# Polymer@gold Nanoparticles Prepared via RAFT Polymerization for Opto-Biodetection
## 3 RAFT Polymerization
### General Concepts
The RAFT polymerization mechanism is based on the reversible chain transfer and equilibrium between active and dormant chains, as illustrated in Figure 7A. It starts with the initiation step where a polymeric active chain (\(P\)n\(\bullet\)) is formed via conventional radical polymerization. Then the primary radical formed reacts with the CTA, thus forming an intermediate radical. In turn, this intermediate radical suffers fragmentation resulting in a dormant polymeric chain (which contains the -S-C(=S)-Z of the CTA) and a radical species (\(R\bullet\)). This radical species reacts with monomer (\(M\)) to form an active polymeric chain (\(P_{1}\bullet\)). The rapid equilibrium established between the active chain (i.e., the propagating radical) and the dormant chain (i.e., the polymeric chain which contains the CTA also referred to as macroCTA or macroRAFT agent) is fundamental to control the molecular weight, as well as to ensure that the concentration of dormant chains is greater than that of active chains [1; 65; 66; 67; 68]. The molecular weight of the polymer chains and its distribution, the composition and the architecture of the resulting polymers can be controlled due to the fact that the termination step is minimized. Suppression, or at least significant reduction of termination is due to the equilibrium that is established between the propagating polymeric chains and a macroCTA, also named macroRAFT agent. The CTA is preserved in the chain giving a \"living\" character to the macrochain which can be extended by adding a second monomer, as illustrated in Figure 7B. In fact, the resulting macroCTA can be isolated and subsequently used in another batch under distinct reaction conditions, for instance aqueous medium [65; 66; 67; 68]. Figure 5: Structures of various chain transfer agent (CTAs)/RAFT agents. Figure 6: Generic structures of RAFT chain-transfer agents. Essentially the control of molecular weight of the polymer chains is achieved by the ratio between the initial concentration of the monomer and the CTA (\([M]_{0}\)/\([\)CTA\(]_{0}\)), at a given initiator concentration. However, the full breast of molecular weight distribution (FMWD) can be rather complex. Even though the termination step is minimized it does occur and different reactions paths can compete as generally illustrated in a condensed manner in step (v) in Figure 7A,B respectively. Moreover, in the case of chain extension, the following types of dead polymers can occur: _R-P_1-P_1-R_, _R-P_1, _R-P_1-P_1-I_, _I-P_1-P_1-I_, _R-P_1-P_1-I_, _R-P_1-P_2-P_2P_1-R_, _R-P_1P_2-P_1-R_, _R-P_1P_2-P_1-R_, _R-P_1P_2-P_2-I_, _R-P_1-P_2-P_2-I_, _I-P_2-P_2-I_, _I-P_1P_2 and _R-P_1P_2-P_2P_1-I_. Furthermore, other types of termination involving intermediate adducts can also take place and different schools of thought have proposed different kinetic schemes and developed advanced calculation tools to simulate the FMWD and get a better understanding of the mechanism [72]. In fact, the kinetics of the various steps and mechanistic subtleties, associated with the type of RAFT agent, and reaction conditions are determinant to achieve tight control of the molecular weight distribution, where diffusional limitations can not be ignored. Nevertheless, broadening of MWD is often reported but it can be often due to poor handling of macroRAFT agents as the thiocarbonylthio groups is sensitive to oxidizing agents present in some solvents, as well as to pH, temperature and light [73]. [PERSON]'s group, in particular, has been studying the kinetics and mechanisms of RAFT polymerization for a long time using a wide range of monomers, chain transfer agents and reaction conditions, including light initiation besides the standard thermal initiation. Studies carried out using pulse laser polymerization combined with size exclusion chromatography (PLS-SEC) and robust kinetic models have been frequently reported in this last decade.