Factores limitantes de los suelos de Encrucijada

become perhaps the most widely used and investigated RDRP process in the field of polymer synthesis as a consequence of its robust synthetic conditions as well as its remarkable versatility in polymerizing a wide variety of monomers.26 Polymers and copolymers synthesized by the RAFT polymerization have a predictable and easily controlled molecular weights with narrow dispersities, typically ÐM ≤ 1.4, hence fulfilling characteristics of a

Commonwealth Scientific and Industrial Research Organization (CSIRO) in Australia, the RAFT polymerization technique uses a thiocarbonyl compound as a chain transfer agent (CTA) to mediate the polymerization and create an equilibrium between active and dormant species which reduces the chance of termination reactions.21,24While the term RAFT is most commonly used to describe this technique, it should be noted that it is also often referred as Macromolecular Design via Interchange of Xanthates (MADIX), in which xanthate compounds are used as chain transfer agents. The MADIX process was concurrently developed in France by Zard et al. with the industrial collaboration of Rhodia, and is now considered as a specific type of RAFT polymerization.27,28The general mechanism of RAFT is similar to the one of a conventional FRP, with the addition of two significant steps: pre- equilibrium and main equilibrium (Scheme 1.3). Similar to the conventional FRP, the process begins with the decomposition of a thermal initiator to produce radical species (I•) which subsequently react with monomers (M) to form the first radical polymer chains (Pn•,

Scheme 1.3a). These oligomeric chains then react with the thiocarbonyl group of the CTA to form a radical polymer intermediate in the pre-equilibrium (Scheme 1.3b). The intermediate can then undergo a reversible fragmentation to either produce a polymeric CTA and release a radical R-group (R•), or revert to the initial growing polymer chain (Pn•) and reform the CTA.

The radical R-group can then reinitiate the polymerization of further monomers to produce other growing polymer chains, Pnand Pm, (Scheme 1.3c). When all the CTA has been reacted

and all polymer chains are capped by the RAFT CTA end-group, the reaction enters the main equilibrium (Scheme 1.3d). During this equilibrium, rapid exchange between active and dormant species occurs allowing all polymer chains to grow at a similar rate and ensure that the concentration of active radicals is kept low, hence reducing the probability of termination reactions occurring. However, some termination can still occur during the polymerization process (Scheme 1.3e) but the occurrence is dramatically reduced compared to conventional

free radical polymerization. Under these conditions, well-defined polymers with controlled molecular weights and narrow dispersities are usually obtained.

Scheme 1.3.Schematic representation of the general mechanism occurring in the RAFT polymerization with (a) initiation, (b) pre-equilibrium, (c) propagation and re-initiation, (d) main

equilibrium and (e) termination.24

1.2.4.1 Importance of the RAFT chain transfer agent

The choice of the CTA is an important parameter of the RAFT process and its structure can significantly affect whether the polymerization will achieve living-like conditions and produce well-defined polymers with controlled molecular weights and narrow dispersities.29,30The type of Z and R groups of the thiocarbonyl compound strongly affect the efficiency of the CTA by influencing the addition and fragmentation rates of the polymerization process. The nature of the Z group influences the stability of the thiocarbonyl group and hence the stability of the radical intermediate formed during the main RAFT equilibrium. Electron withdrawing groups (Z = -Ph, -SR) will tend to increase the reactivity

radical intermediate is favoured as it will be more stable than the propagating radicals. Conversely, electron donating groups (Z = -OPh, -N(Et)2) will have the opposite effect and

the radical intermediate will not be stabilized as the reactivity of the thiocarbonyl bond will be decrease towards the radicals. Based on the nature of the Z group, four different types of CTA have been reported: dithioesters, trithiocarbonates, dithiocarbamates and xanthates (Figure 1.2).22,25,31,32

Figure 1.2.Schematic representation of the four types of chain transfer agents used during the RAFT polymerization process.

More activated monomers (MAMs), such as styrenes, acrylates, methacrylates or acrylamides form more stable propagating radicals and therefore require RAFT CTAs with higher chain transfer constants and a higher ability to fragment such as dithioesters or trithiocarbonates. In comparison, less activated monomers (LAMs) such as vinyl acetate (VAc), N-Vinylpyrolidone (NVP) and other vinyl ester monomers form unstable radicals requiring the use of RAFT CTAs with lower chain transfer constants, such as xanthates and dithiocarbamates. Using dithioester and trithiocarbonate chain transfer agents on less activated monomers such as VAc would lead to a polymerization with inhibited and retarded effects as a consequence of the poor leaving groups of the monomer which will lead to a low fragmentation rate and hence a poor control of the polymerization process. Similarly, using xanthates and dithiocarbamates on more activated monomers such as acrylates would lead to an inefficient control over the polymerization as a consequence of the decreased reactivity of the carbonyl bond. Encompassing the stability of the Z group and differing monomer reactivities, Moad et al. presented guidelines for the selection of the RAFT agents to use (Figure 1.3).33

Z: Ph >> SMe > N ~ Me >> N O

> OPh > OEt ~ N(Ph)(Me) > N(Et)2

MMA, MAM S, MA, AM, AN VAc, NVP, NVC R: CN CH3 CH3 Ph CH3 CH3 ~ > Ph H CN > COOEt CH3 CH3 >> CH2 CH3 CH3 CH3 CH3 CH3 ~ CN CH3 H ~ Ph H CH3 > CH3 CH3 CH3 ~ Ph H H MMA, MAM S, MA, AM, AN VAc, NVP, NVC

Figure 1.3.Schematic representation of the guidelines for the selection of the correct RAFT agent for various monomer systems. For the Z group, the fragmentation rate increases from left to right. For R

group, the fragmentation rates decrease from left to right. Reproduced from33

The nature of the R group of the CTA is also an important parameter affecting the successful outcome of the polymerization. Indeed, the R group should be a good leaving group compared to the propagating polymer chains and also be able to effectively re-initiate the polymerization.23 Hence, caution should be taken for the choice of both the Z and R groups to be use in the chain transfer agent. Correct selection of the Z and R groups enables successful control of the polymerization of a wide range of monomers, hence rendering the RAFT polymerization process a very versatile and applicable technique.