COMPARATIVE POLYMERIZATION IN THE GAS PHASE AND IN CLUSTERS .1. COVALENT DIMER FORMATION AND ENTROPY BARRIERS TO POLYMERIZATION IN ISOBUTENE

摘要

This work presents kinetic and thermochemical data for ionic polymerization in the gas phase, and the following paper bridges toward the condensed phase by cluster measurements. Gas-phase measurements by pulsed high-pressure mass spectrometry involving reactions in the radical and protonated ion branches in isobutene lead to the following observations. (1) i-C4H8.+ + i-C4H8 --> t-C4H9+ is fast (k = 0.5 +/- 0.1 x 10(-9) cm(3) s(-1)) from 250 to 625 K, but the competing condensation to form C8H16.+ decreases with increasing temperature (k(250) = 1.4 x 10(-9) cm(3) s(-1), k(497) = 0.2 x 10(-9) cm(3) s(-1)). The distinct behavior suggests two collision types, leading to direct reaction or complex formation. (2) The product C8H16.+ is identified as a covalent adduct with a dissociation energy > 132 kT/mol (32 kcal/mol), and bracketing experiments confirm it as an octene ion corresponding to an IP of 8.55 +/- 0.15 eV. (3) The adduct undergoes H-2 transfer to i-C4H8 and the rate coefficient has a large negative temperature coefficient of T--4.3. The product is unreactive toward further polymerization. (4) In the protonated (carbonium ion) branch, consecutive association reactions have decreasing rate coefficients. (5) The first step in the protonated branch, forming C8H17+, is reversible and the thermochemistry [Delta H degrees = -95.8 kJ/mol (-22.9 kcal/mol), Delta S degrees = -136.8 J/(mol K) (-32.7 cal/(mol K))] is consistent with the formation of a tertiary carbonium ion. The thermochemistry of the second step exhibits an unusually large negative entropy change [Delta H degrees = -101.3 kJ/mol (-24.2 kcal/mol), Delta S degrees = -203.7 J/(mol K) (-48.7 cal/(mol K))] and a rate coefficient with a very large negative temperature coefficient (T-12 to T-16). Both observations suggest a highly constrained product, whose formation constitutes an entropy barrier to the polymerization sequence. (6) A novel H-2 transfer reaction, from the carbonium ion C8H17+ to i-C4H8, is observed, and the rate coefficient has a negative temperature coefficient. The results suggest that ionic polymerization reactions that involve steric entropy barriers will be slow under flame conditions but fast at low interstellar temperatures. The temperature coefficients explain the need for using low temperatures in the industrial polymerization of isobutene. [References: 29]