Determination of Absolute Rate Constants

Three different types of rate eonstants are used in ideal polymerization kinetics described by Eq. (6.24), namely, those for initiation (kd), propagation (kp), and termination (ki). Till now we have discussed only polymerization kinetics under steady-state conditions and how these allow evaluation of kd (see Dead-end polymerization) and the ratio kp/kj or kyki (but not kp and ki individually) using measurable quantities like Rp, Ri, and M]. As we shall see below, employing nonsteady-state polymerization with photoinitiation and intermittent illumination (Flory, 1953 Walling, 1957), it is possible to evaluate kpjkt so that by combining it with kpjk obtained from steady-state experiments, one can determine kp and kt individually. 

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The determinations of absolute rate constants with values up to ks = 1010 s-1 for the reaction of carbocations with water and other nucleophilic solvents using either the direct method of laser flash photolysis1 or the indirect azide ion clock method.8 These values of ks (s ) have been combined with rate constants for carbocation formation in the microscopic reverse direction to give values of KR (m) for the equilibrium addition of water to a wide range of benzylic carbocations.9 13... 

Since the determination of absolute rate constants is one of the most urgent problems in cationic polymerization, and the styrene-perchloric acid system seemed to be so clean and simple, Gandini and Plesch set out first to check Pepper and Reilly s results by determining spectroscopically the concentration of carbonium ions during polymerization, and they intended then to extend the method to other monomers. However, their findings were not as expected. A comparison of spectroscopic and conductivity measurements with rate measurements in an adiabatic calorimeter showed [4] that in methylene dichloride solution ... 

These considerations show that the determination of the number and nature of growing species in cationic polymerizations is, in general, not easy, and the alleged determinations of absolute rate constants need to be scrutinised most carefully from this point of view. 

Interest within the physical organic community on the mechanism for the formation and reaction of ion-pair and ion-dipole intermediates of solvolysis peaked sometime in the 1970s and has declined in recent years. The concepts developed during the heyday of this work have stood the test of time, but these reactions have not been fuUy characterized, even for relatively simple systems. Richard and coworkers have prepared a short chapter that summarizes their recent determinations of absolute rate constants for the reactions of these weak association complexes in water. This work provides a quantitative basis for the formerly largely qualitative discussions of competing carbocation-nucleophile addition and rearrangement reactions of ion and dipole pairs. 

DETERMINATION OF ABSOLUTE RATE CONSTANTS 3-8a Non-Steady-State Kinetics... 

Much work has been done since the early 1980s on the detailed investigation of the azirine-nitrile ylide interconversion using pulsed-laser photolysis. Thus the azirines 103 (R =R =Ph, R =H R =Me, R = R =Ph R = p-napthyl, R = Me, R = H), on irradiation in isooctane, gave intense long-hved absorptions (250-400 nm) attributed to the nitrile ylides 104 (44). Quenching studies with electon-deficient alkenes led to the determination of absolute rate constants that were similar to those reported earlier for steady-state trapping experiments. The nitrile ylide-olefin reactions are discussed in more detail in Section 7.3.1. 

Determination of Absolute Rate Constants for Reactions Involving... 

Schoneich C, Bonifacic M, Asmus K-D (1989) Reversible H-atom abstraction from alcohols by thiyl radicals determination of absolute rate constants by pulse radiolysis. Free Rad Res Commun 6 393-405... 

Simha, R., On the photodegradation of methyl methacrylate and determination of absolute rate constants, Journal of Polymer Science, 9(5), pp. 465 67 (1952). 

A gas-phase investigation of the system F+HNO3 and the determination of absolute rate constants for the reaction of the NO3 radical,... 

Non-stationary state techniques are required for the determination of absolute rate constants and, almost invariably in the field of free-radical chemistry, absolute rate determinations have been made directly on combination reactions. Ultimately, therefore, absolute rate constants for radical addition reactions depend on a knowledge of the rate of addition relative to the rate of combination of the radical involved. 

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