Polyelectrolytes, and sulfonated polymers (Figure 2-1) in particular, have gradually become key components in important industrial processes such as water purification, oil recovery or fuel cell preparation.82,83,92 Amongst them, PAMPS are of a particular
interest as they combine high thermal stability, resistance to hydrolysis and a high solubility in water over a wide range of pH.93 PAMPS is used as rheology modifier,
dispersant for oil spills, or as biocompatible hydrogels for various biomedical purposes.82,84,86,92 In all these applications, the molecular weight, architecture and
chain-end functionality of the polyelectrolyte plays an essential role in controlling the physical-chemical properties of the resulting material.
Figure 2-1: Examples of sulfonic acid monomers used in the literature.56,94
PAMPS is commonly prepared via conventional radical polymerisation in aqueous solution at low temperature using redox initiation. This process has been widely used for about three decades, albeit with a lack of control over the resulting molecular weight and chain-end functionality of the polymer, typically resulting in dispersities above 1.5.95-99 Today, the emergence of controlled / living radical polymerisation
(NMP,100 ATRP,101 or RAFT42) allows for the preparation of well-defined polymers,
with respect to both molecular weights and architecture.102 Mincheva et al., optimised
the polymerisation of AMPS® by transformation into the sodium salt by addition of
NaOH, forming AMPS(Na) (pH 7.5, 8, 9, 10 and 12). They used copper-mediated ATRP in a mixture of methanol and water and reported the influence of pH and ligand type on the resulting materials.103 The optimal conditions for well-control ATRP of
AMPS(Na) in water and methanol (3:1) was found to be with Me6-TREN ligands at
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within 3 hours obtaining narrow and monomodal SEC molecular weight distribution with a dispersity of about 1.29.
Scheme 2-1: PAMPS synthesised by Mincheva et al. using Me6-TREN ligand.103
In another example, Nikolaou et al. achieved full monomer conversion in less than 30 minutes by polymerising AMPS®2405 using copper-mediated living radical
polymerisation in aqueous solution at 0 °C still with Me6-TREN.33 PAMPSDP≤80 with
narrow and monomodal molecular weight distributions were prepared by tuning the following ratio [I]:[CuBr]:[Me6-TREN] and maximising the amount of deactivating
species formed during the disproportionation step. This method however yielded poor control over the polymerisation when degree of polymerisation (DP) higher than 80 were targeted. Additionally, the use of a metal-catalyst requires a further step of product purification, which might be an issue for associated applications or scale-up manufacturing.
Scheme 2-2: PAMPS synthesised by Nikolaou et al. using Me6-TREN in water.33
Another convenient approach to prepare polysulfonated materials is to use RAFT polymerisation. Only a few examples of polysulfonated polymers synthesised by RAFT polymerisation have been reported in the literature using either polar organic solvents (e.g. alcohol) or aqueous solution. Matyjaszewki et al. synthesised well- defined AMPS® homopolymer in methanol at 60 °C using cumyl dithiobenzoate as
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2-3).104 While a dispersity of 1.36 was obtained the monomer conversion determined
by 1H NMR spectroscopy was only about 60 %, with susbsequent purification by
dialysis being required to remove the remaining monomer.
Scheme 2-3: RAFT polymerisation of AMPS® in methanol.104
In another example, McCormick and co-workers reported the polymerisation of sodium 4-styrenesulfonate in aqueous solution at 70 °C using 4-cyanopentanoic acid, a dithiobenzoate RAFT agent, and 4,4′-azobis(4-cyanopentanoic acid) (ACVA / V- 501) as the initiator.94 Full monomer conversion was obtained within 8 hours and
monomodal SEC chromatograms were obtained depicting dispersities as low as 1.25.
Scheme 2-4: RAFT Polymerisation of sodium styrene sulfonate monomer.94
More recently McCormick et al. focused their research on the aqueous RAFT polymerisation of AMPS(Na) (AMPS® with NaOH in water) still using 4-
cyanopentanoic acid dithiobenzoate as CTA and ACVA as the initiator ([CTA]0/[I]0 =
5).58,105-107 They first reported the polymerisation of AMPS(Na) in aqueous solution at
70 °C and pH 9.5 (Scheme 2-5).105,106 Polymers with low dispersity were obtained (Ð
< 1.3) and a linear increase of both the molecular weight and first order kinetics with time was observed. However, an induction time of about 1 hour was observed, likely due to the slow rates of reinitiation by the radical created from the R-group of the
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CTA. Additionally, the monomer conversion never exceeded 90 %. Finally, an increase in the dispersity was noticeable after 30 % monomer conversion.105
Scheme 2-5: RAFT polymerisation of AMPS(Na) in water using a dithiobenzoate CTA.58,105-107
In 2004, McCormick et al. reported a study on the potential hydrolysis of a CTA (i.e. dithioesther) at 70 °C and that was shown to be a critical parameter on the polymerisation process, especially at pH higher than 7.58 They observed an increase
of dead chain concentration at higher pH, posing a problem for the synthesis of complex architectures (e.g. diblock synthesis). This explained why in 2010, the RAFT polymerisation of AMPS® in aqueous solution was carried out at lower pH (6.5) and
stopped at monomer conversion of approximately 30 % (i.e. to avoid the loss of molecular weight control, Ð ~ 1.2).107 Finally, this AMPS® homopolymer was used as
macroCTA to be further chain extended with N-acryloyl-L-alanine (AAL). The chain
extension was shown to be successful with the observation of a monomodal SEC peak shifted to higher molecular weight than the AMPS® homopolymer used, thus
suggesting a good reinitiation of the macroCTA with observed dispersities being lower than 1.3.
The main disadvantage when using aqueous RAFT polymerisation is mainly linked to the loss of chain end fidelity connected to the potential hydrolysis of the chain transfer agents. This hydrolysis has been shown to be not only pH- and temperature- dependent but also CTA-dependent.58 Indeed, it has been shown in the literature that
dithiobenzoates were more prone to hydrolysis than trithiocarbonates and that the rate of hydrolysis increased at pH higher than 7 and temperatures higher than 50 °C.66,108- 110
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