Advances in Hair Styling 422 resistance and flaking.5 This sparked an evolution of modified polymers to overcome these consumer perceivable drawbacks. The strategy was to copolymerize two or more monomers with various functional groups to make copolymers. Over several decades, a whole family of polymers based on vinyl pyrrolidone (VP) have been created incorporating features for improved performance. Initially VP was copolymerized with vinyl acetate (VA) to increase hydrophobicity and improve high humidity hold. Derivitization with such monomers as dimethylaminopropylmethacrylate (DMAPMA) to improve flexibility and film smoothness as well as humidity resistance followed. Alkylated monomers imparted hydrophobicity which improved mechanical durability under high humidity. Quaternization provided greater substantivity, especially from rinse-off applications.6,7 Terpolymers were made with not only the aforementioned class of monomers but also with acrylic acid derivatives and vinylcaprolactam. An advanced styling polymer derived from VP homopolymer is the tetrapolymer polyquaternium-69. The structure of this polymer is illustrated in Figure 1 and structure-function relationships of the components are listed in Table 1.8 It can also be seen in Table 2 how the derivatization of VP adds important hair attributes that the VP polymer cannot achieve alone. Besides film stiffness, the tetrapolymer provides stiffness under mechanical stress or durability of stiffness, high humidity curl retention, and reduction in flaking, to name a few. Figure 1. Polyquaternium-69