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
The polymerization of DADMAC is a well-known example of cyclopolymerization, a reaction where polymer chain growth is accompanied by the formation of cyclic groups in the polymer backbone.7 Typically, when monomers bearing two nonconjugated carbon-carbon double bonds (C=C) are reacted under addition polymerization conditions, the resulting polymers will either be highly cross-linked or contain residual unsaturation due to unreacted C=C bonds. However, for certain monomers such as diallyl quaternary ammonium salts, polymer chain propagation proceeds with alternating intra- and intermolecular addition of the growing radical chain across the double bonds, providing linear polymers with little or no cross-linking and few unreacted C=C bonds.
For more than two decades, DADMAC and other diallyl quaternary ammonium monomers were believed to form a thermodynamically favored, six-membered ring during cyclo-polymerization; however, as modern structural determination techniques such as carbon-13 nuclear magnetic resonance (13C NMR) spectroscopy became more accessible in the 1970s, it was proven that these monomers instead form a kinetically favored five-membered ring.2 The five-membered ring forms because DADMAC chain propagation occurs rapidly. The growing polymer chain adds across the first C=C bond of the monomer, then quickly adds across the second C=C bond, “locking” the five-membered ring in place before the second allyl group can adopt the conformation required to close the repeat unit as the less strained six-membered ring.
PQ-6 is typically supplied as a pale yellow, transparent aqueous solution with a neutral to slightly acidic pH and mild aldehydic odor.2, 8–10 Most grades supplied to the personal care industry are ca. 40% active polymer solids and have solution viscosities of 7,000–12,000 cP, although the exact solution viscosity of the raw material will depend on polymer molecular weight and concentration. Solutions of PQ-6 are expected to contain residual DADMAC monomer (typical levels of up to 1.5% w/w are possible9), NaCl, and by-products of polymerization initiators such as sulfate salts from the decomposition of persulfate initiators.
The primary industrial applications of PQ-6 are as separation aids in water treatment and mining processes and as additives in paper and textile manufacturing. In the personal care industry, it may be employed as a conditioning agent in rinse-off and leave-on formulations for hair and skin care applications. Most commonly, PQ-6 is found in hair care where it provides lubricity and softness to improve wet and dry combing, reduce static charge buildup, and aid in film formation and style retention. Due to its exceptional chemical stability, PQ-6 is especially preferred for use in high pH and/or highly oxidative formulations such as bleaches, dyes, relaxers and permanent waving products.
Recommended use levels of active PQ-6 typically range from 0.2–1.2% w/w. Due to its relatively high cationic charge density, PQ-6 readily forms strong physical complexes with anionic surfactants and anionic polymers; therefore, special care must be taken when combining PQ-6 with such ingredients to prevent the formation of insoluble precipitates during processing. To solubilize PQ-6 in the presence of anionic surfactants, either a large excess of anionic surfactant is required, or amphoteric, betaine or nonionic secondary surfactants must be added to the formulation. High loads of secondary surfactants are especially necessary when formulating clear liquid cleansers with PQ-6.
PQ-6 should never be combined directly with solutions of anionic polymers or concentrated anionic surfactants. Instead, commercial PQ-6 solutions are preferably pre-diluted with water and added to formulations following the addition and complete dissolution of all other polymers and surfactants. The portion of the water used to pre-dilute PQ-6 will depend on the level of PQ-6 in the formulation, but it typically is about 5–10 times the % w/w of PQ-6 solution being added to the batch. Alternatively, PQ-6 may be added up front to the water phase when there are no anionic polymers, e.g. thickeners, or anionic surfactants that require dissolution first.
Next month’s “Ingredient Profile” column will review PQ-6’s conditioning cousin copolymer, polyquaternium-7. Be sure to check it out. Reproduction of the article without expressed consent is strictly prohibited.
Send e-mail to CT_author@allured.com.
1. Polyquaternium-6, Monograph ID 2442, International Cosmetic Ingredient Dictionary and Handbook, 13th ed, Personal Care Products Council: Washington DC (2010)
2. C Wandrey, J Hermindez-Barajas and D Hunkeler, Diallyldimethylammonium chloride and its polymers, Adv Polym Sci 145, 123–182 (1999)
3. Merquat Polyquaternium-6 Series, Nalco product bulletin PC-PolyQ-6, Nalco Company, Naperville, IL (2007)
4. US 3461163, Synthesis of dimethyl diallyl ammonium chloride, JE Boothe, assigned to Calgon Corp. (Aug 12, 1969)
5. US 4151202, Preparation of dimethyl diallyl ammonium chloride and polydiallyl dimethyl ammonium chloride, WE Hunter and TP Sieder, assigned to Nalco Chemical Co. (Apr 24, 1979)
6. JE Boothe, HG Flock, MF Hoover, Some homo- and copolymerization studies of dimethyldiallylammonium chloride, J Macromol Sci, Part A, Pure Appl Chem 4 6 1419–1430 (1970) 7. GB Butler, Cyclopolymerization and cyclocopolymerization, Marcel Dekker Inc., New York (1992) 8. Genamin PDAC, Clariant Industrial & Consumer Specialties Product Bulletin, Clariant International Ltd., Muttenz, Switzerland (Jan 2010)
9. Mirapol 100, Rhodia Product Data Sheet N000169, Rhodia Novecare, Cranbury, NJ (Jul 2009)
10. Optasense CP6 Conditioning Agent, Croda product bulletin DS-205R-2, Croda Inc., Edison, NJ (Jan 2008)