Mechanism of stereoselective polymerization of propylene oxide with [{MeOCH₂CH(Me)OZnOCH(Me)CH₂OMe}₂ · {EtZnOCH(Me)CH₂OMe}₂] as initiator

Abstract

A series of studies on molecular level elucidation of stereospecific polymerizations of propylene oxide (PO) were carried out with a well-defined organozinc complex of 'chair' type structure, [[CH,0CH,CH(Me)OZnOCH(Me)CH,0CH3]2 . [EtZnOCH(Me)CH,OCH,],]([Zn-MPb,,), which contains two four-coordinated Zn and two six-coordinated Zn atoms, the latter being surrounded, respectively, by three types (endo-coordinated, exo-coordinated and non-coordinated) of D-and L-methoxypropoxy groups. An experiment using a partly deuterated [Zn-MPb,, and PO-a-d showed the initiation reaction to be caused by an attack of the noncoordinated methoxypropoxy (MP) group on a monomer molecule. Another experiment using (L)-PO as monomer showed that the (D)-MP group was preferentially transferred to @.)-monomer to break its 0-CH, bond. A molecular model study of [Zn-MPL,, complex suggested that there may be chiral holes which can accomodate only one sense of enantiomeric PO molecule. The chiral structure of the hole around the six-coordinated zinc atom is presumably determined by the chair type Zn-0 framework together with the coordinated MP groups.

Enantioselective Epoxide Polymerization Using a Bimetallic Cobalt Catalyst

“…Regarding homogeneous catalysts, aluminum porphyrin catalysts have been reported to produce partially isotactic polypropylene oxide (PPO) with an m -dyad content of 69% . A zinc cluster used by Tsuruta enabled the isoselective polymerization of rac -PO with a moderate m -dyad content of 79% and TOF of 0.9 h −1 . Until recently, these were the most successful catalysts for synthesizing isotactic PPO from rac -PO.…”

Stereoregular Polymerization of (R,S)-Propylene Oxide by an Alumoxane−Propylene Oxide Complex

“…The primary focus of recent investigation has involved two general aspects: (1) the development of alternative catalytic systems aimed at the synthesis of a more highly stereoregular and crystalline poly(propylene oxide) (PPO) from the racemic monomer [2][3][4] and (2) the development of an understanding of the mechanisms of the steroregular polymerization as catalyzed by different metal species. [5][6][7][8][9] Racemic PO can be polymerized in a stereoregular manner by a variety of transition and main group metal catalysts, including: FeCl 3 -PO, Al( i Bu) 3 /H 2 O, 10 AlEt 3 / H 2 O, 11 ZnR 2 /H 2 O, 12 and (tetraphenylporphinato)aluminum chloride [AlCl(TPP)]. 13 However, all of these catalysts suffer from the relatively low stereoregularities of the polymers obtained.…”

“…After the discovery in 1955 of the iron(III) chloride catalyst for the stereoregular polymerization of ( R , S )-propylene oxide (PO), research on the preparation of this type of polymer from PO has continued sporadically for several decades. The primary focus of recent investigation has involved two general aspects: (1) the development of alternative catalytic systems aimed at the synthesis of a more highly stereoregular and crystalline poly(propylene oxide) (PPO) from the racemic monomer − and (2) the development of an understanding of the mechanisms of the steroregular polymerization as catalyzed by different metal species. − …”

Designing Sequence Selectivity into a Ring-Opening Metathesis Polymerization Catalyst

“…At each turnover, there is a stereochemical inversion at the metal center, which leads to enantiomorphic site control to form either isotactic or syndiotactic polypropylene, depending on the symmetry of the ligands on the metallocene. A similar mechanism, also identified as enantiomorphic site control, occurs in the stereoselective ring-opening polymerization of propylene oxide by certain multinuclear zinc alkoxide complexes, as reported by Tsuruta . In analogy, therefore, one can designate the chemoselective, alternating ring-opening metathesis copolymerization of two cycloolefins with the tick-tock catalyst to occur with diastereomeric site control.…”