Epoxides coordination polymerization

High-molecular-weight polymers of polyethylene oxide of MW > 100,000 are prepared by a coordination polymerization with alkali earth metal compounds. These catalysts activate the epoxide ring by forming complexes with the oxygen in the oxide with the alkali earth metal ... 

FUK 87] Fukuchi Y., Takahashi T., Noguchi H. et al, Photoinitiated anionic coordination polymerization of epoxides, a novel polymerization process . Macromolecules, o. 20, pp. 2316-2317, 1987. 

The first polymerization of an oxetane, trimethylene oxide, by coordination polymerization has been described. Copolymers of TMO with epoxides were readily made. Such copolymers provide an interesting family of new elastomers. One such elastomer, the im-AGE copolymer, was investigated in some detail and found to have a desirable combination of properties. Such TM) elastomers. 

The first coordination polymerization of an oxetane, trimethylene oxide (TMO), occurs readily, much like propylene oxide (PO), with the Et3Al-H20-acetyl acetone catalyst at 65 C., to give high molecular weight polymer (n jjj up to 12). TMO copolymerizes with epoxides with this coordination catalyst. For example, TMO copolymerized well with allyl glycidyl ether (AGE) which was about seven times more reactive than TMO, in contrast to the PO-AGE copolymerization in which both monomers are of equal reactivity. 

In this chapter, the anionic and related nudeophiUc polymerizations of epoxides are reviewed. The elementary mecharrisms involved in the presence of different initiators and catalysts and the main sjmthetic strategies developed for the preparation of epoxide homopolymers and copolymers are described. In the second section, the anionic polymerization of epoxides involving alkali metal derivatives is described. The use of orgarric derivatives as counterions or catalysts is presented in the third section. The fourth section is devoted to epoxide-coordinated polymerization. Finally, in the last sertion, monomer-activated epoxide polymerization is described. The cationic polymerization of epoxides is described in another chapter. 

The first coordination epoxide polymerization catalytic system was reported by Pmitt and Baggett in 1955. It was based on iron tricbloride. Since that time, metal-based catalysts bave been widely exploited for epoxide polymerization. Tbe most studied systems are those based on zinc or aluminum derivatives. A first group consists of diethylzinc or trialkylaluminum associated to a cocatalyst, wbicb is generally water or an organic compound (alcohol, amine, and other compoimds) that reacts with the alkyl metal to form in situ new metal derivatives as the true catalytic system exploited (see Table 6). For a detailed review on the coordination polymerization of epoxides, see Kman. ... 

Table 6 Main catalytic systems used for the coordination polymerization of epoxides ...

Isotactidty of poly(POx) chains corresponding to the crystalline fraction is explained by steric constraints and orientation of the complexed monomer, which induces stereoselectivity in the nudeophilic attack by the chain end. It was demonstrated that monomer insertion proceeds by attack at the carbon atom of the epoxide ring where it is deaved with inversion of the carbon configuration. This necessitates an attack of the complexed monomer by the nudeophile from the back, which requires the partidpation of two adjacent aluminum atoms and chain transfer from one aluminum atom to the other one at each monomer addition. The coordination polymerization mechanism proposed by Vandenberg for the trialkylaluminum/water system is shown in Scheme 23. 

Anionic and anionic coordination polymerizations of epoxides are often slow processes that require long reaction times to achieve high monomer conversions. Moreover, as reported in previous sections, a majority of these polymerizations suffer from side reactions, illustrated by the chain transfer reaction to monomer in alkali metal anionic polymerization and by a very low initiation efficiency and the formation of several polyether populations in coordination polymerizations. 

In the ionic-coordinative polymerization (17,18) of epoxides, a metal, M, such as Li, Mg, Zn, Ca, Al, Sn, or Fe, with ligands, such as OH, OR, Cl, Br, OSnRa, or NH2, ionically coordinates with the epoxide oxygen, and this is followd by nucleophilic attack of the ligand to open the oxirane ring and form the initiated species that propagates. 

Functional olefins and epoxides, v/here a functional group is separated from the polymerizable moiety by a spacing arm, have been polymerized via coordinative-anionic and coordination polymerization processes. Metathesis polymerization has recently attracted a considerable amount of interest as well. 

The trigonal planar zinc phenoxide complex [K(THF)6][Zn(0-2,6-tBu2C6H3)3] is formed by the reaction of a zinc amide complex, via a bis phenoxide, which is then further reacted with potassium phenoxide. TheoX-ray structure shows a nearly perfect planar arrangement of the three ligands with zinc only 0.04 A out of the least squares plane defined by the three oxygen atoms.15 Unlike the bisphenoxide complexes of zinc with coordinated THF molecules, these complexes are not cataly-tically active in the copolymerization of epoxides with C02. The bisphenoxide complex has also been structurally characterized and shown to be an effective polymerization catalyst. 43... 

The rate of polymerization is heavily dependent upon the size of the epoxide substituents. Consequently, Et-EO polymerizes slower than either PO or EO,936 and (TPP)AICI inserts only one molecule of Bu-EO over a period of 5 days. Reactivity follows the order EO > PO > -Et-EO cis-BO > ECH > Bu-EO. Further, in a 1 1 mixture of cis- and trans-BO, the cis monomer is opened preferentially, possibly because coordination of the monomer to the metal center occurs prior to ring cleavage. It has also been shown that the 1° alkoxide derived from the ringopening of EO is much more reactive than the 2° alkoxide resulting from PO. 

Iodosylbenzene is sufficiendy reactive on its own to epoxidize electron-deficient olefins such as tetracyanoethylene (43). It is possible that coordinated monomeric iodosylbenzene is substantially more reactive than polymeric iodosylbenzene and that complexation of a monomeric form is sufficient to provide the requisite reactivity with normal olefins. 

It should be mentioned that donor substitution of the phenylene backbone of the salphen ligand was shown to have a decreasing effect on activity [103], which explains the overall lower productivity compared with halogen-substituted chromium salphens. However, experiments clearly proved an increased activity upon dimerization. Whereas the monomeric complex m = 4) converts about 30% of p-BL in 24 h, producing a molecular weight of 25,000 g/mol, the corresponding dimer yields up to 99% conversion with > 100,000 g/mol. Moreover, the smaller polydispersity (PD < 2) shows the better polymerization control, which is attributed to the decreased rate of polymer chain termination. This behavior is caused by the stabilization of the coordinated chain end by the neighboring metal center, as recently reported for dual-site copolymerizations of CO2 with epoxides [104-106]. The polymeric products feature an atactic microstructure since the... 

Excluding polymerizations with anionic coordination initiators, the polymer molecular weights are low for anionic polymerizations of propylene oxide (<6000) [Clinton and Matlock, 1986 Boileau, 1989 Gagnon, 1986 Ishii and Sakai, 1969 Sepulchre et al., 1979]. Polymerization is severely limited by chain transfer to monomer. This involves proton abstraction from the methyl group attached to the epoxide ring followed by rapid ring cleavage to form the allyl alkoxide anion VII, which isomerizes partially to the enolate anion VIII. Species VII and VIII reinitiate polymerization of propylene oxide as evidenced... 

For the (coordination) anionic polymerization, metal alkoxides are often employed as initiators. In this system, the ring opening of epoxide takes place by a nucleophilic attack of an alkoxide on the (activated) epoxide carbon to generate another metal alkoxide which behaves as the propagating species (Scheme 3), The nature of metal-alkoxide... 

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