Determination of the Propagation Rate Constant in the Carbocationic Polymerization of 2,4,6-Trimethylstyrene Priyadarsi De, † Laszlo Sipos, † Rudolf Faust,* ,† Michel Moreau, ‡ Bernadette Charleux, ‡ and Jean-Pierre Vairon* ,‡ Polymer Science Program, Department of Chemistry, University of Massachusetts Lowell, Lowell, Massachusetts 01854, and Laboratoire de Chimie Macromole ´ culaire, Universite ´ Pierre et Marie Curie, Case 185 T44 E1, 4 Place Jussieu, 75252 Paris Cedex 05, France Received August 4, 2004; Revised Manuscript Received October 26, 2004 ABSTRACT: The living carbocationic polymerization of 2,4,6-trimethylstyrene (TMeSt) was studied using the 1-chloro-1-(2,4,6-trimethylphenyl)ethane/BCl 3 initiating system in CH2Cl2 from -70 to -20 °C. The equilibrium constant of ionization (Ki) was calculated from the cation concentration determined by UV- visible spectroscopy. From the apparent rate constant of propagation (kp app ) and Ki, the absolute rate constant of propagation for ion pairs kp ( ) 1.4 × 10 4 L mol -1 s -1 was calculated at -70 °C. The rate constant of ion collapse, k-i was determined from the polydispersity vs conversion plots. From Ki and k-i, the rate constant of ionization, ki were also calculated. From the temperature dependence of Ki, and k-i the enthalpy and entropy of the ionization and deactivation were obtained, respectively. Competition experiments i.e., polymerization carried out in the presence of 2-chloropropene as a π-nucleophile, were also carried out at -70 °C to determine kp ( separately. NMR spectroscopy, gel permeation chromatography, and elemental analysis suggested complete capping of the polymeric cation and the absence of side reactions. From the limiting number-average degree of polymerization and the known rate constant of capping k c ) 62.0 L mol -1 s -1 , kp ( ) 8.3 × 10 4 L mol -1 s -1 was determined. This value is similar to that determined from kp app and Ki; thus, the utility of competition experiments is corroborated. Introduction We recently reported the absolute rate constants of propagation for ion pairs (k p ( ) determined by two different diffusion clock methods in the cationic homo- polymerization of isobutylene (IB) 1,2 and styrene (St). 3 The k p ( values agreed well with those predicted by the linear free energy relationship (LFER) by Mayr; 4 how- ever, they were 4-6 orders of magnitude higher than previously accepted values. 5 This discrepancy was as- cribed to largely overestimated values of active center concentration by previous authors. In the case of St and derivatives UV spectroscopy could be used to determine the concentration of active centers and hence the propagation rate constants; however, identification of the fraction of the absorption that is due to propagating species is necessary when various side reactions, e.g., isomerization, inter- and intramolecular alkylation, etc. may take place. In the cationic polymerization of St, one of the major side reactions is indanic cyclization. 6 Intra- and intermolecular alkylation are absent in the cationic polymerization of 2,4,6-trimethylstyrene (TMeSt), which was recognized in an early report on the living polym- erization of TMeSt initiated by the cumyl acetate/BCl 3 initiating system in CH 3 Cl at -30 °C. 7 We have recently reexamined the polymerization of TMeSt initiated by the 2,4,6-trimethylstyryl cation, a model propagating end in CH 2 Cl 2 at -70 °C, and observed living polym- erization yielding polymers with theoretical molecular weights and very low polydispersity indices (M w /M n ) 1.02 to 1.1). 8 Since side reactions of the TMeSt cation are absent, the equilibrium constant of ionization (K i ) could be determined by UV spectroscopy and from the active chain end concentration and the rate of polym- erization, k p ( may be calculated. Therefore, TMeSt is a highly suitable monomer to compare k p ( values pre- dicted by LFER to that obtained by direct measurement and to the propagation rate constant obtained in com- petition experiments (used previously with IB and St). In the present study, we report on results of these kinetic studies in the living cationic polymerization of 2,4,6-trimethylstyrene. Experimental Section Materials. TMeSt (Aldrich, 95%) was freed from inhibitor by washing with 5% NaOH and then repeatedly with water. After drying over anhydrous Na2SO4, it was distilled from calcium hydride under reduced pressure. The initiator 1-chloro- 1-(2,4,6-trimethylphenyl)ethane (TMeStCl) was synthesized by hydrochlorination of TMeSt in CH 2Cl2 (TMeSt/CH2Cl2, 1/10 (v/v)) at 0 °C and after removing CH2Cl2 in the rotary evaporator it was purified by vacuum distillation from calcium hydride. 1 H NMR: 6.92 (s), 5.67 (q), 2.56 (s), 2.29 (s), and 1.95 ppm (d). BCl3 (99.9%, Aldrich) and di-tert-butylpyridine (DTBP, 97+ %, Aldrich) were used as received. Commercial 2-chloro- propene (98%, Aldrich) (may contain HCl as impurity, which can initiate polymerization during the competition experiment) was distilled from NaOH at 20 °C just before use and collected in a round-bottom flask cooled by dry ice. The CH 2Cl2 was shaken three times with 5% NaOH and then repeatedly with water. It was predried with anhydrous Na2SO4, and distilled from calcium hydride under nitrogen. This distilled CH2Cl2 was refluxed under nitrogen overnight with phosphorus pen- toxide and distilled to a round-bottom flask over phosphorus pentoxide. It was refluxed under nitrogen overnight and distilled just before use. Other chemicals and solvents were purified as described previously or used as received. 1,2 Polymerization. Polymerizations were carried out under a dry nitrogen atmosphere ([H2O] < 1.0 ppm) in an MBraun 150-M glovebox (Innovative Technology Inc., Newburyport, MA). Large (75 mL) culture tubes were used as polymerization reactors. The total volume of the reaction mixture was 20 mL. At predetermined time intervals, the polymerizations were * Corresponding authors. † University of Massachusetts Lowell. ‡ Universite ´ Pierre et Marie Curie. 41 Macromolecules 2005, 38, 41-46 10.1021/ma048390n CCC: $30.25 © 2005 American Chemical Society Published on Web 12/15/2004