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By: Michael J. Fevola, PhD, Johnson & Johnson Consumer and Personal Products Worldwide
Posted: April 5, 2011, from the April 2011 issue of Cosmetics & Toiletries.
Last month’s “Ingredient Profile” column reviewed polyquaternium-6 (PQ-6), the cationic homopolymer of diallyldimethylammonium chloride (DADMAC). In relation, this month’s column examines its cousin, polyquaternium-7 (PQ-7), one of the most widely used and cost-effective conditioning polymers for personal care applications.
Chemistry and Manufacture
PQ-7 is the polymeric quaternary ammonium salt derived from the copolymerization of acrylamide (AM) and DADMAC monomers.1 The level of cationic DADMAC monomer incorporated into PQ-7 type copolymers may vary quite dramatically, from 5–80 mol%.2 Personal care grades of PQ-7 are reported to contain from 25–50 mol% DADMAC and a balance of 50–75 mol% AM, and may exhibit molecular weight (MW) values ranging from 1.0 × 105 g/mol to 3.0 × 106 g/mol.3–5 Like PQ-6, PQ-7 is also a strong polyelectrolyte, yet its charge density is significantly lower than that of PQ-6 due to the incorporation of nonionic, hydrophilic AM repeat units in the polymer chain (see Figure 1).
AM monomer: AM monomer is a small volume commodity chemical that can be traced back to propylene, a petrochemical feedstock derived from natural gas. Figure 2 shows the commercial synthesis route for producing AM, which involves the ammoxidation of propylene to produce acrylonitrile, followed by the catalytic hydration of the acrylonitrile to AM. In the first reaction, propylene gas is reacted with oxygen and ammonia over a solid bismuth-molybdate catalyst to generate acrylonitrile, itself a useful monomer and chemical intermediate. The acrylonitrile is then heated with water over a copper catalyst to yield the AM monomer, which may be supplied as a white crystalline solid or a 50% aqueous solution stabilized with 25–50 ppm Cu2+ ion to prevent autopolymerization during storage and shipping.6
Monomer reactivity ratios: No discussion of PQ-7 synthesis is complete without mentioning the concept of monomer reactivity ratios. When two ethylenically unsaturated monomers, M1 and M2, are reacted together via free radical addition copolymerization, the composition of the resulting copolymer will be governed by the reactivity of each monomer with the propagating radical chain end (see Figure 3), which in turn depends upon the ultimate repeat unit of the growing chain—i.e., whether there is an M1 or M2 radical species on the chain end.7 The reactivity ratios, r1 and r2, are defined as:
r1 = k11/k12 and r2= k22/k21