Polyethylene transition metal salts

Another method for photodegrading polyethylene is to include metal salts, which catalyze photooxidation reactions, in the solid polymer. The compounds most generally used for that purpose are divalent transition-metal salts of higher aliphatic acids, such as stearic acid or dithiocarbonates or acetoacetic acid. The photochemical reaction is an oxidation-reduction reaction that forms free radicals capable of reacting with polyethylene, RH, to initiate an autooxidation chain reaction, as follows ... 

Coordination polymerization n. Polymerization of vinyl monomers using a catalyst comprising a transition metal salt and a metal alkyl Ziegler-Natta catalysts for polymerization of ethylene to produce polyethylene. 

Ziegler-Natta catalysts consist of a complex of a base metal alkyl or halide with a transition metal salt. Base metals from groups I-III of the periodic table may be used in combination with transition metals from groups IV-VIII. Naturally, some of these combinations are preferred for one type of polymerization or another. A classic example of a Ziegler-Natta catalyst suitable for the polymerization of ethylene to high density polyethylene is the complex of triethyl aluminum (AlEts) with titanium tetrachloride (TiCU). The proposed reaction mechanism is shown in Figure 3. 

In 1954, Ziegler and coworkers observed that the course of the reaction of ethene with trialkylalanes was drastically altered by the presence of traces of nickel salts [25]. Instead of low molecular weight polyethylene, the only product was 1-butene. Obviously, the transition metal strongly supports the displacement reaction of the alkyl group bonded to the aluminum by ethylene, a reaction which can be formally described as transfer of a hydridoalane. 

In 1954, Ziegler and coworkers [11,12] reported that traces of nickel salts dramatically alter the course of the growth reaction of ethylene with trialkylalanes, the Aufbau process. Instead of the low molecular weight polyethylene which was expected, the only product was butene. This observation culminated in Ziegler s discovery of transition metals that were highly effective in polymerizing ethylene, an accomplishment for which he later shared the Nobel Prize. It also opened the door to transition metal catalyzed hydroalumination reactions. In 1968, Eisch and Foxton showed that addition of nickel(II) salts increased the rate of the hydroalumination of alkynes by approximately 100-fold [13]. The active catalyst was believed to be a nickel(O) species. 

Transition metal cations can be made organically soluble by complexation with a crown ether, a polyethylene glycol) or its dimethyl ether (an open crown) or tris(3,6-dioxaheptyl)amine (TDA-1, an open cryptand, 1). TDA-1 is very hydrophilic and is most useful for the solubilization of solid salts. On the other hand, it also forms complexes with some metal carbonyls. Alternatively, a very lipophilic anion (for instance stearate) can make a salt organic. Finally, some other special ligand (e.g., a bipyridine-N,N -dioxide derivative) can be used. In all these cases positively charged species are brought into the organic phase for reaction. 

Recently, our group has found that functionalized ethylene oligomers can be used as ligands to prepare recoverable, reusable homogeneous catalysts from both transition metals like rhodium and nickel and from lanthanide salts. In complimentary studies of polyethylene functionalization, we have found that ethylene oligomers of My > 1200 are quantitatively entrapped in polyethylene precipitates when a solution of polyethylene and a functionalized ethylene oligomer is cooled to room temperature or when polyethylene and the functionalized oligomer are co-precipitated by addition of a second solvent such as methanol Suitably... 

OBPs based on a full-aliphatic-carbon backbone such as polyethylene (PE) and polypropylene (PP), when doped with relatively small amounts of pro-oxidant/pro-degradant additives (0.5-1 % by weight of fatty acid salts of transition metals such as Fe, Mn, or Co or their combination), become susceptible to an oxo-biodegradation process occurring at the end of their predetermined service life. 

The general consensus on the mechanistic details of transition metal-catalyzed polyethylene formation is that the active site comprises a metal with an alkyl group as active chain end and a free coordination site, with the metal incorporated in a ligand or in a salt crystal [32]. Ethylene is inserted in a syn fashion into the metal-carbon bond. Iron bis(iminoaryl)pyridyl dichloride (BI P FeCl2, where R denotes the ortho substituents on the aryl entity Fig. 1) in combination with MAO or (tri)alkyl aluminum compounds (AIR3) yields active ethylene polymerization systems [23]. Both the free coordination site and the alkyl group of the iron center thus originate from the interaction with the aluminum compounds. 

---