4.5
Protocol 4.1: Synthesis of A8-35
Whereas unlabeled A8-35 is commercially available, experimenters may want to use variants thereof
that are either tagged or labeled isotopically or otherwise to suit their special needs. The synthesis of
A8-35 is simple on paper, but it has its pitfalls. As has been described in § 4.3.1.2.2, polymers that do
not match the nominal composition are at risk of presenting an abnormal solution behavior, which, in
turn, will entail suboptimal properties of MP/A8-35 complexes, such as the formation of small
aggregates (Gohon et al. 2008). This can be disastrous, for instance, for SANS or NMR experiments
(see Chaps. 9 and 10, respectively). We present here a detailed protocol with comments and caveats
(in italics and marked with a pointing hand ).
A8-35 is obtained by hydrophobization of a poly(acrylic acid) (PAA) precursor. This is achieved
by successively grafting octylamine (yielding A8-75 as an intermediate) and isopropylamine onto the
precursor dissolved in N-methyl-2-pyrrolidone (NMP), in the presence of N,N’-dicyclohexylcarbodiimide (DCI) as activating agent (Tribet et al. 1996; Gohon et al. 2004, 2006). After purification,
which is performed in aqueous media, the basic form of the polymer is obtained by neutralization in
water with sodium hydroxide. The procedure can be decomposed in three steps: (i) PAA preparation
and characterization, (ii) PAA modification, and (iii) purification and characterization of the final
product.
4.5.1
Preparation and Characterization of the PAA Precursor
The starting material, PAA, is available from two different manufacturers, Aldrich and Acros, as
50% w/w aqueous solutions of a partial sodium salt form (Aldrich) or acidic form (Acros). The PAA
from Aldrich presents a slightly different weight-average molar mass, hM w i (see § 4.6.1, Annex 4.1,
for definition), from that from Acros (~5 vs. ~5.5 kDa, respectively). Because of the broad dispersion
of masses around the average one, this is without consequences on the properties of the final A8-35.
Disaggregation of the Precursor
Both commercial solutions are viscous and turbid, due to polymer aggregation. Extensive disaggregation is an essential preliminary step. Indeed, because hydrophobization must occur randomly, it is
critical to achieve a thorough dispersion of the PAA chains in the reacting medium. This requires to
break all weak intermacromolecular interactions, such as those resulting from H-bonding (Henke
et al. 2011; Swift et al. 2016) and/or from chain entanglement (Harrington 2008). The disentanglement
of the PAA chains can be achieved by heating at 90
C a 10Â diluted commercial solution, but better
results are obtained by bringing the pH of a dilute solution to 10, followed by neutralization on an
acidic resin column. PAA is recovered under its solid acidic form after removing water by freezedrying. As expected (Yamamoto et al. 2000a, b), lyophilization does not lead back to polymer
aggregation, and the resulting PAA readily yields limpid solutions in aqueous or organic polar
solvents.
Characterization of the Disaggregated Precursor
Dry mass. Being hygroscopic, PAA is never totally water-free, even after freeze-drying. Because the
chemical modification must be performed on a precisely known amount of polymer, the dry mass is
commonly determined by total organic carbon analysis (TOC) or by simple acid/base titration of the
carboxylic acid functions.
4.5 Protocol 4.1: Synthesis of A8-35
205
Protocol 4.1: Synthesis of A8-35
Whereas unlabeled A8-35 is commercially available, experimenters may want to use variants thereof
that are either tagged or labeled isotopically or otherwise to suit their special needs. The synthesis of
A8-35 is simple on paper, but it has its pitfalls. As has been described in § 4.3.1.2.2, polymers that do
not match the nominal composition are at risk of presenting an abnormal solution behavior, which, in
turn, will entail suboptimal properties of MP/A8-35 complexes, such as the formation of small
aggregates (Gohon et al. 2008). This can be disastrous, for instance, for SANS or NMR experiments
(see Chaps. 9 and 10, respectively). We present here a detailed protocol with comments and caveats
(in italics and marked with a pointing hand ).
A8-35 is obtained by hydrophobization of a poly(acrylic acid) (PAA) precursor. This is achieved
by successively grafting octylamine (yielding A8-75 as an intermediate) and isopropylamine onto the
precursor dissolved in N-methyl-2-pyrrolidone (NMP), in the presence of N,N’-dicyclohexylcarbodiimide (DCI) as activating agent (Tribet et al. 1996; Gohon et al. 2004, 2006). After purification,
which is performed in aqueous media, the basic form of the polymer is obtained by neutralization in
water with sodium hydroxide. The procedure can be decomposed in three steps: (i) PAA preparation
and characterization, (ii) PAA modification, and (iii) purification and characterization of the final
product.
4.5.1
Preparation and Characterization of the PAA Precursor
The starting material, PAA, is available from two different manufacturers, Aldrich and Acros, as
50% w/w aqueous solutions of a partial sodium salt form (Aldrich) or acidic form (Acros). The PAA
from Aldrich presents a slightly different weight-average molar mass, hM w i (see § 4.6.1, Annex 4.1,
for definition), from that from Acros (~5 vs. ~5.5 kDa, respectively). Because of the broad dispersion
of masses around the average one, this is without consequences on the properties of the final A8-35.
Disaggregation of the Precursor
Both commercial solutions are viscous and turbid, due to polymer aggregation. Extensive disaggregation is an essential preliminary step. Indeed, because hydrophobization must occur randomly, it is
critical to achieve a thorough dispersion of the PAA chains in the reacting medium. This requires to
break all weak intermacromolecular interactions, such as those resulting from H-bonding (Henke
et al. 2011; Swift et al. 2016) and/or from chain entanglement (Harrington 2008). The disentanglement
of the PAA chains can be achieved by heating at 90
C a 10Â diluted commercial solution, but better
results are obtained by bringing the pH of a dilute solution to 10, followed by neutralization on an
acidic resin column. PAA is recovered under its solid acidic form after removing water by freezedrying. As expected (Yamamoto et al. 2000a, b), lyophilization does not lead back to polymer
aggregation, and the resulting PAA readily yields limpid solutions in aqueous or organic polar
solvents.
Characterization of the Disaggregated Precursor
Dry mass. Being hygroscopic, PAA is never totally water-free, even after freeze-drying. Because the
chemical modification must be performed on a precisely known amount of polymer, the dry mass is
commonly determined by total organic carbon analysis (TOC) or by simple acid/base titration of the
carboxylic acid functions.
4.5 Protocol 4.1: Synthesis of A8-35
205