Containing Nontransition Metals

Aluminum porphyrins are versatile initiators which are applicable to controlled ring-opening polymerization of various heterocyclic monomers (Table 1) such as epoxides (11), oxetanes (12) lactones with four-, six-, and seven-membered rings (13-15), lactide (16), six-membered cyclic carbonates (17) and cyclic siloxanes (19).- They are also excellent initiators for the controlled addition polymerization of unsaturated monomers such as acrylates (20), methacrylates (21)-- and methacrylonitrile (22).- -  

---

Compounds of nontransition metals form solid solutions in relatively narrow ranges of concentration and, consequently they usually contain few ions of "abnormal" valence. However, the number of such ions is still sufficient to alter the thermoelectric properties of such compounds. 

In the following discussion it will be assumed that the polymerization takes place at the transition metal-carbon bond. Both monometallic and bimetallic mechanisms have been proposed for the propagation reaction. A mechanism is defined as monometallic when only the transition metal is concerned in the propagation reaction, whereas in bimetallic mechanisms both transition and nontransition metals are involved. Thus, in the monometallic mechanism, it is irrelevant whether the complex contains one or two metal centers. 

Nickel, containing 0.6 rf-electron vacancy per atom (as measured magnetically), when alloyed with copper, a nontransition metal containing no rf-electron vacancies, confers passivity on the alloy above approximately 30-40 at.% Ni. Initiation of passivity beginning at this composition is indicated by corrosion rates in sodium chloride solution (Figs. 6.12 and 6.13), by corrosion pitting behavior in seawater (Fig. 6.13), and, more quantitatively, by measured values of /critical and passive (Fig. 6.14), [41-43] or by decay (Flade) potentials (Fig. 6.15) [44] in IN H2SO4. 

Monomers with electron-rich double bonds produce one-to-one copolymers with monomers having electron-poor double bonds in reaction systems that also contain certain Lewis acids. These latter are halides or alkyl halides of nontransition metal elements, including AlCb, ZnCh, SnCL, BF3, AI(CH2CH3)Cl2, alkyl boron halides, and other compounds. The acceptor monomer generally has a cyano or carbonyl group conjugated to a vinyl double bond. Examples are acrylic and methacrylic acids and their esters, acrylonitrile, vinyl ketones, maleic anydride, fumaric esters, vinylidene cyanide, sulfur dioxide, and carbon monoxide. The variety of donor molecules is large and includes various olefins, styrene, isoprene, vinyl halides and esters, vinylidene halides, and allyl monomers [30]. 

A natural objection to such an approach is the problem of whether conduction electrons in the given compounds can be considered to be quasi-free. As is known, the conductivity of refractory compounds is comparable to that of the corresponding transition metals and is considerably less than the conductivity of typical nontransition metals. In the case of high densities of states at the Fermi level this indicates a partial localisation of electrons, which appears when the crystal contains impurities, vacancies and other types of translational symmetry disturbances. There are indications, however, that partial localization of conduction electrons does not lead to qualitatively different relationships between the... 

In the polymers prepared with catalytic systems containing organo-metallic compounds of nontransition metals e.g. AIR3, where R = alkyl, aryl or aralkyl radical) it has been established that y=R, at least for some macromolecules (52). In this case, initiation clearly consists of the insertion of the first propylene molecule into an M— R bond. In catalysts prepared in the absence of organometallic compounds an M—R bond is probably formed at first (it has not been ascertained throu which reactions, although many examples of reactions forming metal-carbon bonds like the one proposed here have been reported in the literature) (53— 57). 

Transition and nontransition elements c-bonded to a carbon-containing residue (an alkyl or an aryl group), which constitute the class of hydrocarbyl metal complexes, may undergo insertion reactions. The term insertion should be clarified, since without further qualification, it simply refers to the stoichiometric result of the reaction, no mechanistic details (see 11.3.2.1.1) being involved. 

Colors of ceramics are due to compounds containing the so-called transition (metallic) elements such as iron and copper, as seen above. Some of the colors of such compounds might be familiar to you. Iron oxide can be brown (rust) or red (hematite ore), and maybe you have seen a nice blue copper sulfate crystal. Compounds of other elements (nontransition elements) are rarely colored. What special is there about compounds of transition elements ... 

As we go from the second period to the third and then the fourth nontransition period, the s-like valence energies drop compared to p-like ones. This process (which one may call dehybridization, or loss of directed bonding character, or metallization) is a gradual one until we reach the fifth period, which contains Cs, Pb, and Bi. Then, quite sharply, the 6s electrons are much more strongly bound than the 6p ones. (This accounts, for example, for the fact that Pb tends to be divalent rather than quadrivalent.) The quantum mechanical explanation for this involves relativistic energy shifts of the s electrons, which are of order 1 eV even for the 6s electrons. 

---