Propagation and initiation

Chain Initiation and Propagation In general, two mechanisms have been suggested for initiation and chain propagation in ring-opening polymerization by the cationic process. One mechanism involves the formation of an onium ion by interaction of the catalyst system with the monomer, as shown here for the cationic polymerization of tetrahydrofuran  

The a-carbon of the oxonium ion being electron-deficient because of the adjacent positively charged oxygen, propagation involves a nucleophilic attack of the oxygen of a monomer molecule on the a-carbon of the oxonium ion. In each case, the product of propagation has a terminal cyclic oxonium ion formed from the newly added monomer molecule. 

The second mechanism postulates that the catalyst causes monomer ring cleavage forming an ionic species, which is followed by attack of another monomer 

Though useful polymers can be made by these reactions, their low ceiling temperatures (see p. 599) and consequent tendency to undergo facile depolymerization by an unzipping mechanism pose serious limitations. To overcome this problem the technique of end-capping or end-blocking may be used. Thus poly-oxymethylene (polyacetal), an engineering plastic, prepared from the cyclic acetal 

5-trioxane (cyclic trimer of formaldehyde) using boron trifluoride etherate as the initiator (in the presence of small amount of water required for polymerization) can be stabilized by capping the thermolabde hydroxyl end groups of the macromolecule by esterification using acetic anhydride  

This type of lactam polymerization is initiated under anhydrous conditions with acids or acid salts which do not split off water at the polymerization temperature (e.g., lactam or amine hydrochloride) as well as with some Lewis acids [176, 177]. The activated species is the monomer cation which takes part both in the initiation and propagation reactions. 

The acidic initiator increases the reactivity of the lactam carbonyl towards nucleophiles. Although protonation of amides occurs preferentially at the oxygen atom [47] a small amount of A-protonated lactam is assumed to be present in the tautomeric equilibrium [178—181] 

Because of the lack of resonance stabilization, the amidium ion (XXIX) has an increased acylating ability and may acylate the nucleophiles present. At the beginning of the polymerization initiated with lactam salts of strong acids (e.g. lactam hydrochloride), the neutral lactam is a stronger nucleophile (pXb = 13.6) than the chloride anion (pXb = 21). Acylation of the lactam with the amidium cation (XXIX) results in the formation of an aminoacyllactam [178—181] 

This reaction is similar to the disproportionation (23) occurring in the anionic polymerization except that the nucleophiles are activated in the latter case. The protonated primary amine group is in equilibrium with the lactam, viz. 

Acylation of the amine group with lactam cations (XXIX), viz. 

---

FIGURE 4 21 The initiation and propagation steps in the free radical mechanism for the chlorination of methane Together the two propaga tion steps give the overall equation for the reaction... 

Write equations for the initiation and propagation steps for the formation of dichloromethane by free radical chlorination of chloromethane... 

Chain-Growth Associative Thickeners. Preparation of hydrophobically modified, water-soluble polymer in aqueous media by a chain-growth mechanism presents a unique challenge in that the hydrophobically modified monomers are surface active and form micelles (50). Although the initiation and propagation occurs primarily in the aqueous phase, when the propagating radical enters the micelle the hydrophobically modified monomers then polymerize in blocks. In addition, the hydrophobically modified monomer possesses a different reactivity ratio (42) than the unmodified monomer, and the composition of the polymer chain therefore varies considerably with conversion (57). The most extensively studied monomer of this class has been acrylamide, but there have been others such as the modification of PVAlc. Pyridine (58) was one of the first chain-growth polymers to be hydrophobically modified. This modification is a post-polymerization alkylation reaction and produces a random distribution of hydrophobic units. 

An important part of hazard analysis and risk assessment is the identification of the scenario, or design basis by which hazards result in accidents. Hazards are constandy present in any chemical faciUty. It is the scenario, or sequence of initiating and propagating events, which makes the hazard result in an accident. Many accidents have been the result of an improper identification of the scenario. 

One aspect of pressure vessel design which has received considerable attention in recent years is the design of threaded closures where, due to the high stress concentration at the root of the first active thread, a fatigue crack may quickly initiate and propagate in the radial—circumferential plane. Stress intensity factors for this type of crack are difficult to compute (112,113), and more geometries need to be examined before the factors can be used with confidence. 

Anionic Polymerization. Addition polymerization may also be initiated and propagated by anions (23—26), eg, in the polymerization of styrene with -butyUithium. The LL gegen ion, held electrostatically in... 

Fig. 3. Polymerization initiation and propagation by radiation-generated free radicals. A is the initiating radical produced by irradiating the Hquid coating. (1) represents the Hquid monomer—unsaturated polymer reactive coating system. R is functional. (2) is the growing polymer chain (free radical). The cured...

The chemical species that can lead to EIC in each alloy system are fairly well known although the exact mechanisms of crack initiation and propagation are not thoroughly understood. The species that promote EIC in one alloy system do not necessarily promote EIC in others. A discussion of EIC and the chemical species that promote it can be found in the Hterature (34). 

Separated Anode/Cathode Realizing, as noted in the preceding, that locahzed corrosion is usually active to the surrounding metal surface, a stress specimen with a limited area exposed to the test solution (the anode) is elec trically connec ted to an unstressed specimen (the cathode). A potentiostat, used as a zero-resistance ammeter, is placed between the specimens for monitoring the galvanic current. It is possible to approximately correlate the galvanic current 7g and potential to crack initiation and propagation, and, eventually, catastrophic fail-... 

Highly ductile materials tend to be more resistant to thermal fatigue and also seem more resistant to crack initiation and propagation. 

Generally the oxidant is compounded in one part of the adhesive, and the reductant in the other. Redox initiation and cure occur when the two sides of the adhesive are mixed. There also exist the one-part aerobic adhesives, which use atmospheric oxygen as the oxidant. The chemistry of the specific redox systems commonly used in adhesives will be discussed later. The rates of initiation and propagation are given by the following equations ([9] p. 221). 

Pitting region. The potential-pH region above E, at which pits can both initiate and propagate. 

Of great significance is the velocity of the sea-water, and the initiation and propagation of pits is more likely under stagnant conditions. Obviously... 

Duquette, D. J., Mechanisms of crack initiation and propagation in corrosion fatigue , Proc. Int. Conf. on Mechanisms of Environmental Cracking in Materials, University of Surrey, 4-7 April 1977, The Metals Society, pp. 305-21 (1977)... 

Abrasion-resistant duties may involve abrasion in an aqueous phase or abrasion by dry particulate materials. The selection of the polyurethane type is most important to obtain the best results. Polyester-based polyurethanes perform best in dry abrasion due to their low hysteresis properties and excellent resistance to cut initiation and propagation. However, polyester polyurethanes are susceptible to hydrolytic degradation, and therefore polyether polyurethanes are normally used for aqueous abrasion duties. 

In the period 1910-1950 many contributed to the development of free-radical polymerization.1 The basic mechanism as we know it today (Scheme 1.1), was laid out in the 1940s and 50s.7 9 The essential features of this mechanism are initiation and propagation steps, which involve radicals adding to the less substituted end of the double bond ("tail addition"), and a termination step, which involves disproportionation or combination between two growing chains. 

---