Aluminium polymeric hydrolysis species

As indicated, the solubihty of these aluminium oxide and hydroxide phases is important in the production of aluminium at elevated temperature and high pH, where the formation of the hydrolysis species, Al(OH) ", predominates and the behaviour of NaAKOH) is particularly important. The lesser hydrolysed monomeric species of aluminium, AlOH to Al(OH)3(aq), are also known to form. In more concentrated aluminium solutions, a number of polymeric aluminium hydrolysis species have been identified, including Al2(OH)2, Al3(OH)4 and Alj3(OH)32 (or, more correctly, Alj304(0H)24 ). The reaction describing the formation of these species is given by Eq. (2.5) (M = Al ). 

The second termination reaction is alkyl chain end transfer from the active species to aluminium [155]. This termination becomes major one at lower temperatures in the catalyst systems activated by MAO. XH and 13CNMR analysis of the polymer obtained by the cyclopolymerization of 1,5-hexadiene, catalyzed by Cp ZrCl2/MAO, afforded signals due to methylenecyclopentane, cyclopentane, and methylcyclopentane end groups upon acidic hydrolysis, indicating that chain transfer occurs both by /Miydrogen elimination and chain transfer to aluminium in the ratio of 2 8, and the latter process is predominant when the polymerization is carried out at — 25°C [156]. The values of rate constants for Cp2ZrCl2/MAO at 70°C are reported to be kp = 168-1670 (Ms) 1, kfr = 0.021 - 0.81 s 1, and kfr = 0.28 s-1 [155]. 

A key to the high polymerization activity of metallocenes are the cocatalysts. Methylaluminoxane (MAO) is mostly used and is synthesized by controlled hydrolysis of trimethyl aluminium [30]. Other bulky anionic complexes which show a weak coordination, such as borates, play an increasing role too. One function of MAO is the alkylation of halogenated metallocene complexes. In the first step, the monomethyl compound is formed within seconds even at — 60 °C [31]. Excess MAO leads to the dialkylated species as NMR measurements show. In order for the active site of form, it is atleast necessary that one alkyl group is bonded to the metallocene [32],... 

The exact nature of the polymeric spedes in solutions of aluminium salts at the pH conditions for charge reversal is not known with certainty. It is possible that several species coexist, depending on their staUlity constants, and that these also change with time with the ultimate product of hydrolysis being aluminum hydroxide particles. A number of species have been proposed in the literature and Matijevic et a . (1964) have provided evidence for the existence of AIg(OH)fo from coagulation studies. The higher valence of this type of species again reduces the concentration of ions... 

The reactions are reversible. The relative concentrations of particular species depend on the pH of the medium. In these reactions, polymerization processes also occur. Polynuclear complex compounds are formed they are also called hydroxo polymers with a certain range of existence. Depending on the pH of the medium they are charged either positively or negatively. During the hydrolysis of aluminium, polynuclear complexes such as... 

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