Magnesium dialkyls, polymeric

The half-sandwich complex is much less prone to polymerize ethylene when combined with magnesium dialkyl. We observed that Cp Nd(BH4)2(THF)2/10 BEM is quite active but 20% vinyl end groups are detected by H NMR, highlighting a substantial occurrence of /3-H abstraction [26 unpublished results]. 

The stereoregularity of butadiene based polymers prepared in cyclohexane with Ba-Mg-Al catalysts depends on polymerization temperature and catalyst concentration. Trans-1,4 content increases nonlinearly with a decrease in polymerization temperature over the range of 80° to 30°C (Figure 11) and/or a decrease in the initial molar ratio of butadiene to dialkyl-magnesium from 3400 to 400 (Figure 12). For polybutadienes prepared with relatively large amounts of catalyst at 30°C, the trans-1,4 content approaches a limiting value of about 907.. 

As discussed in Sects. 2.1 and 2.2.8 control of molar mass is an important aspect in the large-scale polymerization of dienes. In Nd-catalyzed polymerizations the control of molar mass is unique amongst Ziegler/Natta catalyst systems as standard molar mass control agents such as hydrogen, 1,2-butadiene and cyclooctadiene which are well established for Ni- and Co-systems do not work with Nd catalysts [82,206,207]. The only known additives which allow for the regulation of molar mass without catalyst deactivation are aluminum alkyls, magnesium alkyls, and dialkyl zinc. 

The compound [Mn(CH2SiM63)2], for example, is polymeric in the crystal, -with a structure (75) like those of dialkyls of beryllium and magnesium (MR2) (M = Be or Mg R = Me or Et), Each metal atom, tetrahedrally coordinated. 

Langer (13) has also disclosed the use of alkyllithium and dialkyl-magnesium tertiary diamine complexes as catalysts for copolymerization of ethylene and other monomers such as butadiene, styrene, and acrylonitrile to form block polymers. Examples are given in which polybuta-dienyllithium initiates a polyethylene block, as well as vice-versa. Random copolymers of these two were also prepared, and other investigators have used not only tertiary diamines but hexamethylphosphoramide (14) and tetramethylurea (15) as nitrogenous base cocatalysts in such polymerizations. Antkowiak and co-workers (11) showed the similarity of action of diglyme and TMEDA in copolymerizations of styrene and... 

To this end, we have recently found that low-coordinate magnesium alkyl complexes can be stabilized by bulky A,A -diisopropylphenyl P-diketiminate ligands, and that these compounds initiate a rapid polymerization of MMA. The alkyl complexes are prepared either by reaction of the diketimine with dialkyl precursors such as MgMe or by treatment of RMgX (R = Me, i-Pr, t-Bu, Ph X = Cl, Br) with the lithium salt of the ligand (Scheme 23.15). ... 

When combined with large excesses of dialkyl magnesium, borohydride rare earth derivatives display a very versatile character, as they afford the chain transfer polymerization of various monomers. By comparison with the stoichiometric addition of MgR2 to borohydride precatalysts, significant changes are noticed. 

Scheme 27.4 Coordinative chain transfer polymerization (CCTP) of myrcene with neodymium borohydride/dialkyl magnesium catalysts.

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