Chemistry and Manufacture
Polyquaternium-6 (PQ-6) is the polymeric quaternary ammonium salt derived from the homopolymerization of diallyldimethylammonium chloride (DADMAC) monomer, as shown in Figure 1.1, 2 The grades of PQ-6 supplied to the personal care industry typically have weight-average molecular weight (Mw) values of ca. 150,000 g/mol, although grades with Mw values as low as 15,000 g/mol are available.3 PQ-6 is a strong polyelectrolyte, i.e. it is comprised of repeating units that remain fully ionized in aqueous solutions independent of the solution pH value. In addition, PQ-6 has a relatively high cationic charge density because each repeating unit bears a positive charge.
DADMAC monomer is synthesized commercially by the reaction of two key commodity precursors, dimethylamine and allyl chloride (see Figure 2).4, 5 Overall, two equivalents of allyl chloride are reacted with dimethylamine to yield the quaternary ammonium monomer and one equivalent of sodium chloride (NaCl). In this process, the hydrochloride salt of the monoallyldimethylamine intermediate, i.e. allyldimethylammonium chloride, is neutralized in situ with one equivalent of sodium hydroxide to facilitate the addition of the second equivalent of allyl chloride. Depending on the specific process employed and the desired monomer purity, the NaCl may be left in the monomer product or separated from the DADMAC.
PQ-6 is produced by the free-radical addition polymerization of DADMAC in aqueous solution, as shown in Figure 3.2, 5 The polymerization is typically initiated using thermal initiators such as ammonium persulfate at temperatures of 60–80°C, or at lower temperatures using redox initiators such as potassium bisulfate (reducer) in combination with potassium persulfate (oxidizer). The reaction is conducted under a nitrogen atmosphere to prevent radical inhibition by oxygen. Chelating agents such as disodium ethylenediamine tetraacetic acid may also be added to the reaction to bind any free metal ions that would otherwise interfere with polymerization.6