Propagational mechanism

Polypropylene polymerized with triethyl aluminum and titanium trichloride has been found to contain various kinds of chain ends. Both terminal vinylidene unsaturation and aluminum-bound chain ends have been identified. Propose two termination reactions which can account for these observations. Do the termination reactions allow any discrimination between the monometallic and bimetallic propagation mechanisms ... 

The mechanism of the polycondensation reaction remains unclear. A vanety of possible reactive intermediates have been suggested, including sdyl radicals and sdyl anions. An anionic propagation mechanism (100,101,103) has been strongly suggested, although the case is by no means setded (104). Other Synthetic Methods. 

This type of stall occurs when all the blades around the compressor annulus stall simultaneously without the occurrence of a stall propagation mechanism. The circumstances under which individual blade stall is established are unknown at present. It appears that the stalling of a blade row generally manifests itself in some type of propagating stall and that individual blade stall is an exception. 

They chemically interfere with the flame propagation mechanism. 

If the combustion process within a gas explosion is relatively slow, then expansion is slow, and the blast consists of a low-amplitude pressure wave that is characterized by a gradual increase in gas-dynamic-state variables (Figure 3.7a). If, on the other hand, combustion is rapid, the blast is characterized by a sudden increase in the gas-dynamic-state variables a shock (Figure 3.7b). The shape of a blast wave changes during propagation because the propagation mechanism is nonlinear. Initial pressure waves tend to steepen to shock waves in the far field, and wave durations tend to increase. 

Ishizuka, S., Hamasaki, T., Koumura, K., and Hasegawa, R., Measurements of flame speeds in combustible vortex rings Validity of the back-pressure drive flame propagation mechanism, Proceedings of the Combustion Institute, 17, 727-734,1998. 

M. Summerfield, S.H. Reiter, V. Kebely, and R.W. Mascolo 1955, The structure and propagation mechanisms of turbulent flames in high speed flows. Jet Propulsion 25(8) 377-384. 

Depending on the obstacle height and spacing, as well as on the vertical height of the channel, one or more of the above-described mechanisms can occur. However, the propagation mechanism comprises continuous reinitiation and attenuation by diffraction around the obstacles. This mechanism essentially is identical to that of a normal detonation, where reinitiation occurs when the transverse waves collide and the reinitiated wave fails between collisions. In quasi-detonations, the reinitiation is controlled by obstacles. In general, the obstacles and walls provide surfaces for the reflection and diffraction of shock and detonation waves. 

Propagation mechanism of quasi-detonations, Proc. Combust. Inst., 22,1723, 1988. 

Photographic study of the structure and propagation mechanisms of quasi-detonations in rough tubes. Prog. Astr. Aeron., 138, 223, 1990. 

Analogous principles should apply to ionically propagated polymerizations. The terminus of the growing chain, whether cation or anion, can be expected to exhibit preferential addition to one or the other carbon of the vinyl group. Poly isobutylene, normally prepared by cationic polymerization, possesses the head-to-tail structure, as already mentioned. Polystyrenes prepared by cationic or anionic polymerization are not noticeably different from free-radical-poly-merized products of the same molecular weights, which fact indicates a similar chain structure irrespective of the method of synthesis. In the polymerization of 1,3-dienes, however, the structure and arrangement of the units depends markedly on the chain-propagating mechanism (see Sec. 2b). 

To elucidate the chain propagation mechanism of alternating copolymerization of TBSM with MA and to quantitatively estimate the contribution of complex-bound monomers to the propagation reaction, a kinetic approach described in Ref.91) was employed. 

Improved control was observed, however, upon addition of benzyl alcohol to the dinuclear complexes.887 X-ray crystallography revealed that whereas (296) simply binds the alcohol, (297) reacts to form a trinuclear species bearing four terminal alkoxides. The resultant cluster, (298), polymerizes rac-LA in a relatively controlled manner (Mw/Mn=1.15) up to 70% conversion thereafter GPC traces become bimodal as transesterification becomes increasingly prevalent. NMR spectroscopy demonstrates that the PLA bears BnO end-groups and the number of active sites was determined to be 2.5 0.2. When CL is initiated by (298) only 1.5 alkoxides are active and kinetic analysis suggests that the propagation mechanisms for the two monomers are different, the rate law being first order in LA, but zero order in CL. 

Termination is by the recombination of H and Cl. There is complete corroboration of this mechanism from photochemical studies. This is undoubtedly a free-radical chain propagation mechanism. 

Chien JCW, Rausch MD, Llinas GH, Lin Y, Winter HH (1991) Two-state propagation mechanism for propylene polymerization catalyzed by rac-anti-ethylidene(l-r 5-tetramethylcy-clopentadienyl)(l-r 5-indenyl).dimethyltitanium. J Am Chem Soc 113 8569-8570... 

Consider now the case of a combined action of two inhibitors, one of which (InH) breaks the chains by reacting with peroxyl radicals and the other (S) breaks down ROOH [9]. The radical In formed in the reaction of InH with R02 is trapped by the peroxyl radical and, therefore, does not contribute to chain propagation (mechanism III, Chapter 14). If kSi -C k3, the inhibitor InH is consumed with a rate of v-Jf, where the initiation rate is Vi = Vio + A 3[ROOH]. After the time ts = ( s[S]o)-1> oxidation becomes quasistationary with the quasistationary hydroperoxide concentration (see Chapter 14)... 

We have synthesized such a material, which is called perfectly imperfect polyethylene, where each branch is a methyl group and its frequency along the backbone is controlled by the nature of the symmetrical diene used in the ADMET polycondensation reaction [37]. Equation 12 illustrates the chemistry used to produce polyethylene by a step polycondensation route rather than a chain propagation mechanism. 

---