Three samples of poly(1,2-cyclohexene oxide) were prepared with ZnEb, (EtZnOMe),, and [[MeOCH,CH(Me)OZnO(Me)CHCH,OMe~ . (EtZnO(Me)CHC~OMe],] ([ Zn-MPh,,), respectively, as initiator. Each polymer sample was treated with butyllithium to form various decomposition products having double bonds or hydroxyl groups as end groups. Two dimeric diols, were obtained from the decomposition products and were found to have isotactic (SS-SS or RR-RR) and syndiotactic (RR-SS) structure, respectively. By comparison of the relative yield of the two dimeric diols from each sample with the relative intensity of the three major signals in their ' H NMR spectra, the latter were assigned to methine protons of isotactic, syndiotactic, and heterotactic triads (from downfield to upfield). It was found that most parts of the main chain of the polymer prepared with [Zn-MPh,, were composed of syndiotactic triads, SS-RR-SS (or RR-SS-RR), while the polymer prepared with ZnEG or (EtZnOMe), contained syndiotactic and heterotactic triads in comparable amounts.
1,2-Epoxycyclohexane (1) was found to behave differently from propylene oxide (PO) in polymerization reactions with organozinc compounds as initiators. A chair-type complex, [Z~I-MP],~, is the only compound that shows high catalytic activity for both polymerization of 1 and PO, following an anionic coordination mechanism. On the other hand, the polymerization of 1 with ZnEt, or (EtZnOMe), as initiator proceeds according to a cationic mechanism. Cationic polymerization of 1 with ZnEt, has two modes of termination reaction resulting in the formation of terminal units containing vinyl ether and ally1 ether moieties. The initiation and propagation mechanism of 1 by [Zn-MP]22 is similar to that of PO, but chain transfer reaction takes place in the polymerization of 1 owing to the low stability of the growing chain end. By using [ZXI-MP],~ as initiator, it was possible to prepare a block copolymer consisting of an isotactic sequence of monomeric units of PO and a syndiotactic sequence of monomeric units of 1. 0 1990, HUthig & Wepf Verlag, Basel CCC 0025-1 16X/90/$03.00 P -
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.
A ring-opening polymerization of racemic iert-butylethylene oxide (t-BEO) was carried out with a well-defined organozinc complex, [CH,0CH2CH(CH3)OZnOCH(CH3)CH20CH3],* [C,H5ZnOCH(CH3)CH2OCH3], ([Zn-MP],,,), as initiator. The I3C NMR analysis showed that the main chain of poly(t-BEO) is isotactic although the initiation reaction between CBEO and [Zn-MP],,, is much less enantioselective compared with the very high selectivity in the propagation stage. In the R,S-copolymerization reaction of t-BE0 (R/S = 64/36), no change in the optical purity of the residual monomer was observed at any extent of conversion. It was concluded from these results that the enantioselection in the propagation stage at any particular site is perfectly governed by the chirality of the first t-BE0 molecule that entered and reacted at the site of [Zn-MP],,,. The mechanism of the stereocontrol processes for the [Zn-MP],,, initiated polymerization of t-BE0 is discussed in comparison with that for the polymerization of propylene oxide.
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