Slow initiation

As mentioned in Sect. 2.2.1.1 slow initiation is controlled either by the rate of ion-generation or cationation (cf. Eqs. (1)—(4). Based on earlier results [58,59] a convenient diagnostic method has been developed [1,29] to elucidate the nature of the rate determining step for rapid carbocationic polymerizations and in IMA (incremental monomer addition) experiments. For only one monomer addition, i.e., for AMI (all monomer in or for the first monomer addition in IMA) experiments, the relevant expressions are as follows  

According to available evidence in the systems investigated to date [29,30,56], the rate of cationation controls the rate of initiation. In line with this information therefore, we assume that upon addition of a stoichiometric excess of MtXn, all the initiator becomes active for cationation, see Eqs. (1) and (2), and Eqs. (3) 

In this treatment we have neglected the number of chains produced by chain transfer to monomer however, it should be kept in mind that the number of polymer chains calculated by Eq. (22) includes those generated by both initiation and chain transfer. Neglecting chains that arose by chain transfer may lead to increased kc, as if initiation were faster (see Sect. 2.3.2 and 2.3.3). 

By the use of the above differential method for the determination of k,. and kp, we have in fact determined kG and kp as a function of time (see Eqs. (27) and (31)) thus, these rate coefficients are not the usual rate constants which are commonly obtained by the conventional integral method. (The reader is reminded, that in the fundamental scenarios, kc and kp were treated as real constants to enable the integration of differential equations.) Our experiments (see later) have proven that kc and kp are not true rate constants but are functions of time or conversion (see e.g., Figs. 13A and 13C and sect. 4.1.1.1). 

Iteak io- (rtiotow Fig. 13A-C. Isobutylene pol iKrization by the 15 0 TMPCl/TiClt system at various TEA concentra- 

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The Landolt reaction (iodate + reductant) is prototypical of an autocatalytic clock reaction. During the induction period, the absence of the feedback species (Irere iodide ion, assumed to have virtually zero initial concentration and fomred from the reactant iodate only via very slow initiation steps) causes the reaction mixture to become kinetically frozen . There is reaction, but the intemiediate species evolve on concentration scales many orders of magnitude less than those of the reactant. The induction period depends on the initial concentrations of the major reactants in a maimer predicted by integrating the overall rate cubic autocatalytic rate law, given in section A3.14.1.1. 

A protonic acid derived from a suitable or desired anion would seem to be an ideal initiator, especially if the desired end product is a poly(tetramethylene oxide) glycol. There are, however, a number of drawbacks. The protonated THF, ie, the secondary oxonium ion, is less reactive than the propagating tertiary oxonium ion. This results in a slow initiation process. Also, in the case of several of the readily available acids, eg, CF SO H, FSO H, HCIO4, and H2SO4, there is an ion—ester equiUbrium with the counterion, which further reduces the concentration of the much more reactive ionic species. The reaction is illustrated for CF SO counterion as follows ... 

Besides the solvent composition, the vehicle system is responsible for various drying deficiencies associated with water-borne coatings, such as slow initial dry time, loss of dry, poor through drying, and hardness (see Coatings). 

These acids can be used alone or as mixtures. It is especially advantageous to use a mixture of liquid and gaseous acids. The gaseous acid will stabilize free monomer in the headspace of a container, while the liquid acid will prevent premature polymerization of the bulk monomer or adhesive. However, it is important to use only a minimum amount of acid, because excess acid will slow initiation and the formation of a strong adhesive bond. It can also accelerate the hydrolysis of the alkyl cyanoacrylate monomer to 2-cyanoacrylic acid, which inhibits the polymerization of the monomer and reduces molecular weight of the adhesive polymer. While carboxylic acids inhibit the polymerization of cyanoacrylate monomer, they do not prevent it completely [15]. Therefore, they cannot be utilized as stabilizers, but are used more for modifying the reactivity of instant adhesives. 

If the rate of addition to monomer is low, primary radical termination may achieve greater importance. For example, in photoinitiation by the benzoin ether 12 both a fast initiating species (13, high k) and a slow initiating species (14, low... 

The magnitude of t0 can be measured from the intercept of a f(a)—time plot. The existence of the induction period can introduce uncertainty into a reduced time analysis if the temperature coefficient of t0 differs from that later applicable, and it is necessary to plot (t — t0)/(tb — t0) against a where tb is the time at which the selected common value of a is attained. The occurrence of a slow initial process can be reflected in deviations from linearity in the f(a) time plot, though in favourable systems the contribution may be subtracted before analysis [40]. 

The seeding procedure is described in Fig. 20 by the line abed. Its inspection shows that the concentration of the monomer left at the time when all the initiator is consumed remains the same whether the monomer is added at once or in two portions. Moreover, the total concentration of the added monomer must exceed a critical value to allow for quantitative consumption of the initiator. Thus, the seeding technique does not eliminate the broadening of molecular weight distribution caused by slow initiation of a virtually irreversible polymerization. This conclusion is confirmed experimentally 133). 

Ed 37Kcal) are generally regarded as increasing in "slowness" in the direction 1isted, because Ad decreases or Ed increases, or both, their value of b increases in the order shown, all other factors remaining equal. Consequently, we must conclude that slow initiators are more likely to produce unstable R-A s than fast ones. The above conclusions involving Tq and initiator choice have been observed experimentally. 

The experiments were carried out with two initiators. According to published data (B.), at the base temperature, Tb, the fast initiator, II, has a half-life of 3.5 minutes, and the slow initiator, 12, has a half-life of 95 minutes. A minor modification of the monomer mass balance (Equation 7) is required for the case of two initiators. 

The more usual pattern found experimentally is that shown by B, which is called a sigmoid curve. Here the graph is indicative of a slow initial rate of kill, followed by a faster, approximately linear rate of kill where there is some adherence to first-order reaction kinetics this is followed again by a slower rate of kill. This behaviour is compatible with the idea of a population of bacteria which contains a portion of susceptible members which die quite rapidly, an aliquot of average resistance, and a residue of more resistant members which die at a slower rate. When high concentrations of disinfectant are used, i.e. when the rate of death is rapid, a curve ofthe type shown by C is obtained here the bacteria are dying more quickly than predicted by first-order kinetics and the rate constant diminishes in value continuously during the disinfection process. 

A, obtained if the disinfection process obeyed the first-order kinetic law. B, sigmoid curve. This shows a slow initial rate of kill, a steady rate and finally a slower rate of kill. This is the form of curve most usually encountered. C, obtained if bacteria are dying more quickly than first-order kinetics would predict. The constant, K, diminishes in value continuously during the process. 

Initiation is slow, while chain growth occurs very rapidly. Activators, such as triethylaluminium, can be added to assist in the reduction of the Cr site and to provide the initial ethyl group, such that the slow initiation step is avoided. 

There are two distinctly different mechanisms for a surface reaction between two species [8], for example toluene (T) and an active surface species ( ). In the Langmuir-Hinshelwood (LH) mechanism, reaction occurs between toluene emd the active surface species when both are adsorbed on the catalyst surface. If this step is the slow initiation step, the rate is proportional to the product of the coverages of toluene and the active site species ... 

When the amount of coke formed as a function of time on stream is compared to the decrease in catalytic activity (see Fig. 3), two regimes of deactivation can be noticed for the strongly deactivating catalysts, i e, a slow initial deactivation which is followed by a rapid loss of activity This first phase is characteristic of a slow transformation of the reactive carbon into less reactive coke. The second phase is attributed to carbon formed on the support which accumulates there and rapidly covers the Pt particles when its amount reaches a critical value causing the sudden decay of catalytic activity. 

Coke formation on these catalysts occurs mainly via methane decomposition. Deactivation as a function of coke content (see Fig. 3 for Pt/ y-AljO,) seems to involve two processes, i e, a slow initial one caused by coke formed from methane on Pt that is non reactive towards CO2 (see Table 3) In parallel, carbon also accumulates on the support and given the ratio between the support surface and metal surface area at a certain level begins to physically block Pt deactivating the catalyst rapidly. The coke deposited on the support very close to the Pt- support interface could be playing an important role in this process. 

Ozone diffuses readily into amorphous region of the polyethylene (32) and oxidation probably occurs much deeper in the solid sample. Ozone also attacks the crystalline part of polyethylene but it has a slow initiation stage followed by more rapid oxidation (13). Because ozone does not diffuse into the crystalline regions (13.32). oxidation is restricted to the surface. The resulting oxidized functional groups on the crystalline regions will remain at the surface, whereas those formed in the amorphous region can diffuse into the bulk. 

A radical initiator based on the oxidation adduct of an alkyl-9-BBN (47) has been utilized to produce poly(methylmethacrylate) (48) (Fig. 31) from methylmethacrylate monomer by a living anionic polymerization route that does not require the mediation of a metal catalyst. The relatively broad molecular weight distribution (PDI = (MJM ) 2.5) compared with those in living anionic polymerization cases was attributed to the slow initiation of the polymerization.69 A similar radical polymerization route aided by 47 was utilized in the synthesis of functionalized syndiotactic polystyrene (PS) polymers by the copolymerization of styrene.70 The borane groups in the functionalized syndiotactic polystyrenes were transformed into free-radical initiators for the in situ free-radical graft polymerization to prepare s-PS-g-PMMA graft copolymers. 

The polymerization of styrene by Zr (benzyl) 4 has the characteristics of producing high molecular weight polymers by a very slow polymerization process. This can be reasonably explained by a slow initiation process followed by a fast propagation reaction. These two processes are probably chemically similar but differ significantly in rate for structural reasons. 

Example 5.1. If the inner wall of a transfer tube is stainless steel of 4 mm outer diameter, 0.25 mm wall thickness and 1.5 m length, the room temperature enthalpy of the tube is about 2700 J. Should only the latent heat be used to precool the tube, about 11 of liquid 4He would be needed. However, the heat capacity of the evaporated gas contributes to the cooling. The room temperature enthalpy of the gas produced by 1 cc of liquid is about 190 J. If half of this enthalpy is used (slow initial transfer), then only about 30 cc of liquid is needed for precooling the transfer line. 

To transfer the liquid, a small overpressure is created in the dewar from which the liquid is to be transferred. A special care must be taken when transferring LHe into either a warm dewar or a partially filled dewar to avoid waste of liquid. If the dewar is warm (77 K), a slow initial transfer reduces the liquid consumption. If liquid is already in the dewar, the risk of inflating a warm stream of He gas must be avoided. The simplest transfer tubes are U shaped. There are flexible and demountable transfer tubes. 

Gellan gum had a slow initial take up by the food industry. Some bakery industry uses are in pie and bakery fillings, bakery icings, frostings and glazes. Gellan gum is often used with other gums and thickeners. 

Choice of a very rapidly initiating cation, e.g., an aroyl or diarylmethylium salt, to avoid the complication from slow initiation. 

The same idea will explain an experiment in which a styrene phial (20 mmole) was broken at -10 °C into 100 ml of a 6 x 10 3 M solution (C104" content) of the coloured ionic reaction product between AgC104 and 1-phenylethyl bromide (see above). Because of the rather slow initial mixing, the colour of the solution was not completely discharged by the styrene, and a violent polymerisation ensued, at least 100 times faster than the reaction catalysed by an equivalent amount of perchloric acid would have been. 

In this section, we will introduce one of the two common methods for solving problems. (You will see the other method in Chapter 5.) This is the Unit Conversion Method. It will be very important for you to take time to make sure you fully understand this method. You may need to review this section from time to time. The Unit Conversion Method, sometimes called the Factor-Label Method or Dimensional Analysis, is a method for simplifying chemistry problems. This method uses units to help you solve the problem. While slow initially, with practice it will become much faster and second nature to you. If you use this method correctly, it is nearly impossible to get the wrong answer. For practice, you should apply this method as often as possible, even though there may be alternatives. 

Kinetic curves relative to polymerization reactions in the solid state commonly show a sigmoidal shape with a slow initiation step followed by a steep increase, even by two orders of magnitude, of the reaction rate. A reaction with this kind of kinetic curve is said to have an autocatalytic behavior. 

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