Efficiency of initiation

Not all the radicals generated by the dissociation of the initiator can initiate polymerization. The effectiveness of initiation,/, is defined by the ratio of the initiation rates and initiator decomposition 

Thus/is the standardized fraction of initiating radicals referred to all radicals generated in the system. 

The fraction of radical reactions in the cage is not necessarily the same for photochemical and thermal dissociation of the initiator. Photochemically generated radicals are usually excited and so they have less tendency to produce stable molecules. They may be flung away from each other by excess energy at the moment of generation, and the probability of mutual collision is reduced [124]. 

The cage effect has a considerable influence on the course of radical polymerizations. It is held responsible for many kinetic anomalies. 

Initiation efficiency, f and the fraction of mutually reacting primary radicals (1 — F) [112b] 

Metal carbene complexes used as initiators (I) are not always fully consumed before the monomer (M) has been completely polymerized. This can happen when the propagation rate constant kp is somewhat greater than the initiation rate constant 

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Higher than first order for monomer, such as the 3/2 power suggests that VP is involved in initiation (17). If the efficiency of initiation is a function of the monomer concentration, then f = P [M], and substituting in equation 2 gives... 

It is the aim of this chapter to describe the nature, selectivity, and efficiency of initiation. Section 3.2 summarizes the various reactions associated with initiation and defines the terminology used in describing the process. Section 3.3 details the types of initiators, indicating the radicals generated, the byproducts formed (initiator efficiency), and any side reactions (e.g. transfer to initiator). Emphasis is placed on those initiators that see widespread usage. Section 3.4 examines the properties and reactions of the radicals generated, paying particular attention to the specificity of their interaction with monomers and other components of a polymerization system. Section 3.5 describes some of the techniques used in the study of initiation. 

If the rate constant kd for spontaneous decomposition of the initiator is known, the efficiency / of initiation may be determined. (This is a refinement of the molecular weight method set forth in Sec. Ic) The spontaneous decomposition rate of benzoyl peroxide in styrene, according to the work of Swain, Stockmayer, and Clarke is 3.2X10- sec. at 60°C. Hence the efficiency of initiation of the polymerization of styrene by benzoyl peroxide at 60°C is indicated to be about 0.60. 

There are initiators that split into two radicals only and initiators that dissociate with the formation of two radicals and one or two molecules. The formation of molecules simultaneously with radicals influences the efficiency of initiation (T= 318 K [93,94]). 

The efficiency of initiation is controlled, essentially, by the reactivity of the cationic moiety of the initiator towards the monomer and the stability of the growing ends can... 

The rate constant /c, contains a factor that allows for the efficiency of initiation not all the radicals generated by the initiator are capable of starting polymer chains, some are lost by combination or other reactions. The initiator efficiency is defined as the ratio of the number of initiator molecules that start polymer chains to the number of initiator molecules decomposed under the given conditions of the polymerization. With most radical initiators the efficiency lies between 0.6 and 0.9 it also depends on the nature of the monomer. 

For the rate of initiation, a half-order was usually obtained, and a first order with respect to the reagent as with tetrahydrofuran (THF) (13). For a few reagents, these relations do not apply—e.g.9 for m-xylene the rate of oxidation is between half and first order with respect to V and less than first order with respect to xylene (11). (In every case Vo2 represents the rate of consumption of 02 corrected for N2 evolution, and efficiency of initiation is assumed to be 0.7). 

H. Kast (104-07) Determination of Sensitivity to Friction (Medicion de la sensibili-dad al rozamiento) (107) Determination of Sensitivity to Initiation by Detonation (110-12) Determination of Sensitivity to Initiation by Influence (112-13) Determination of Power of Explosives using Trauzl Test (113-17), Small Lead Block Test (117), Quinan Apparatus (118), Guttmann Apparatus (118-19), Ballistic Pendulum (119-20), Mortar (Mortero probeta) (120-21) Determination of Efficiency of Initiating Devices by Lead Plate Test (121-23), Nail Test (123), Sand Test (124) and Acoustic Tests (124) Determination of Characteristics of Flames Produced on Explosion (125-29)... 

In the kinetic experiments a special method of mixing was required. This revealed a further experimental variable order of addition. The initiator solution was frozen into the bottom of the NMR tube. Monomer was frozen on the upper walls of the tube. On initiation the monomer melted first and ran down onto the frozen initiator, the initiator solution thus melted into a high concentration of monomer. If however molten initiator solution was run on to frozen monomer the rate of polymerization was drastically reduced. The concentration of unreacted t-BuMg groups was also significantly reduced. The efficiency of initiation of active polymerization sites was much lower when this event occurs at high initiator and low monomer concentration. 

Preliminary results on the kinetics of the polymerization and the efficiency of initiation of the isotactic polymerizations initiated by t-BuMgBr in toluene solution are consistent with the Bateup mechanism proposed for the stereoblock and syndio-tactic-like polymerizations initiated by n-BuMgBr in THF-rich solution — a mechanism which involves initiation and propagation through monomer — active centre complexes (5,8). 

The tests are essentially the same as listed under Initiating Efficiency of Initiating Explosives, etc... 

Blasting caps and detonators, test of see Esop s test for efficiency of detonators 1 XI Grotta s test for detonators 1 XV Initiating efficiency of initiating expls, blasting caps detonators 1XVII... 

It is clear from Eq. 28-1 that the efficiency of initiation depends upon both the affinity Kt and the rate constant k for opening of the double helix. Notice that the Pribnow sequence is AT-rich therefore, opening of the helix at this point would be easier than in a GC-rich region. Tlius, the Pribnow sequence may represent a point of entry of RNA polymerase to form the open complex.67 Other upstream A T tracts are often present frequently at about the -43 position in the UP element. They also seem to strengthen promoter activity.68 Tire open complex is thought to undergo some kind of isomerization to form an initial transcribing... 

The radiotracer method for estimating efficiency of initiation was applied by Bevington and Eaves (32) to polymerization in benzene and in carbon tetrachloride. Whereas they had calculated that about 47% of the radicals from AIBN initiate polymer chains in DMF solvent, efficiency in benzene was about 50% and in carbon tetrachloride about 30%. This low efficiency in carbon tetrachloride is attributed to attack of radicals from AIBN on the carbon tetrachloride solvent, especially at high concentrations of solvent. Chains initiated by secondary radicals derived in this way from the solvent would not be detected by the tracer method. 

R. A. Gregg and M. S. Matheson Chain transfer in the polymerization of styrene. VI. Chain transfer with styrene and benzoyl peroxide the efficiency of initiation and the mechanism of chain termination. J. Am. Chem. Soc. 73, 1691 (1951). 

Promoters differ by up to 1000-fold in their efficiency of initiation of transcription so that genes with strong promoters are transcribed very frequently... 

The preceding discussion has led us to the conclusion that the surface is the only locus of polymerization which needs to be considered in the heterogeneous polymerization of acrylonitrile. Radicals arrive at the surface at a rate determined by the decomposition of the initiator and efficiency of initiation. Propagation occurs on the surface at a rate determined by the activity of monomer at the surface. By analogy with emulsion polymerization, where monomer diffuses into the particles rapidly enough to maintain near equilibrium activity (14), we assume that the activity of the monomer adsorbed on the particle surface is approximately equal to the mole fraction in solution. The propagation rate constant is presumably influenced somewhat by the presence of the solid surface. 

Initiation. The initiator (I) decomposes to form free radicals (R-), which initiate the growth of polymer by sequential addition of monomer molecules. The fraction (f) which successfully initiate chains is called the efficiency of initiation. 

There are a number of factors that determine whether a protonic acid can initiate polymerization of alkenes. Their acidity (pKa), and therefore the basicity of the resulting counteranion, determines the efficiency of initiation. Although reliable pKa values of acids stronger than sulfuric or hy-droiodic (pKa < -9) are difficult to obtain in aqueous solutions due to their nearly complete dissociation, the pKa values of acetic acid (4.75) and trichloroacetic acid (0.7) in water provide useful references. Conductometric and potentiometric estimates of the pK values of selected protonic acids in various organic solvents are summarized in Table 11 in descending acid strength. These values are not very precise, however, because the amount of moisture in each system was not monitored precisely. 

To assure a sufficient initiation rate, it is recommended to use a compound that ionizes more readily than the macromolecular covalent species. This is the case for 1-alkoxyhaloethanes used as initiators for a-methylstyrene and cumyl halides for isobutene polymerizations [5,40], The latter system may, however, lead to intramolecular cyclization. Blocking either the ortho or meta position in the aromatic ring improves the efficiency of initiation with cumyl derivatives [5]. 

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