Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

APPENDIX 2 409Appendix 2In this appendix, we discuss briefly some of the results of a typical kineticinvestigation carried out in the author's laboratory by a number of collaborators".This work illustrates several of the points raised in the text. For example, the workshows how complicated kinetic behaviour may be turned into something mucheasier of interpretation by identifying and eliminating the factors responsible forthe complication. Furthermore, although the data yield an homogeneous set of firstorder rate coefficients covering a four-fold range of initial concentration, the dataof a sub-set of replicate experiments are non-homogeneous indicating the presenceof some partially uncontrolled factor. These points will be discussed and thecalculations set out in some detail.The data under discussion were obtained in the course of a study of the anionicpolymerization of isoprene in benzene at 35.1" C. Earlier work on the polymerizationusing n-butyl lithium as initiator in a number of hydrocarbon solvents hadshown that the kinetic behaviour was complicated. For example, the isopreneconcentration-time curves were sigmoidal and the ratio of the number of molesof isoprene consumed to the number of moles of n-butyl lithium reacted increasedas the reaction proceeded. These and other complications showed that the rate ofthe reaction could not possibly be represented by an equation of the typed[isoprene] - = k[isoprene]" [n-butyl lithium] *dtIn principle, it would have been possible to fit the data to a more complexexpression and to proceed, thereby, to propose a reaction mechanism. However,in this case-and this must be typical of many analogous situations-it waspossible to suggest the reqson for the appearance of many of the complex featuresalready referred to. Without going into the detailed arguments [these are discussedin two review articles 12*13], it was thought that the complications weredue in part to the presence of two competitive reactions consuming isoprene,namely,CH,= CH-C=CH,I k ICH,CH,+ C,H,Li 4 C4H,CH,-CH=C-CH,-Li+CH, CHI CH,I I ICH,=CH-C= CH,+C,H, {CH*-CH= C-CH,}, CH,-CH=C-CH,-Li+._ CHaCH,kirI.+ CaH, {CH,-CH=G-CH,}i + 1 CH,--CH=(!LCH,-Li+It followed that, if this explanation were correct, the kinetic behaviour would