Metals, polymer chains

V. STEPWISE PREPARATION OF LINEAR n-CONJUGATED BIS(TERPYRIDINE)METAL POLYMER CHAINS ON GOLD SURFACE... 

One of the interesting things about the redox polymers is their use in the creation of the molecular electronic devices.3-5 Redox polymer films on electrodes have been fabricated using chemical modification, electrochemical polymerization, polymer coating, and so on.88 Recently, stepwise complexation methods have been employed to fabricate multiple complex layers.89,90,91 In this section, the stepwise preparation of bis(tpy)metal polymer chains by combining terpyridine (tpy) ligand self-assembled monolayer (SAM) formation and metal-tpy coordination reactions is described as an example. This method realized the formation of a desired number of polymer units and a desired sequence of Co-Fe heterometal structures in the polymer chain.92... 

According to the Cossee mechanism, the two key steps in Ziegler—Natta polymerizations are monomer coordination and migratory insertion into the metal-polymer chain bond (Scheme 10). In analogy to heterogeneous Ziegler—Natta catalysis, a first-order reaction rate with respect to monomer concentration is generally assumed also for metallocene-based catalysts, that is. 

To quench a polymerization reaction catalyzed by a metathesis catalyst, addition of a methanol/toluene mixture e.g., 1 4 v/v) is elfective. Pouring the polymerization mixture into a large volume of methanol is useful to precipitate the formed polymer and to remove oligomers and catalyst residues that are soluble. In the case of Rh catalysts, acetic acid is added to quench the polymerization, giving rise to the cleavage of the metal-polymer chain bond. 

Lewis acids, such as the haUde salts of the alkaline-earth metals, Cu(I), Cu(II), 2inc, Fe(III), aluminum, etc, are effective catalysts for this reaction (63). The ammonolysis of polyamides obtained from post-consumer waste has been used to cleave the polymer chain as the first step in a recycle process in which mixtures of nylon-6,6 and nylon-6 can be reconverted to diamine (64). The advantage of this approach Hes in the fact that both the adipamide [628-94-4] and 6-aminohexanoamide can be converted to hexarnethylenediarnine via their respective nitriles in a conventional two-step process in the presence of the diamine formed in the original ammonolysis reaction, thus avoiding a difficult and cosdy separation process. In addition, the mixture of nylon-6,6 and nylon-6 appears to react faster than does either polyamide alone. 

These appHcations are mosdy examples of homogeneous catalysis. Coordination catalysts that are attached to polymers via phosphine, siloxy, or other side chains have also shown promise. The catalytic specificity is often modified by such immobilization. Metal enzymes are, from this point of view, anchored coordination catalysts immobilized by the protein chains. Even multistep syntheses are possible using alternating catalysts along polymer chains. Other polynuclear coordination species, such as the homopoly and heteropoly ions, also have appHcations in reaction catalysis. 

Carboxylated polymers such as AF use similar but not identical compounds. The higher strength, especially hot bond strength, is due to the interaction of the carboxyl groups on the polymer chain with the metal oxides. The crystallization rate of AF is low and does not contribute to bond strength. Manufacture of adhesive compounds from AF is more demanding than manufacture of those from AD. 

Creep of polymers is a major design problem. The glass temperature Tq, for a polymer, is a criterion of creep-resistance, in much the way that is for a metal or a ceramic. For most polymers, is close to room temperature. Well below Tq, the polymer is a glass (often containing crystalline regions - Chapter 5) and is a brittle, elastic solid -rubber, cooled in liquid nitrogen, is an example. Above Tq the Van der Waals bonds within the polymer melt, and it becomes a rubber (if the polymer chains are cross-linked) or a viscous liquid (if they are not). Thermoplastics, which can be moulded when hot, are a simple example well below Tq they are elastic well above, they are viscous liquids, and flow like treacle. 

Today the term anionic polymerisation is used to embrace a variety of mechanisms initiated by anionic catalysts and it is now common to use it for all polymerisations initiated by organometallic compounds (other than those that also involve transition metal compounds). Anionic polymerisation does not necessarily imply the presence of a free anion on the growing polymer chain. 

The final variable to be mentioned here is the presence of impurities. These may be metallic fragments residual from Ziegler-type processes or they can be trace materials incorporated into the polymer chain. Such impurities as catalyst fragments and carbonyl groups incorporated into the chain can have a serious adverse influence on the power factor of the polymer, whilst in other instances impurities can have an effect on aging behaviour. 

Another method for slowing oxidation of rubber adhesives is to add a compound which destroys the hydroperoxides formed in step 3, before they can decompose into radicals and start the degradation of new polymer chains. These materials are called hydroperoxide decomposers, preventive antioxidants or secondary antioxidants. Phosphites (phosphite esters, organophosphite chelators, dibasic lead phosphite) and sulphides (i.e. thiopropionate esters, metal dithiolates) are typical secondary antioxidants. Phosphite esters decompose hydroperoxides to yield phosphates and alcohols. Sulphur compounds, however, decompose hydroperoxides catalytically. 

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