ABSOLUTE RATE CONSTANTS FOR HYDROCARBON AUTOXIDATION: III. Α-Methylstyrene, Β-Methylstyrene, AND INDENE

Abstract

Absolute rate constants for the copolymerization of a-methylstyrene and oxygen have been measured from 13 to 50 OC. The propagation and termination rate constants can be represented by k, = (6.6 Z+Z 0.7) X lo6 exp (-(8 070 + 800)lRT) 1 mole-' s-I kt = (1.6 =t 0.7) X lo8 exp (-(3 700 ZI Z 1 GOO)/RT) 1 m~l e -~s -] . Experiments with 2.6-di-t-butyl-4-methylphenol a t 65 "C have shown that C G H~C ( C H~) : C H~ and C O H~C ( C D~) : C D~ have the same propagation rate constant but that cham termlnatlon involves a deuterium isotope effect ( k t ) n / ( k t )~ = 1.5.Absolute rate constants for the copolymerization of oxygen with a-methylstyrene and with indene at 30 "C showed that a significant fraction of the oxidation chains were terminated by a lcinetically first order process (rate constant k,). The rate constants for 8-methylstyrene and indene a t 30 "C are k, = 51 and 142 1 mole-Is-', k t = 1.6 X lo7 and 2.5 X 107 1 mole-1s-1, and k, = 0.61 and 1.2 s-l, respectively. The propagation rate constant for indene can be separated into a rate constant for the copolymerization with oxygen (kndd = 128 I mole-' s-l) and a rate constant for hydrogen atom abstraction (k,b,t, = 14 1 mole-' s-I). In the presence of heavy water the first order process for indene had a deuterium isotope effect (k,)/(k,)~?o = 3. IXTRODUCTIONThe effects of temperature and solvents on the absolute rate constants for the autoxidation of styrene, i.e., for its copolylnerization with oxygen, were described in the first paper in this series (1). The second paper gave the absolute rate constants for four deuteriunl-labelled styrenes and a nurtlber of ring-substituted styrenes (2). The present paper extends this xvork t o a-and P-methylstyrene and indene. Both a-methylstyrene and iridelie have been shown to copolylnerize with oxygen in tllermally initiated oxidations (3, 4). a-PIethylstyrene is of particular interest since it yields a tertiary peroxy radical. The mechanis~n of chain termination for con-lpounds giving tertiary peroxy radicals has beell under very active study in the past 2 or 3 years (5-9) but absolute rate constants have been lneasured in only a few cases, i.e., for chain propagation (k,) and chain termination ( k t ) in the autoxidation of culnene (9-12) and 2,4,6-trimethyl heptane (13) and for the mutual destruction of t-butylperoxy radicals derived from t-butylhydroperoxide (9). a-A,leth>,lstyrene is also interesting in that the rate of the a,al-azo-bis-isobutyronitrile (AIBN) therlnally initiated oxidation has been reported to increase very noticeably as the oxygen pressure is increased, e.g., the rate a t 50' with 0.01 mole/l AIBN is 0.076, 0.12, and 0.13 mole 1-I h-I a t 100, 750, and 3 200 n~m oxygen pressure, respectively (3). This variation in the rate with the oxygen pressure has been attributed to a contribution from crossed chain termination, involving aralkyl (R') and aralliylperoxy (RO;) radicals, to the overall rate of chain terlnination a t oxygen pressures below 3 200 mm (3). This paper describes the effects o...

Mechanism of “Autoacceleration” in the Thermal Oxidative Polymerization of α-Methylstyrene

“…The E d value, thus obtained, could be taken as the experimentally derived E i value for AMS polymerization in the presence of acetophenone. From the observed E a (14 kcal/mol) for AMS polymerization, in the presence of acetophenone after the break point, together with the reported E p2 and E t values, 25 the calculated value of E i from eq 1 was found to be 16 kcal/ mol, which is consistent with the experimentally observed E i (18 ( 2 kcal/mol) from DSC experiments as discussed above. This strongly suggests that the mechanism proposed in Scheme 1 is operative even after the knee point formation, provided E i has to be modified for the catalytic effect of acetophenone on the cleavage of PMSP.…”

“…The E a for thermal oxidative polymerization of AMS, based on Scheme , is given by where E p2 , E i , and E t are the activation energies for propagation, initiation, and termination respectively. The E p2 and E t values for AMS polymerization are 8.1 and 3.7 kcal/mol, respectively . Using the experimentally obtained E a value (18 kcal/mol), before the knee point, the E i value calculated from eq 1 was found to be 24 kcal/mol.…”

“…The E p2 and E t values for AMS polymerization are 8.1 and 3.7 kcal/mol, respectively. 25 Using the experimentally obtained E a value (18 kcal/mol), before the knee point, the E i value calculated from eq 1 was found to be 24 kcal/mol. This is close to the E i value for the thermal oxidative polymerization of styrene (27 kcal/mol), calculated in a similar fashion.…”

Quantitative studies of the autoxidation of linoleate monomers sequestered in phosphatidylcholine bilayers. Absolute rate constants in bilayers

“…1-4, gave good agreement of the k, and 2k, for tetralin (lit. values (33) are k, = 6.4 M-' s-' and 2k, = 7.6 X lo6 M-' s-I) and reasonable agreement for methyl linoleate in chlorobenzene (lit. values (34) are k, = 62 M-' s-I and 2k, = 8.8 X lo6 M-' s-I), considering the large errors that occur in the determinations of k, and 2k, (24).…”

“…Absolute rate constants for propagation, k,, and termination, 2 k, Some exploratory experiments were conducted to (1) determine if the rotating sector technique (22), widely used to study photoinitiated reactions in homogeneous solution (33,34) can be applied to determine absolute rate constants, k, and 2k,, in microenvironments such as bilayers and micelles, and (2) to determine the applicability of water-soluble initiators and inhibitors (6) to this technique. Trial experiments carried out in homogeneous media, Table 4, Nos.…”