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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.
page 2 of 4
and indicate the ratio of the rate constant for a reactive propagating species adding its own type of monomer (k11 or k22) to the rate constant of its addition to the other monomer (k12 or k21). In the ideal case, r1 = r2 ≈ 1, meaning that both monomers M1 and M2 will react at similar rates regardless of whether there is an M1 or M2 on the growing chain end, and the resulting copolymer will contain a random distribution of the two monomers.
However, if M1 reacts preferentially over M2 (i.e., r1 > 1 and r2 < 1), more M1 will be consumed earlier in the reaction. Thus, copolymer chains formed early in the reaction will be rich in M1 and those formed later in the reaction will be rich in M2, a phenomenon known as compositional drift. When compositional drift occurs, the resulting copolymer product will be a heterogeneous mixture of individual copolymer chains of varying comonomer composition; such heterogeneity will influence the properties and performance of the copolymer product.
AM-DADMAC copolymerization: For the copolymerization of AM (M1) and DADMAC (M2), the reactivity ratios differ by up to two orders of magnitude, with reported values of r1 = 4.8–7.1 and r2 = 0.03–0.22.8 Thus, if AM and DADMAC are simply mixed and reacted together in a batch copolymerization, significant compositional drift will occur, resulting in a PQ-7 product with a heterogeneous distribution of AM and DADMAC in the copolymer chains. To obtain PQ-7 with a more uniform comonomer distribution, semi-batch copolymerization processes must be employed, wherein the more reactive AM monomer is slowly fed into the copolymerization reaction to ensure uniform consumption of both monomers and a more homogeneous comonomer distribution in the final product (see Figure 4).9, 10
PQ-7 for personal care applications is usually produced via the aqueous solution copolymerization of AM and DADMAC in a semi-batch process like that shown in Figure 4, where AM monomer and initiator are fed continuously into a reactor initially charged with water, DADMAC, a portion of AM, a portion of initiator, and other additives—e.g., chelating agents, pH adjusters, etc. This reaction is conducted in an oxygen-free environment, either under vacuum or nitrogen blanket, at temperatures of 30–60°C and is initiated using common radical initiators such as persulfate salts or water-soluble azo compounds.
A critical step in the process is the monomer-scavenging treatment that occurs following the polymerization. In this step, the PQ-7 solution is treated with excess initiator or other compounds at higher temperatures (70–80°C) to eliminate any unreacted monomer in the product. While the (co)polymers are essentially nontoxic, the unreacted AM monomer is a neurotoxin,3 and only PQ-7 materials with residual levels of less than 10 ppm AM have been indicated as safe by the Cosmetic Ingredient Review expert panel.11 In practice, many suppliers of personal care grade PQ-7 strive to maintain residual AM levels of < 1 ppm to ensure it is virtually undetectable in finished goods.