Epoxide polymerization alkoxide

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... 

Problem 10.4 Discuss the effect of exchange reaction on epoxide polymerizations initiated by metal alkoxides in alcoholic solution, for the cases where the added alcohol is (a) equally acidic, (b) more acidic, and (c) less acidic than the polymeric alcohol formed by the exchange reaction. What would be the result of the use of HCl or RCOOH in place of ROH (or H2O) ... 

Exchange Reactions In epoxide polymerizations initiated by alkali metal alkoxides and hydroxides, protonic snbstances sneh as water or aleohol are often added to dissolve the initiator and prodnce a homogeneous system. However, these snbstances can contribute to an exchange reaction with the propagating ehain. In the presenee of aleohol (ROH), for example, the exchange reaction can be represented by... 

Epoxide polymerization can be described in terms of three different mechanisms (1) anionic (base-catalyzed), (2) cationic (acid-catalyzed), and (3) coordinate. The third actually combines features of the first two extremes, since it involves coordination of the monomer oxygen at a Lewis acid catalyst site (L), followed by attack on the thus activated monomer by an alkoxide already bound to the site. 

Besides direct hydrolysis, heterometaHic oxoalkoxides may be produced by ester elimination from a mixture of a metal alkoxide and the acetate of another metal. In addition to their use in the preparation of ceramic materials, bimetallic oxoalkoxides having the general formula (RO) MOM OM(OR) where M is Ti or Al, is a bivalent metal (such as Mn, Co, Ni, and Zn), is 3 or 4, and R is Pr or Bu, are being evaluated as catalysts for polymerization of heterocychc monomers, such as lactones, oxiranes, and epoxides. An excellent review of metal oxoalkoxides has been pubUshed (571). 

Yasuda, T., Aida, T., and Inoue, S., Reactivity of (por-phinato)aluminum phenoxide and alkoxide as active initiators for polymerization of epoxide and lactone. Bull. Chem. Soc. Jap.. 9, 3931-3934, 1986. 

Diiminatc zinc complexes are highly active catalysts in the copolymerization of epoxides and C02. Complexes that are catalytic are of the form ZnLX, where X is alkoxide, acetate, or bis(tri-methylsilyl)amide. Changing the ligand geometries of the complexes allows variation in the catalytic behavior and activity.941 The polymerization of lactide with diiminate zinc has also been studied.942... 

Equation 1 expresses a state of equilibrium between an alcohol A. on a molecule whose degree of polymerization is j, the catalyst C and the alkoxide anion A.C. In Relation 2 this activated intermediate reacts with monomeric anhydride A, forming an acid adduct A.AC, which dissociates, forming an unassociable carboxylic acid A.A. Reactions 3-5 depict the union of a carboxylic intermediate with a monomeric epoxide E, or with pendant oxiranes on macromole- ... 

Block copolymers of /3-PL and /3-BL have been synthesized using (251), although reaction times of several weeks are required.782 Since (TPP)Al-based carboxylates are also known to polymerize epoxides (see Section 9.1.7.2), the sequential addition of /3-BL and propylene oxide (PO) results in formation of a p(/3-BL-/3-PO) diblock.782 However, reversing the order of addition fails to produce the block copolymer since the propagating alkoxide (TPP)Al(OCHMeCH2)nCl does not initiate the ROP of /3-BL. 

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. 

The corresponding thiolate (324) will also polymerize epoxides following photoinitiation 944 For example, using 190 equivalents of EO, a conversion of <2% was observed after 205 min in the dark. However, this rises to 97% if the system is irradiated for 80 min (Mn = 8,700, Mn calc = 8,100, Mw/Mn= 1.05). 111 NMR studies confirm that the propagating species is a metal-alkoxide and as a result, if irradiation is stopped once initiation has occurred, the polymerization will persist, although... 

The anionic polymerization of epoxides such as ethylene and propylene oxides can be initiated by metal hydroxides, alkoxides, oxides, and amides as well as metal alkyls and aryls, including radical-anion species such as sodium naphthalene [Boileau, 1989 Dreyfuss and Drefyfuss, 1976 Inoue and Aida, 1984 Ishii and Sakai, 1969]. Thus the polymerization of ethylene oxide by M+A involves initiation... 

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... 

Epoxides readily undergo anionic copolymerization with lactones and cyclic anhydrides because the propagating centers are similar—alkoxide and carboxylate [Aida et al., 1985 Cherdron and Ohse, 1966 Inoue and Aida, 1989 Luston and Vass, 1984]. Most of the polymerizations show alternating behavior, with the formation of polyester, but the mechanism for alternation is unclear. There are few reports of cationic copolymerizations of lactones and cyclic ethers other than the copolymerizations of [5-propiolactone with tetrahydrofuran and... 

The polymerization of an epoxide by hydroxide or alkoxide ion is often carried out in the presence of an alcohol. Why How is the degree of polymerization affected by alcohol Discuss how the presence of alcohol affects both the polymerization rate and molecular weight distribution. 

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... 

Thus kp for lithium counterion is 1/300 of kp for potassium counterion. The low reactivity and association of lithium alkoxide was reported in the anionic polymerization of epoxides.We have found that two fold increase of the lithium initiator concentration has led to a decrease of the kp nearly to one half. This indicates that the kinetic order with respect to the initiator would be near to zero, suggesting a very high degree of association of the active species. Thus the propagation reaction appears to proceed in practice through a very minor fraction of monomeric active species in case of lithium catalyst. 

Alkoxide-Type Initiators. Using the guide that an appropriate initiator should have approximately the same structure and reactivity as the propagating anionic species (see Table 1), alkoxide, thioalkoxide, carboxylate, and silanolate salts would be expected to be useful initiators for the anionic polymerization of epoxides, thiiranes, lactones, and siloxanes, respectively (106—108). Thus low molecular weight poly(ethylene oxide) can be prepared... 

The direct reaction between alcohols and epoxides is very slow so alkaline catalysts are employed and are presumed to increase the rate by converting the growing chain to the more nucleophilic alkoxide ion. Polymerizations of this kind have recently been carefully investigated by Gee and coworkers (21). The reactions are still not rapid even at elevated temperatures so the molecular weights obtained in this way are normally quite low (< 10,000). The most rapid reactions of epoxides are those catalysed by acids but even they must be regarded as of the nucleophilic type with acceleration due to the increased reactivity of the protonated monomer,... 

Since this brief summary covers most of our present knowledge of the stereospecific polymerizations, it must be very evident that any mechanism now proposed must be based largely on intuition and cannot be detailed. Most authors agree that propagation probably occurs via an anionic reaction between a metal alkoxide and an epoxide,... 

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