Initiator fragmentation

For linear alkanes, the initial fragment lost is an ethyl group (never a methyl group), followed by propyl, butyl, and so on. An intense peak at mass 43 suggests a chain longer than butane. 

Propagation. The initiator fragment reacts with a monomer M to begin the conversion to polymer the center of activity is retained in the adduct. Monomers continue to add in some way until molecules are formed with degree of polymerization n ... 

If n is large enough, the initiator fragment-an end group-need not be written explicitly. 

While this reaction with solvent continues to provide free radicals, these may be less reactive species than the original initiator fragments. We shall have more to say about the transfer of free-radical functionality to solvent in Sec. 6.8. 

Next we assume that a fraction f of these initiator fragments actually reacts with monomer to transfer the radical functionality to monomer ... 

The degree of polymerization values in the two combining radicals can have any value, and the molecular weight of the product molecule will be considerably higher on the average than the radicals so terminated. The polymeric product molecule contains two initiator fragments per molecule by this mode of termination. 

This mode of termination produces a negligible effect on the molecular weight of the reacting species, but it does produce a terminal unsaturation in one of the dead polymer molecules. Each polymer molecule contains one initiator fragment when termination occurs by disproportionation. 

From the ratio of activities and measured values of n, the average number of initiator fragments per polymer can be determined. Carry out a similar argument for the ratio of activities for polystyrene and evaluate the average number of initiator fragments per molecule for each polymer from the following datat ... 

Finally, the dielectric properties of a nonpolar polymer are modified by inclusion of even small amounts of a polar comonomer. In coatings applications the presence of polar repeat units in an otherwise nonpolar polymer reduces the tendency for static buildup during manufacture, printing, and ultimate use. On the other hand, in dielectric applications this increases the power loss and must be kept to a minimum, even to the exclusion of polar initiator fragments. 

Four-membered heterocycles prefer to cleave, upon ionization, into two fragments, each containing two of the ring atoms. Further cleavages commence from these initial fragments (Scheme 5). Specific details can be found as follows azetidines (B-71MS296), oxetanes... 

Initial Fragment Velocity for Ideal-Gas-Filled Vessels... 

A theoretical upper limit of initial fragment velocity can be calculated if it is assumed that the total internal energy E of the vessel contents is translated into... 

The results of a parameter study were used to compose a diagram (Figure 6.33) which can be used to determine initial fragment velocity (Baker et al. 1978a and 1983). 

Initial Fragment Velocity for Vessel Filled with Nonideal Gases... 

The fraction of explosion energy which contributes to fragment generation is unclear. Its effect on initial fragment velocity deserves more attention in relation... 

This equation is well known and often used to calculate initial fragment velocity, but its application can result in gross overestimation. Assuming adiabatic expansion of the ideal gas, it can be derived that ... 

As a vessel ruptures, its fragments accelerate rapidly to a maximum velocity. This value is the initial fragment velocity Vj. It is used to calculate either the range of fragment travel or, if collision with an obstacle occurs before maximum range is attained, impact velocity. 

Figure 9.7. Fragment velocity versus scaled pressure (Baker et al. 1983). V = initial fragment...

Then the mean initial fragment velocity will be ... 

In earlier investigations chain ends were suggested to be important initiation sites for dehydrochlorination. Provided there are no transfer reactions during polymerization, at least half the polymer chain ends will carry initiator fragments. In practice, transfer reactions swamp the normal termination processes and <30% of the chain ends carry initiator residues [59]. 

Berger and Mcycrhoff150 also reported that termination involves substantial disproportionation. They determined the initiator fragments per molecule in PS prepared with radiolabeled AIBN and conducted a detailed kinetic analysis of the system. They also found a marked temperature dependence for k k. Values of kt fkK ranged from 0.168 at 30 °C to 0.663 at 80 °C. 

H1 NMR spectroscopy was found to be unsuitable for head-group analysis of HSi-PaM-eSt. The resonance associated with the Si-H proton at the head-group is broadened by multiple splitting and the resonances of the aromatic protons of the initiator fragment are buried in the aromatic proton resonances of phenyl rings of the aMeSt repeating units. 

Sn = solvent n Rc = initiator fragment R = radical of chain length i or J M = monomer... 

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