Stereoelective polymerization of β-butyrolactone

Racemic β-butyrolactone was polymerized using chiral initiators obtained from the reaction of organometallic derivatives (ZnEt₂, CdMe₂, AlEt₃) with R(?) 3,3 dimethyl-1,2 butanediol. With the zinc initiator, R(+) enantiomer is preferentially incorporated in the polymer chain with a stereoelectivity ratio r_R equal to 1.6. Crude polymer was fractionated into a crystalline, predominantly isotactic, part and an amorphous heterotactic part, both optically active. Sites of different stereospecificities, present in the initiator, are all active for the stereoelective polymerization. With the cadmium initiator, S(?) enantiomer is preferentially polymerized (r_s = 1.01), extending homosteric-antisteric rules previously established for thiiranes. Aluminium initiator leads to an homosteric process (r_R = 1.1). Chiroptical properties (o.r.d. and c.d.) of polymers prepared with zinc initiator show a predominance of R-configurational units, indicating that ring-opening occurs by O-acyl cleavage with retention of configuration.