On the Mechanism of Initiation

Once this new family of catalysts was proven to show excellent polymerization activity and versatility (see below) in the polymerization of norbornenes, we immediately set out to understand fhe mechanism by which the polymerization occurs. The mechanism is of importance not only in fhe design of other, and possibly better, catalysts but also in helping to establish fhe microstructure of the polymers formed. 

By contrast, when a sample of catalyst 2 was dissolved in CDCI3 and about 1 equiv of norbornene were added, the resulting reaction was so slow that the insertion reaction could be followed by NMR spectroscopy. After addition of an equivalent of norbornene, the NMR spectrum was taken within 30 min. Along with 

The NMR spectra of the poly(norbornene) produced by catalyst 1 (naked nickel) and 2 (naked palladium) are given in Fig. 4.9 a and b, respectively. Based on the different NMR signatures of these two polymers, their microstructures are obviously different However, the absence of resonances upheld of 28 ppm (see 2,3-dimethylnorbornane model compounds above) in either spectrum proves that subsequent monomer insertions occur on the exo face of norbornene. Therefore, any microstructural differences between the poly(norbornene)s must be attributed to differences in tacticity. 

The polymers were studied in more detail using H- C correlation spectroscopy. The results of these studies are presented in Fig. 4.10 a and 4.10 b for poly(norbornene) made using catalyst 1 and 2, respectively. 

The spectra of both polymers can be readily separated into methine and methylene regions those resonances appearing upheld of 40 ppm in the C NMR spectrum are due to methylene carbons and those downheld are due to methine carbons. Of special interest are the cross-peaks due to the bridging methylene group C7 (between 36 and 39 ppm, circled in the spectra). As expected, the carbons in this region correlate with the two inequivalent protons II , and Hy, attached to the bridge carbon, C7. However, in the nickel-based polymer, there are two distinct types of bridging carbons, while in the palladium polymer there is only one 

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The earlier data on initiators used, and on the mechanism of initiation are comprehensively collected in the Lundberg s review [9]. Unfortunately, the evidence presented till now on the mode of initiation and bond cleavage (0-alkyl or 0-acyl) as a function of initiator and monomer structures ... 

There are two discrete views on the mechanism of initiation. The initiation by nitromethane is claimed to be conducted via the formation of a proton, whereas... 

A survey of current opinion on the mechanism of initial C—C bond formation in MTG has been presented. Proposals for the reactive C intermediate range from carbocationic to radical. However, the various intermediates may be shown to be related to one another as follows ... 

Fundamental research into the nature cS the simplest member of the alkylaluminum/coinitiatcv systems, Al(CH3)3/t-C4H9Cl, led to the discovery of an interesting solvent effect which may have an important bearing on the mode of action of these initiator/coinitiator systems in general and on the mechanism of initiation in particular. It was found that the NMR spectrum of AlfCHjlj, which displays two peaks in... 

The investigations carried out earlier characterise the reactivity of different classes of polymers with NO. However, the mechanisms of free-radical processes proposed on the basis of the results considered are rather formal. As a rule, they take account of changing molecular weights and the composition of final molecular products of the nitration. In connection with this, the study of structures of free radicals forming in primary and intermediate stages of polymer conversions attracts special interest. Such research allows drawing conclusions on the mechanism of initiation of free-radical conversions dependent on the nature of functional groups of macromolecules. As... 

Incremental monomer addition studies, bearing on the mechanism of initiation and termination, were done under a nitrogen atmosphere in bottles. The bottles were fitted with a perforated cap having a butyl gasket which was extracted using a mixiure of ethanol with 32 by weight of toluene. These polymerizations were carried out at an initial catalyst concentration of O.OW weight percent and at 50°C. 

Detailed studies on the mechanisms of initiation and propagation of epoxide polymerization have been conducted using well-defined and stable initiators. Ethylene oxide (EO) was generally chosen as the reference monomer, owing to the living character of its polymerization, whereas the anionic polymerization of most other epoxides, including propylene oxide (POx), is subject to side reactions. 

Catalysis (qv) refers to a process by which a substance (the catalyst) accelerates an otherwise thermodynamically favored but kiaeticahy slow reaction and the catalyst is fully regenerated at the end of each catalytic cycle (1). When photons are also impHcated in the process, photocatalysis is defined without the implication of some special or specific mechanism as the acceleration of the prate of a photoreaction by the presence of a catalyst. The catalyst may accelerate the photoreaction by interaction with a substrate either in its ground state or in its excited state and/or with the primary photoproduct, depending on the mechanism of the photoreaction (2). Therefore, the nondescriptive term photocatalysis is a general label to indicate that light and some substance, the catalyst or the initiator, are necessary entities to influence a reaction (3,4). The process must be shown to be truly catalytic by some acceptable and attainable parameter. Reaction 1, in which the titanium dioxide serves as a catalyst, may be taken as both a photocatalytic oxidation and a photocatalytic dehydrogenation (5). 

The thiophthalimide (CTP) and sulfenamide classes of retarders differ from the organic acid types by thek abiUty to retard scorch (onset of vulcanization) without significantly affecting cure rate or performance properties. Much has been pubUshed on the mechanism of CTP retardation. It functions particularly well with sulfenamide-accelerated diene polymers, typically those used in the the industry. During the initial stages of vulcanization, sulfenamides decompose to form mercaptobenzothiazole (MBT) and an amine. The MBT formed reacts with additional sulfenamide to complete the vulcanization process. If the MBT initially formed is removed as soon as it forms, vulcanization does not occur. It is the role of CTP to remove MBT as it forms. The retardation effect is linear with CTP concentration and allows for excellent control of scorch behavior. 

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). 

In catalytic polymerization the reactivity of the propagation center depends on the catalyst composition. Therefore, the dependence of the molecular structure of the polymer chain mainly on the catalyst composition, and less on the experimental conditions, is characteristic of catalytic polymerization. On the other hand, in polymerization by free-radical or free-ion mechanisms the structure of a polymer is determined by the polymerization conditions (primarily temperature) and does not depend on the type of initiator. 

We heated the substrate of zinc oxide containing 10 cm 2 of silver atoms (in this case there was already no emission after completion of deposition) at 300 C. Such thermal treatment results in formation of microcrystals, rather than evaporation adatoms on the surface of the substrate made of zinc oxide. In paper [34] it was shown that microcrystals with diameter 100 A deposited on the zinc oxide surface are acceptors of electrons, therefore the formation of microcrystals results in increase of resistivity of a sensor substrate above the initial value (prior to silver deposition). In this case the initial value of the resistance of sensor-substrate was 2.1 MOhm, after adsorption of silver atoms it became 700 kOhm, and as a result of heating at 300°C and formation of microcrystals - acceptors of electrons it in increased up to 12 MOhm. If such a substrate is subject to deposition of 3-10 5 cjjj-2 silver again, then emission of silver atoms gets detected. From the change of resistivity of sensor-detector due to deposition of silver atoms one can conclude that in this case the emission of atoms is 4 times as low than in experiment with pure substrate made of zinc oxide, which confirms the supposition made on the mechanism of emission of adatoms. 

Thus, ellipsometry gives direct evidence for a model of the initial stages of polythiophene growth, disproving the conclusions based purely on coulo-metry. In the same paper, Hamnett and Hillman were able to obtain valuable and complementary information not just on the initial stages of the polymerisation but also on the mechanism of the subsequent nucleation and growth. The unique piece of information that the ellipsometer was able to extract, the changes in film thickness (in real time), when combined with coulometric data allowed a wealth of information to be deduced, e.g. with respect to the film composition, and ably showed the power of the technique. 

DPMs offer a viable tool to study the macroscopic behavior of assemblies of particles and originate from MD methods. Initiated in the 1950s by Alder and Wainwright (1957), MD is by now a well-developed method with thousands of papers published in the open literature on just the technical and numerical aspects. A thorough discussion of MD techniques can be found in the book by Allen and Tildesley (1990), where the details of both numerical algorithms and computational tricks are presented. Also, Frenkel and Smit (1996) provide a comprehensive introduction to the recipes of classical MD with emphasis on the physics underlying these methods. Nearly all techniques developed for MD can be directly applied to discrete particles models, except the formulation of particle-particle interactions. Based on the mechanism of particle-particle interaction, a granular system may be modeled either as hard-spheres or as soft-spheres. ... 

An often useful discriminant is initial rate data as a function of total pressure. The equations are simpler because terms for the products are absent. Depending on the results of initial rate analysis, the number of complete equations that may need to be investigated could be narrowed. For instance it may be determined initially if a reactant is adsorbed with or without dissociation. Since the number of possible mechanisms sometimes can be 15 or 20 or more, every bit of preliminary assistance is desirable. Typical initial... 

It is interesting to note that a Cu(II) precatalyst gives an active catalyst in the absence of reducing agent (phenylhydrazine, etc.) contrary to observations made with bis(oxazolines) as ligands and to the accepted mechanism of this reaction. This reaction occurs at temperatures as low as 0°C, and contradicts the conventional belief that slow thermal homolytic cleavage of the oxidant (tert-butyl per-benzoate) is responsible for initiation of the catalytic cycle. Clearly, further work on the mechanism of this transformation is warranted. 

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. 

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