Molecules initiator

One distinction that should be pointed out involves the comparison of Eqs. (5.1) and (5.40). In the former we considered explicitly the AB monomer, while the latter is based on the polymerization of AA and BB monomers. In both instances is obtained by dividing the total number of monomer molecules initially present by the total number of chains after the reaction has occurred to extent p. Following the same procedure for different reaction... 

Determine the average functionality of the mixture. The total number of functional groups is 6.00 mol, but the total number of molecules initially present... 

Molecules initially in the J = 0 state encounter intense, monochromatic radiation of wavenumber v. Provided the energy hcv does not correspond to the difference in energy between J = 0 and any other state (electronic, vibrational or rotational) of the molecule it is not absorbed but produces an induced dipole in the molecule, as expressed by Equation (5.43). The molecule is said to be in a virtual state which, in the case shown in Figure 5.16, is Vq. When scattering occurs the molecule may return, according to the selection mles, to J = 0 (Rayleigh) or J = 2 (Stokes). Similarly a molecule initially in the J = 2 state goes to... 

The mechanism of the anionic polymerization of styrenes and 1,3-dienes initiated by alkaU metals has been described in detail (3,20) as shown in equations 3—5 where Mt represents an alkaU metal and M is a monomer molecule. Initiation is a heterogeneous process occurring on the metal surface. The... 

When the styrene has been consumed, to give living polymers of narrow molecular mass distribution, more styrene and more catalyst is added. The styrene adds to the existing chains and also forms new polymer molecules initiated by the additional sec-butyl-lithium. 

Stabilizer molecule Monomer molecule Initiator molecule Primary radical Oligomeric radical... 

Basically the kinetic results are consistent with the first (rapid) reaction being the addition of a hydroxide ion to the diazonium ion followed by the very fast deprotonation of the (Z)-diazohydroxide to give the (Z)-diazoate (steps 1 and 2 in Scheme 5-14). In addition, however, the stopped-flow experiments showed that the diazonium ion also reacts with the water molecule, initially forming the conjugate acid of the (Z)-diazohydroxide (ArN2OH2), which is then very rapidly deprotonated (reaction 1 in Scheme 5-14). The rate of the relatively slow (Z/E)-isomerization (reaction 5 in Scheme 5-14) can in general be measured by conventional spectrophotometry. 

The dimerization of ketones to 1,2-diols can also be accomplished photochemi-cally indeed, this is one of the most common photochemical reactions. The substrate, which is usually a diaryl or aryl alkyl ketone (though a few aromatic aldehydes and dialkyl ketones have been dimerized), is irradiated with UV light in the presence of a hydrogen donor such as isopropyl alcohol, toluene, or an amine. In the case of benzophenone, irradiated in the presence of 2-propanol, the ketone molecule initially undergoes n — k excitation, and the singlet species thus formed crosses to the T, state with a very high efficiency. 

The molecule initially sits in potential energy minimum. A lateral force F is required to overcome attractive interac-... 

Maleimides Alkyl and aryl maleimides in small concentrations, e.g., 5-10 wt% significantly enhance yield of cross-link for y-irradiated (in vacuo) NR, cw-l,4-polyisoprene, poly(styrene-co-butadiene) rubber, and polychloroprene rubber. A-phenyhnaleimide and m-phenylene dimaleimide have been found to be most effective. The solubihty of the maleimides in the polymer matrix, reactivity of the double bond and the influence of substituent groups also affect the cross-fink promoting ability of these promoters [82]. The mechanism for the cross-link promotion of maleimides is considered to be the copolymerization of the rubber via its unsaturations with the maleimide molecules initiated by radicals and, in particular, by allyfic radicals produced during the radiolysis of the elastomer. Maleimides have also been found to increase the rate of cross-linking in saturated polymers like PE and poly vinylacetate [33]. 

E. B. Nelson, Exudate molecules initiating fungal responses to seeds and roots. The Rhizosphere and Plant Growth (D. L. Keister and P. B. Cregan, eds.) Kluwer Academic Publishers, Dordrecht, The Netherlands, 1991, p. 197. 

We have used different anionic, cationic, and metathesis initiators but after the reaction of the initiator with one monomer molecule (initiation, kj) no subsequent propagation was observed ... 

Anthraquinones are nearly perfect sensitizers for the one-electron oxidation of DNA. They absorb light in the near-UV spectral region (350 nm) where DNA is essentially transparent. This permits excitation of the quinone without the simultaneous absorption of light by DNA, which would confuse chemical and mechanistic analyses. Absorption of a photon by an anthraquinone molecule initially generates a singlet excited state however, intersystem crossing is rapid and a triplet state of the anthraquinone is normally formed within a few picoseconds of excitation, see Fig. 1 [11]. Application of the Weller equation indicates that both the singlet and the triplet excited states of anthraquinones are capable of the exothermic one-electron oxidation of any of the four DNA bases to form the anthraquinone radical anion (AQ ) and a base radical cation (B+ ). 

The polymerization rates of styrene and acrylonitrile monomer are not equal. If we were to initiate polymerization in an equimolar solution of the two monomers, the styrene monomer would initially be depleted at a faster rate than the acrylonitrile. Thus, the copolymer molecules initially produced would contain a higher concentration of styrene than acrylonitrile. As the reaction progressed, the styrene would be depleted from the solution and the comonomer ratio in the copolymer would gradually shift towards a higher acrylonitrile content. The final product would consist of polymer chains with a range of comonomer compositions, not all... 

Figure 8.11 Sequential STM images showing a 2-butanol molecule initially adsorbed on a Ti5c site dissociating at an Ob-vac. (a) Before dissociation, the 2-butanol molecule is indicated with a green arrowhead, (b) After the reaction, the 2-butoxy takes the position...

Finally, we note a very clever isotope experiment done by Tatsuo Matsushima (2 4). The results are summarized in Figure 14 and show that when 1JC0 is preadsorbed on Pt and then a mixture of - - C0 and O2 is introduced into the gas phase, the product CO2 molecules initially formed all are labelled with - C. In fact the experiment is not quite so simple. An initial 002/ C02 ratio is measured (unity in... 

If N0 is the number of reactant molecules initially present then... 

Photochemical reactions are the reactions of excited-state molecules initiated by photon absorption whereas thermal reactions are the reactions of ground-state molecules usually initiated by heat. The energy of photoexcitation of molecules can be provided by photon absorption even at very low temperatures and is of the same order as the activation energies for ground-state molecules. Provided the process of photoexcitation can be utilised in order to... 

Although the propagation reactions are only shown once, you should be aware that they occur in a sequence a very large number of times before the termination reactions remove the reactive radicals. Thus, free-radical chain reactions are characterised by the formation of a very large number of product molecules initiated by the absorption of a single photon in the initiation step that is, chain reactions act as chemical amplifiers of the initial absorption step. 

Signaling molecules initiate a process of signal transduction. Transduction implies that a signal received in one form is translated into a signal of another form. Typically, here is what happens. A signal from a molecule, insulin for example,... 

In Eq. (5.25) H2O represents a water molecule initially present outside the coordination sphere of the metal ion, which, as a result of the exchange, has entered the first coordination sphere. It follows that the degree of kinetic reactivity of aquometal ions with complexing agents parallels their kinetic lability toward water exchange. Moreover, since the water exchange rate constants of most metal ions are known, predictions on the rate of complex formation of aquometal ions can be made. 

The temperature dependency of enzymatic activity is usually asymmetric. With increasing temperature, the increased thermal movement of the molecules initially leads to a rate acceleration (see p.22). At a certain temperature, the enzyme then becomes unstable, and its activity is lost within a narrow temperature difference as a result of denatu-ration (see p. 74). The optimal temperatures of the enzymes in higher organisms rarely exceed 50 °C, while enzymes from thermophilic bacteria found in hot springs, for instance, may still be active at 100 °C. 

Scheme 10.8 Proton transfer to a hydride ligand via dihydrogen bonds with participation of two proton-donor HX molecules the dimer (HX)2 forms the dihydrogen bond [pathway (3)] the second HX molecule initiates the formation of the solvent-separated or contact ion pair [pathway (1) or (2), respectively].

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