Mechanism of initiation

The mechanism of initiation in cationic polymerization using Friedel-Crafts acids appeared to be clarified by the discovery that most Friedel-Crafts acids, particularly haUdes of boron, titanium, and tin, require an additional cation source to initiate polymerization. Evidence has been accumulating, however, that in many systems Friedel-Crafts acids alone are able to initiate cationic polymerization. The polymerization of isobutylene for instance can be initiated, reportedly even in the absence of an added initiator, by AlBr or AlCl (19), TiCl ( )- Three fundamentally different... 

Styrene readily copolymerizes with many other monomers spontaneously. The styrene double bond is electronegative on account of the donating effect of the phenyl ring. Monomers that have electron-withdrawiag substituents, eg, acrylonitrile and maleic anhydride, tend to copolymerize most readily with styrene because their electropositive double bonds are attached to the electronegative styrene double bond. Spontaneous copolymerization experiments of many different monomer pair combiaations iadicate that the mechanism of initiation changes with the relative electronegativity difference between the monomer pairs (185). 

The reaction between benzoyl peroxide and A,A-dimethylaniline has been the subject of many examinations over the years. The following mechanism of initiation is fairly well accepted in the polymerization of styrene. It seems likely that a similar mechanism is followed for other free-radical polymerizations (Scheme 5). 

Addition of styrene to a green solution of naphthalene" Na+ in tetrahydrofuran leads to an instantaneous change of color from green to red. Styrene polymerizes rapidly and quantitatively within a few seconds, and when the reaction is completed, addition of water converts the red solution of polystyryl carbanions into colorless solution of polystyrene. After precipitation of the polymer it was shown spectroscopically25 that the residual solution contains an amount of naphthalene equal to that used in the preparation of the initiating catalyst. This observation confirms the proposed mechanism of initiation of the polymerization. 

This mechanism of initiation is confirmed by the fact that, when the PAN-PEO block copolymer is treated with diisocyanate in benzene in the presence of pyridine acting as catalyst, copolymers lose their solubility in DMF as a result of the formation of intermolecular chemical bonds75). 

Various mechanisms have been proposed to explain the initiation processes. The self-initiated copolymerizations of the monomer pairs S-MMA and S-AN proceed at substantially faster rates than pure S polymerization. For S-AN333 and S-MAHJJ the mechanism of initiation was proposed to be analogous to that of S homopolymerization (Scheme 3.62) but with acrylonitrile acting as the dicnophile in the formation of the Diels-Alder adduct (Scheme 3.66). 

The proposed mechanism of initiation with the dithiuram disulfide 4 is shown in Scheme 9.8, The dithiuram disulfide decomposes thermally or photochcmically to give dithiocarbamyl radicals 6, These radicals 6 add monomer only slowly and... 

Since initiation with conventional Friedel-Crafts halides cannot be controlled, the fine-tuning of reactions becomes extremely cumbersome. In contrast, by the use of alkylaluminum compounds elementary events (initiation, termination, transfer) become controllable and thus molecular engineering becomes possible. Indeed, by elucidating the mechanism of initiation etc., a large variety of new materials, i.e., block3, graft4-6 bigraft7 copolymers, have been synthesized and some of their physical-chemical properties determined. 

Let us consider the mechanism of initiation in greater detail, taking benzoyl peroxide as an example. It may be assumed to dissociate initially into benzoate radicals, as previously indicated by reaction (2). 

It hag been shown that transition of a backbone carbon from the sp to sp state is promoted by tensile stresses and inhibited by compressive strains (10,44). The acceleration of the process of ozone oxidation of the polymers under load is not associated with the changes in supramolecular structure or segmental mobility of the chain. The probably reason of this effect is a decreasing of the activation energy for hydrogen abstraction (44). The mechanism of initial stages of the reaction of ozone with PP can be represented as ... 

Hershey, J. W. B., and Merrick, W. C. (2000). Pathway and mechanism of initiation of protein synthesis. In Translational Control of Gene Expression (N. Sonenberg, J. W. B. Hershey, and M. B. Mathews, eds.), pp. 33—88. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York. 

Jackson, R. J. (2005). Alternative mechanisms of initiating translation of mammalian mRNAs. Biochem. Soc. Trans. 33, 1231-1241. 

In order to determine the physical mechanism of initial ET including other rapid kinetics in photosynthetic RCs, it is necessary to construct a vibronic model that comprises the electronic and vibrational states of the system. It is also important to take into account temperature effect in both experiments and theories. In particular, we should stress that most of MO calculations carried out for RCs are based on the crystallographic structures. However, the structure at room temperature may be different from that obtained from the X-ray analysis,... 

YuB Shilov. Kinetics and Mechanism of Initiated Oxidation of Polypropylene. Ph.D. thesis Dissertation, Institute Chemical Physics, Chemogolovka, 1979. 

In the field of cationic polymerisation a notoriously intractable problem is the mechanism of initiation by aluminium halides. Despite much work on the polymerisation itself, there are few studies of the initiation mechanism. Existing theories are shown to be inadequate to explain the most characteristic features of the reactions when a solution of an aluminium halide in an alkyl halide is introduced into a solution of isobutene, there ensues a fast polymerisation which generally stops at incomplete conversion, and the number of moles of polymer formed is much smaller than the number of moles of initiator these features are found over a very wide range of conditions. 

This brings us to the complicated and difficult question concerning the mechanism of initiation by titanium tetrachloride. There is extensive evidence, obtained with many different systems, that under certain circumstances titanium tetrachloride will only act as initiator in the presence of a co-initiator. There is also good evidence from different laboratories that in other circumstances a co-initiator is apparently not required, and in at least one system both types of initiation go on simultaneously [15]. 

The mechanism of initiation of cationic polymerisations by metal halides was clarified and systematized to some extent by the discovery of the phenomenon of co-catalysis or co-initiation. But, whereas there was, by the mid-1960s, good evidence that at any rate in many systems the halides of boron, titanium, and tin required a co-initiator, the position with regard to the best-known and most popular initiator, and the one which was of greatest economic significance, aluminium chloride, remained obscure. Of the vast number of published experiments on the system, aluminium chloride + isobutylene, hardly any could provide evidence concerning the initiation reaction, because they were almost exclusively concerned with measurements of yields and degree of polymerisation (DP). 

It now appears that much more careful thought is required in the selection of systems which may offer the prospect of yielding unambiguous results. In particular, the choice of solvent and initiator must be considered very carefully, and features such as mechanisms of initiation, catalyst efficiency, nature of end-groups, and the internal order of reactions need to be explored much more thoroughly than has been customary in the past. Too much effort has been wasted by ignoring the precept First the Chemistry, then the Kinetics . 

The sensitivity to ionizing radiation is maximal in those cells able to activate a co-ordinate program of cell death (apoptosis) primed by the radiation-induced oxidative stress. Albeit apoptosis is a nuclear event and radiation-induced DNA damage is probably the most relevant mechanism of initiation of apoptosis, the control of the execution phase (and sometime also the initiation) takes place at the mitochondrial level. Radioresistance occurs... 

Pantaloni et al. (1981) investigated the mechanism of initiation of tubulin assembly, and the role of MAPs in the stabilization of microtubules. For their work they used microtubule protein carried through two cycles of assembly/disassembly, followed by treatment with 1 M sodium chloride for 1 hour, and subsequent centrifugation to remove the 70,000-, 168,000-, and 210,000-molecular weight proteins commonly... 

The solution of the riddle posed by Kornblum s dark Sj l reaction is as follows. The nucleophile does work as a single electron-transfer initiator of the chain process. However, the mechanism of initiation does not consist of a mere outer-sphere electron transfer from the nucleophile to form the anion-radical of the substrate. Rather, it involves a dissociative process in which electron transfer and bond breaking are concerted (Costentin and Saveant 2000). Scheme b at the beginning of Section 7.8 illustrates the concerted mechanism. 

G. L. Collingridge (1985). Long term potentiation in the hippocampus mechanisms of initiation and modulation by nemo transmitters. Trends Pharmacol. Sci. 6 407-411. 

Refs 1) W.A. Taylor A. Weale, PrRoySoc 138A, 92-116(1932) (Mechanism of initiation and propagation of detonation in solid explosives) la) E. Burlot, CR 197, 1223-25 (1933) CA 28, 2538 (]934) (Ignition of explosives in vacuo) lb) A.F. Belyaev, DoklAkadN 18, 267 (1938) (Origination of detonation in explosives under the action of a thermal impulse) 2) A.F. Belyaev et al, KhimReferatZh 1940, No 10-11 CA 37, 1270(1943) (Investigation of initiation of small quantities of expls by heat with an impulse lasting for 10 3-10 secs, showed that the more volatile the expl, the harder... 

CA 46, 2266(1952 (Initiation of deton in gases. Published theories of deton are reviewed, and some new thoughts on the structure of deton waves and the mechanism of initiation of deton by flames are presented) 14a) F.R. Bowden A.D. Yoffe, "Initiation and Growth of Explosion in Liquids and Solids , Cambridge-UnivPress, England (1952) 14b) J.A. Fay... 

PrRoySoc 246A, 257-68(1958) (Some considerations on mechanism of initiation of detonation in explosives) 30) G.P. 

M.A. Cook etal, TrFaradSoc 56, 1028-38(1960 Promotion of shock initiation of detonation by metallic surfaces) 36a) Andreev Belyaev (i960), 265-68 (Starting impulse and mechanism of initiation) 268-70 (Initiation by heat) 270-73 (Initiation by flame) 273-86 (Initiation by shock or friction) 287-89 (Initiation of expln in projectiles on hitting a target) 36b) J. Favier C. Fauquignon, MP 42, 65-81(1960) (Initiation of expls. and transmission of detonation) 37) D.B. Moore J.C. Rice, Detonation of Secondary Explosives by Lead Azide , SRI (Stanford Research Institute), Poulter Laboratories, Technical Report 004-60(1960) 37a) S.J. Jacobs, AmRocket-... 

SocJ 30, 151-58(1960) (Recent advances in condensed media detonation) 37b) Dunkle s Syllabus (1960-1961), pp 4a 4b (Initiation of shock waves) lOa-lOg (Initiation of deflgrn and deton) p 12a (Frank-Kamenet-skii formulation) p 13b (Initiation by electric discharge) p 13f (Thermal Decomposition and Initiation of Explosives, as discussed by B. Reitzner) pp 17a to 17e (Mechanism of initiation and propagation of detonation in solid explosives) pp 17e 17f [Marlow Skidmore (Ref 31) concluded from their investigations that the problem of shock initiation is somehow related to the temperature distribution in the shock pulse and its effect on the chemical reaction rate. They used an Arrhenius type relationship for the rate increase in the frac-... 

Detonation (and Explosion), Mechanism of Initiation and Propagation of. See under Detonation (and Explosion), Initiation (Birth) and Propagation (Growth or Spread) in Explosive Substances... 

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