Formation of the Initiating Species

In a few cases, the actual polymerization starter is not identical with the added initiator. In anionic polymerizations with strong bases such as t-C H OK in dimethyl sulfoxide, the base first reacts with the sulfoxide and forms the DMSO anion as the actual polymerization-starting species  

The subsequent reactions of the tetrahydrofuran cation are not known in detail. However, a protonic acid is produced, and this is the actual polymerization-starting species. 

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The rate of formation of the initiating species and, accordingly, the molecular weight distribution can be controlled by variation of the incident light intensity. 

It is in many ways unfortunate that the study of cationic polymerization has, from its very start, been so intimately linked with the very complicated and ill-understood chemistry of the metal halides. This connection is largely fortuitous and there is the promise of much progress in this field when these two problems can be attacked independently. On the one hand, we need to know much more about the complex acids and esters which are formed when water, alcohols, carboxylic acids, and alkyl halides react with metal halides on the other hand, a study of olefin polymerizations catalysed by simple acids such as HBr [14], HC104 [25], and H2S04 [26] should be rewarding, because they would presumably be unobscured by the complications and uncertainties accompanying the formation of the initiating species when this involves a metal halide. 

Lipid peroxidation may beinitiated by any primary free radical which has sufficient reactivity to extract a hydrogen atom (Fig. 2.10) from a reactive methylene group of an unsaturated fatty acid. For example, species such as hydroxyl radicals OH, alkoxyl radicals RO peroxyl radicals ROO and alkyl radicals R may be involved. The formation of the initiating species is accompanied by bond rearrangement that results in stabilization by diene conjugate formation. The lipid radical then takes up oxygen to form the peroxyl radical. Peroxyl radicals can... 

This reaction seems to be specific for monomers containing amide groups (acrylamide or methacrylamide), but once these monomers are present in the electrolytic medium, other monomers, e. g., acrylonitrile, can be polymerized. The authors attribute the polymerization initiation to the electrogenerated metal ions only, but it is possible that even the perchlorate ion plays a role in the formation of the initiating species. The polyacrylamide thus obtained has an electrical conductivity 3 to 4 times higher than that of polymers obtained by the usual methods. This is due to the presence of metallic cations coordinatively bound in the polymer bulk. The presence of these cations increases the thermal stability of the polymer by 20—40%. 

There is also some indirect evidence for the interaction of cyclohexene with catalyst systems. First, the presence of cyclohexene assists the formation of the initiating species in the WCl6/Ph4Sn-catalyzed ROMP of 28 (Stonich 1991b). Second, the presence of cyclohexene increases by 30% the rate of ROMP of cycloocta-1,5-diene catalyzed by WCl6/Me4Sn at 25°C, without itself being consumed (Patton, P.A. 1987). However, there is no NMR spectroscopic evidence... 

Hydroxides. The hydrolysis of uranium has been recendy reviewed (154,165,166), yet as noted in these compilations, studies are ongoing to continue identifying all of the numerous solution species and soHd phases. The very hard uranium(IV) ion hydrolyzes even in fairly strong acid (- 0.1 Af) and the hydrolysis is compHcated by the precipitation of insoluble hydroxides or oxides. There is reasonably good experimental evidence for the formation of the initial hydrolysis product, U(OH) " however, there is no direct evidence for other hydrolysis products such as U(OH) " 2> U(OH)" 2> U(OH)4 (or UO2 2H20). There are substantial amounts of data, particulady from solubiUty experiments, which are consistent with the neutral species U(OH)4 (154,167). It is unknown whether this species is monomeric or polymeric. A new study under reducing conditions in NaCl solution confirms its importance and reports that it is monomeric (168). 8olubihty studies indicate that the anionic species U(OH) , if it exists, is only of minor importance (169). There is limited evidence for polymeric species such as Ug(OH) " 25 (1 4). 

Usually, a rapid binding step of the inhibitor I to the enzyme E leads to the formation of the initial noncovalent enzyme-inhibitor complex E-I. This is usually followed by a rate determining catalytic step, leading to the formation of a highly reactive species [E—I ]. This species can either undergo reaction with an active site amino acid residue of the enzyme to form the covalent enzyme-inhibitor adduct E—I", or be released into the medium to form product P and free active enzyme E. 

However, as can also be seen in Fig. 11, primary and secondary amines do not perform very effectively as primers, compared to tertiary amines, even though they also contain long alkyl chains. It has been demonstrated that, instead of directly initiating ECA polymerization, primary and secondary amines first form aminocyanopropionate esters, 12, because proton transfer occurs after formation of the initial zwitterionic species, as shown in Eq. 7 [8,9]. 

A major complication in applying radiation chemical techniques to ion-molecule reaction studies is the formation of nonionic initial species by high energy radiation. Another difficulty arises from the neutralization of ions, which may also result in the formation of free radicals and stable products. The chemical effects arising from the formation of ions and their reactions with molecules are therefore superimposed on those of the neutral species resulting from excitation and neutralization. To derive information of ion-molecule reactions, it is necessary to identify unequivocally products typical of such reactions. Progress beyond a speculative rationalization of results is possible only when concrete evidence that ionic species participate in the mechanism of product formation can be presented. This evidence is the first subject of this discussion. 

As is the case for cationic polymerisation, anionic polymerisation can terminate by only one mechanism, that is by proton transfer to give a terminally unsaturated polymer. However, proton transfer to initiator is rare - in the example just quoted, it would involve the formation of the unstable species NaH containing hydride ions. Instead proton transfer has to occur to some kind of impurity which is capable for forming a more stable product. This leads to the interesting situation that where that monomer has been rigorously purified, termination cannot occur. Instead reaction continues until all of the monomer has been consumed but leaves the anionic centre intact. Addition of extra monomer causes further polymerisation to take place. The potentially reactive materials that result from anionic initiation are known as living polymers. 

Thermal initiation and ordinary bimolecular termination also occur during polymerization in addition to initiation by the dissociation of the adduct or the active polymer chain-end dissociation and reversible temination (formation of the dormant species). Therefore, the degree of the control of the molecular weight and the molecular weight distribution is determined by the ratio of the polymer chains produced under control and uncontrol. If the contribution of the thermal initiation and bimolecular termination is very small, the molecular weight distribution is close to the Poisson distribution, i.e., Mw/Mn=1 + 1/Pn, where Pn is the degree of polymerization. It was shown that when the number of... 

These workers examined the mechanism of this reaction in detail (258). Burst kinetics were observed suggestive of the formation of an initial species from the catalyst precursor with subsequent slow turnover. The reaction was found to be pH sensitive, with a break point at pH 7.4, indicating a change in mechanism under these conditions. This pH corresponds to the expected value for secondary Cu(II) alkoxides. Based on this evidence, a formulated mechanism was advanced for this reaction, illustrated in Scheme 29. 

In this chapter we will discuss some aspects of the carbonylation catalysis with the use of palladium catalysts. We will focus on the formation of polyketones consisting of alternating molecules of alkenes and carbon monoxide on the one hand, and esters that may form under the same conditions with the use of similar catalysts from alkenes, CO, and alcohols, on the other hand. As the potential production of polyketone and methyl propanoate obtained from ethene/CO have received a lot of industrial attention we will concentrate on these two products (for a recent monograph on this chemistry see reference [1]). The elementary reactions involved are the same formation of an initiating species, insertion reactions of CO and ethene, and a termination reaction. Multiple alternating (1 1) insertions will lead to polymers or oligomers whereas a stoichiometry of 1 1 1 for CO, ethene, and alcohol leads to an ester. 

When carbon monoxide is bubbled through a methanol solution of (dppp)Pd(triflate)2 a carbomethoxy-palladium species is formed, which can undergo insertion of alkenes and hence this is a feasible alternative initiation route to chain-growth polymerisation (Figure 12.4) [13], To ensure a clean formation of the carbomethoxy species, however, exclusion of water is a prerequisite. If during the preparation water was present the formation of a palladium hydride complex (dppp)PdFT was observed (reaction (1), Figure 12.2). 

These observations suggest a reaction scheme for bismuth molybdate catalysts where the allylic species is formed initially at a bismuth center and then reacts further at a molybdenum site to produce acrolein. Thus, once the allylic complex is formed, the MoO polyhedra are highly active and selective for acrolein formation. This hypothesis was tested by investigating the oxidation of bromoallyl (C3HjsBr) over molybdenum oxide 116). Since the C—Br bond in bromoallyl is much weaker than the C—H bond in propylene, the ease of formation of the allylic species should be significantly enhanced with bromoallyl compared with propylene. If the initial propylene activation occurs on bismuth, then the reaction of bromoallyl over molybdenum oxide should approach the activity and selectivity of propylene over bismuth molybdate. This was the observed result, and the authors concluded that the bismuth site was responsible for the formation of the allylic intermediate. 

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