Magnesium and Zinc Initiators

Early reports document the use of Zn(OR)2 and EtZn(OR), (R = CH2Br, CH2CH2CH = CH2, CH2CH2NEt2)817 for well-controlled polymerizations of CL. Subsequently, the use of zinc(II) lactate and related carboxylates was described.818,819 For simple magnesium derivatives, low activities and epimerization are generally observed.820 In more recent years, research in this field has been directed towards the development of single-site initiators. 

A more active zinc catalyst has been developed containing a /3-diketiminate ancillary ligand.823 Complex (272) converts 200 equivalents rac-LA. in less than 20 min at room temperature to highly heterotactic PLA (the probability of a racemic junction between monomer units, PR = 0.90 at 25 °C and 0.94 at 0°C) of narrow polydispersity (Mn = 37,900, Mn calc = 28,800, Mw/Mn= 1.10). Kinetic studies reveal that the zinc initiators display a fractional rate law order (1.56 0.06). In accord with the observation that kR/ss kR/RR (or ks/RR ks/ss), the polymerization of L-LA proceeds significantly slower than rac-LA (kapp L-LA = 0.031 min-1 kapp rac-LA = 0.22 min-1).824 

The analogous magnesium alkoxide dimer, (273), is more active than (272), although polydispersities are generally higher (ca.1.5), a result of transesterification side reactions.824 An increased level of control may be acquired by the addition of 1 equivalent of PrOH (Mw/Mn 

The magnesium initiators (273) and (274) do not display the same stereocontrol as their zinc analogs over the ROP of rac-LA in either CH2C12 or benzene. However, in THF highly heterotactic PLA is produced.826 In non-coordinating solvents the Mg and Zn initiators are believed to adopt dimeric and monomeric resting states respectively, whereas both exist as monomeric forms in THF and hence give rise to similar stereoselectivity. 

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