Análisis Microbiológicos al producto final

The livingness of RAFT polymerisation permits the sequential addition of monomer to obtain highly defined structures called block copolymers comprised of blocks of different

monomers.73-74 The RAFT mechanism allows all polymer chains to grow using only a small

fraction of active radical species, meaning a high proportion of the polymer chains are “living” i.e. possessing the thiocarbonylthio moiety, as expressed by Equation 1.6.53

𝐿 (%) = [𝐶𝑇𝐴]0

[𝐶𝑇𝐴]0+ 2𝑓[𝐼]0(1 − 𝑒−𝑘𝑑𝑡)(1 − 𝑓_{2 )}𝑐

Equation 1.6 - Theoretical determination of the relative amount of living polymer chains using an azo-initiator compound.

The fraction of living chains, L (%), can be calculated using Equation 1.3, where [CTA]0 and [I]0 are the initial concentrations of CTA and initiator, respectively, kd and f describe the thermal decomposition of the initiator and 1-fc/2 the termination mechanisms. Utilising a high ratio of [CTA]/[I]consumed results in a very high fraction of the “living” ω-chain ends, offering the possibility of efficient chain extension(s). In this regard, a low flux of radical species is beneficial. However, the rate of propagation (Rp) in RAFT polymerisation is proportional to the concentration of active radical species, which itself is related to the initial initiator concentration and the rate at which is decomposes, as for conventional radical polymerisation (Equation 1.7).63

𝑅𝑝 = 𝑘𝑝[𝑀][𝑃˙] = 𝑘𝑝[𝑀]√

𝑓. 𝑘𝑑[𝐼]0𝑒−𝑘𝑑𝑡

𝑘𝑡

Equation 1.7 - Theoretical determination of the rate of propagation for RAFT polymerisation.

Where [M] and [P˙] are the concentrations of monomer and polymeric radical species, respectively; kp andkt arethe rate of propagation and termination, respectively; f is the initiator efficiency and [I]0 is the initiator concentration at time t. Therefore, monomer conversion is strongly correlated to the initiator concentration. In order to successfully conduct a RAFT

Page | 18 polymerisation to high monomer conversion, yielding polymers with a narrow molar mass distribution and also a high fraction of living chains, a fine optimisation of the [M]/[CTA]/[I] ratio has to be established.

When synthesising a block copolymer by RAFT, one of the key parameters to take into account is the choice of the Z group of the CTA, since its ability to stabilise the intermediate radical must be compatible with the radical derived from each monomer.70 Additionally, the order of the chain extensions is governed by the reactivity of the monomers.75 The first block acts as a macroCTA for the polymerisation of the second block, hence the reactivity of the propagating radical of the first block will affect the chain extension with the second monomer. Indeed, conditions which allow to proceed to the main equilibrium (i.e. R being an efficient homolytic leaving group and R˙ adding to monomer) must again be satisfied. The order of the synthesis of block copolymers require careful optimisation: the more activated monomers, which polymers are better homolytic leaving group, should be polymerised prior to the LAMs in order to ensure the successful chain extension of all the polymer chains.64, 70

The other aspect to take into consideration in the preparation of block copolymers using a degenerative transfer mechanism such as RAFT is that a continuous source of exogenous radicals are required.70 _{The radicals generated from the decomposition of the} initiator will react with the newly added monomer and propagate, yielding homopolymers of the second monomer. The proportion of this undesired homopolymer is related to the ratio of (macro)CTA to radicals introduced. Under conventional RAFT conditions, this ratio will be relatively low, and therefore the majority of polymer chains in the final population will be the desired AB block copolymer. However, when targeting further block extensions, the presence of undesired polymer chains (dead chains and initiator derived chains) is compounded and can become non-negligible, typically leading to a poorly defined population.

When targeting multiple block extensions via RAFT, to yield multiblock copolymers, conditions must be carefully considered in order to minimise the contribution of dead and initiator derived chains.76 Traditionally, this entails using a relatively high ratio of CTA to initiator and stopping each polymerisation cycle at moderate monomer conversion (<70 %), thereby requiring timely and costly purification steps between each block extension.77-78 However, in 2013 Gody and co-workers demonstrated how well-defined multiblock copolymers could be readily prepared via RAFT in a one-pot system through a rigorously considered approach.53, 76, 79 By polymerising acrylamide monomers, which possess a high

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kp/(kt)1/2 compared to most other vinyl monomers, and working at high monomer concentrations, near-quantitative monomer conversion could be achieved for each polymerisation cycle with very low initiator concentrations. This system could be further optimised by using water as solvent, which further increases the kp of acrylamides. Finally, it was shown how targeting a low degree of polymerisation for each RAFT polymerisation cycle and using rapidly decomposing azo-initiators permitted the one-pot synthesis of a well-defined multiblock copolymer with an unprecedented number of blocks (21) in a short time frame (2 h per block).79 Well-defined polyacrylate and polymethacrylate multiblock copolymers have also since been successfully prepared by carefully optimising the conditions of the RAFT process.80-82 _{With such time and resource-efficiency, complex multiblock copolymer} architectures are becoming increasingly accessible and offer enormous potential for industrial applications.83-84

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