are the thiocarbonylthio compounds, with a general structure Z-(C=S)-S-R (e.g., dithioesters, xanthates, dithiocarbamates and trithiocarbonates--Figure 6), which have been used for the synthesis of different homopolymers and copolymers. [PERSON] et al. 2005 and 2006 [65; 66] summarized a wide range of these CTAs that were used until then, and [PERSON] et al. 2007 [70] focused on CTAs used in the synthesis of water-soluble copolymers. The choice of the CTA is a key aspect for a successful RAFT polymerization and it depends not only on the properties of the R and Z groups but also on the monomer that will be polymerized, as well as the reactions conditions. Indeed, [PERSON] et al. [65; 66] have outlined some guidelines for the selection of RAFT agents based on the efficiency of the Z and R groups in the polymerization of some monomers such as methyl methacrylate (MMA), vinylacetate (Vac), styrene (S), methyl acrylate (MA), acrylamide (AM), and acrylonitrile (AN).