Nitroxide-Mediated Polymerization:  The Pivotal Role of the<i>k</i>_dValue of the Initiating Alkoxyamine and the Importance of the Experimental Conditions

Abstract

In nitroxide-mediated polymerization, NMP, the role of the initiation and therefore the role of the initiating dissociation rate constant kd1 on the control and livingness of a styrene and n-butyl acrylate polymerization was investigated. Different polymerizations were carried out, initiated with two different alkoxyamines based on the same nitroxide (1• or SG1). Kinetic modeling studies using the PREDICI software are used to explain these results. The use of a new tertiary SG1-based alkoxyamine 1a instead of the secondary SG1-based alkoxyamine 1b improved the control of the polymerization of both styrene and n-butyl acrylate. In particular, the polymerization of n-butyl acrylate can be carried out without adding any extra initial amount of nitroxide and the controlled polymerization of styrene at 90 °C is now possible. The complete kinetic analysis showed that the main role of the alkoxyamine with a high dissociation rate constant is to produce quickly in situ an extra amount of free nitroxide, which is necessary to obtain a good control of a polymerization. Moreover, we linked theoretically the threshold kd1 value necessary to have a good controlled and living polymerization in a true monocomponent system with the value of the propagation rate constant kp. This new statement (kp/kd1 ≤ 6.0 × 105 L·mol-1) has to be added to Fischer's equations to determine the optimum experimental conditions to obtain a successful polymerization. Furthermore, the results showed clearly the importance of the experimental procedure (heating with a ramp of temperature or not, purity of the monomers and the vessels) to obtain a successful living/controlled polymerization.