Polymerization epoxide

Propylene oxide and other epoxides polymerize by ring opening to form polyether stmctures. Either the methine, CH—O, or the methylene, CH2—O, bonds ate broken in this reaction. If the epoxide is unsymmetrical (as is PO) then three regioisomers are possible head-to-tad (H—T), head-to-head (H—H), and tad-to-tad (T—T) dyads, ie, two monomer units shown as a sequence. The anionic and... 

Organic Network Formers. As indicated above, an additional organic network can be built up by organic polymer synthesis within an inorganic network. The basic principles are shown in Equations 3 to 5 with a vinyl, methyl methacrylate and epoxide polymerization ... 

Kim et al. [67] recently reported the synthesis of heterometallic chiral polymer (salen) Co-(Al, Ga, ln)Cl3 complexes 26-32 (Figure 10) and their use in the HKR of racemic epoxides. Polymeric salen catalysts showed very high reactivity and enantioselectivity at substantially lower catalyst loadings for the asymmetric ring opening of terminal epoxide to obtain the enantio-enriched products. The performance of catalysts is retained on multiple-use and do not suffer the problems of solubility and deactivation (Scheme 5). 

The polymer has a predominantely head-to-tail structure with propagation occurring almost exclusively by attack at the P-carbon—the less sterically hindered site (Eq. 7-11), that is, an Sn2 attack [Kasperczyk and Jedlinski, 1986 Oguni et al., 1973 Price and Osgan, 1956 Quirk, 2002], Propylene oxide and other substituted epoxides polymerize more slowly than does ethylene oxide because of steric hindrance. 

Epoxide polymerizations taking place in the presence of protonic substances such as water or alcohol involve the presence of exchange reactions. Examples of such polymerizations are those initiated by metal alkoxides and hydroxides that require the presence of water or alcohol to produce a homogeneous system by solubilizing the initiator. Such substances increase the polymerization rate not only by solubilizing the initiator but probably also by increasing the concentration of free ions and loose ion pairs. In the presence of alcohol the exchange... 

In the following sections, we describe the recent development of catalyst systems for epoxide polymerization, focusing on homopolymerization, (alternating) co-polymerization with CO or GO2 reported from 1993 to 2004. Although aluminum and zinc are not classified as transition metals, polymerization catalyst systems using those metals will be discussed since they greatly contribute to the field of epoxide polymerization. 

Polymerization using epoxides as monomers includes the ring opening of epoxides via C-O bond cleavage. Thus, a mode of G-O bond cleavage (Sn2 or SnI) and selectivity, that is, which C-O bond is cleaved, coupled with the symmetry of epoxides (symmetrical or unsymmetrical), cause regio- and stereochemical issues to be controlled in the epoxide polymerization. 

In 2000, Mason and Perkins reported a unique aluminum-based epoxide polymerization system. When an aluminum complex prepared from a 2 1 mixture of APBus and methylphosphonic acid is used, EGH polymerizes rapidly at room temperature to give high molecular weight poly-EGH [7l/n= 130 000gmol ... 

Some mechanism aspects of epoxide polymerization. Stereochemical structure of the crystalline polyethers from the 2,3-epoxybutanes. J. Polymer Sci.B2,1085 (1964). 

Ishimori,M., Nakasugi,0., Takeda,N, Tsuruta,T. Studies on organometallic compounds as polymerization catalysts. II. Diethylzinc/water system for epoxide polymerization. Makromol. Chem 115,103 (1968). 

Highly fluorinated ethenes react readily with oxygen to yield epoxides, polymeric peroxides, perfluorocyclopropane, and, by rearrangement, fluorinated acetyl halides carbonyl fluoride and carbon dioxide are undesired byproducts, the amount of which is dependent on the method used. 

Hexacyanometalate Salt Complexes as Catalysts for Epoxide Polymerizations... 

Reasoning from the success of Price (16) and others with zinc compounds as catalysts for epoxide polymerizations, the zinc hexacyano-... 

The zinc cation gives by far the most active catalyst. Iron, cobalt, and nickel cations also gave salts with considerable catalytic activity. Cadmium, because of its chemical similarity to zinc, and aluminum, because of its use in other epoxide polymerization catalysts, were considered as likely candidates to give active catalysts. However, complexes of the salts of these cations were only slightly catalytic. The salts used as cation sources in catalyst preparations also affected catalytic activity. Zinc salts, especially zinc chloride and zinc bromide, were retained in considerable amounts in the finished complexes, and the use of these salts gave the most active catalysts. 

Fig. 10. General principle of epoxide polymerization with iron arene complexes...

From the DSC experiment, it is evident that the iron cation is less reactive in epoxide polymerization then Brensted acids obtained from, e.g. sulfonium salts (see Fig. 11). Therefore, a heat treatment after the illumination step is necessary. The narrow enthalpy peak observed could refer to a very uniform pathway. 

The oxidation state of the complexed iron in these photoinitiators is 2 before and after exposure to actinic radiation. Oxidation of the complexed iron to the 3" state is possible after the irradiation step. Irradiation in the presence of an oxidant yields a stronger Lewis acid with increased reactivity for the epoxide polymerization. 

At the beginning of this materials technology the curing reaction was mainly based on the thermally induced epoxide polymerization demanding high temperatures and long ciuring times [5,6]. 

St. Pierre and Price [23] have also shown that anhydrous KOH is an interesting initiator of epoxide polymerization. Anhydrous RbOH and CsOH were also effective but, interestingly, NaOH and LiOH were not. 

Alkali metal naphthalene complexes have also been used to initiate epoxide polymerizations. Solov yanov and Kazanski [25] studied the polymerization of EO in tetrahydrofuran using sodium, potassium or cesium naphthalene as initiator. A living polymer was produced there is no chain rupture or transfer. The rate of polymerization depends on the concentration of active centres in a complex manner. The kinetic order varies from 0.23 for Na" (or 0.33 for K and Cs" ) up to full first order as initiator concentration decreases. The polymerization is first order in monomer, but deviations are observed at high concentrations. 

Three types of onium salts most often mentioned in the literature as photoinitiators for epoxide polymerization are diazonium salts, aryliodoniurn salts and onium salts of the elements of Groups Va and Via. Representative examples of each type are shown in Table II. 

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