Metal alkoxides polymerization

Precaution Combustible Incompat. with oxidizers, acids, peroxides, azo compds., metal alkoxides, polymerization initiators avoid excessive heat, ignition sources... 

Precaution Combustible incompat. with oxidizers, adds, peroxides, azo compds., metal alkoxides, polymerization initiators avoid excessive heat, ignition sources Hazardous Decomp. Prods. CO , NO , possibly amnwnia Storage Store in well-ventilated area in tightly closed containers Ken-React NZ 39 [Kenrich Petrochems.]... 

GopolymeriZation Initiators. The copolymerization of styrene and dienes in hydrocarbon solution with alkyUithium initiators produces a tapered block copolymer stmcture because of the large differences in monomer reactivity ratios for styrene (r < 0.1) and dienes (r > 10) (1,33,34). In order to obtain random copolymers of styrene and dienes, it is necessary to either add small amounts of a Lewis base such as tetrahydrofuran or an alkaU metal alkoxide (MtOR, where Mt = Na, K, Rb, or Cs). In contrast to Lewis bases which promote formation of undesirable vinyl microstmcture in diene polymerizations (57), the addition of small amounts of an alkaU metal alkoxide such as potassium amyloxide ([ROK]/[Li] = 0.08) is sufficient to promote random copolymerization of styrene and diene without producing significant increases in the amount of vinyl microstmcture (58,59). 

Figure 17 summarizes the avadable sol—gel processes (56). The process on the right of the figure involves the hydrolysis of metal alkoxides in a water—alcohol solution. The hydrolyzed alkoxides are polymerized to form a chemical gel, which is dried and heat treated to form a rigid oxide network held together by chemical bonds. This process is difficult to carry out, because the hydrolysis and polymerization must be carefully controlled. If the hydrolysis reaction proceeds too far, precipitation of hydrous metal oxides from the solution starts to occur, causing agglomerations of particulates in the sol. 

In anionic polymerization the reaction is initiated by a strong base, eg, a metal hydride, alkah metal alkoxide, organometaHic compounds, or hydroxides, to form a lactamate ... 

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). 

From the preceding discussion, it is easily understood that direct polyesterifications between dicarboxylic acids and aliphatic diols (Scheme 2.8, R3 = H) and polymerizations involving aliphatic or aromatic esters, acids, and alcohols (Scheme 2.8, R3 = alkyl group, and Scheme 2.9, R3 = H) are rather slow at room temperature. These reactions must be carried out in the melt at high temperature in the presence of catalysts, usually metal salts, metal oxides, or metal alkoxides. Vacuum is generally applied during the last steps of the reaction in order to eliminate the last traces of reaction by-product (water or low-molar-mass alcohol, diol, or carboxylic acid such as acetic acid) and to shift the reaction toward the... 

This scheme is remarkably close to the coordination insertion mechanism believed to operate in the metal alkoxide-catalyzed ring-opening polymerization of cyclic esters (see Section 2.3.6). It shares many features with the mechanism proposed above for the metal alkoxide-catalyzed direct polyesterification (Scheme 2.18), including the difficulty of defining reaction orders. 

Kricheldorf, H. R., Berl, M., and Scharnagl, N., Poly(lac-tones). 9. Polymerization mechanism of metal alkoxide initiated polymerizations of lactide and various lactones. Macromolecules. 21, 286-293, 1988. 

Either particulate sol or polymeric sol has been used for thin film coatings. The polymeric sol was fabricated by partial hydrolysis of corresponding metal alkoxide. If the rate of hydrolysis or condensation is very fast, then some kinds of organic acids, beta-dicarbonyls, and alkanolamines have been used as chelating agent in sol-gel processes to control the extent and direction of the hydrolysis-condensation reaction by forming a strong complex with alkoxide. [2]. 

The basic sol-gel reaction can be viewed as a two-step network-forming polymerization process. Initially a metal alkoxide (usually TEOS, Si(OCIl2CH )4) is hydrolyzed generating ethanol and several metal hydroxide species depending on the reaction conditions. These metal hydroxides then undergo a step-wise polycondensation forming a three-dimensional network in the process. The implication here is that the two reactions, hydrolysis and condensation, occur in succession this is not necessarily true (8.9). Depending on the type of catalyst and the experimental conditions used, these reactions typically occur simultaneously and in fact may show some reversibility. 

Anionic polymerization can be initiated by a variety of anionic sources such as metal alkoxides, aryls, and alkyls. Alkyllithium initiators are among the most useful, being employed commercially in the polymerization of 1,3-butadiene and isoprene, due to their solubility in hydrocarbon solvents. Initiation involves addition of alkyl anion to monomer... 

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... 

For polymeric CSD processes, three classes of metal organic (metallo-organic) compounds are used most often as starting reagents metal alkoxides, metal carboxylates, and metal beta-diketonates. These species differ in their solubility and reactivity, as well as their tendency to react with one another, all of which are factors that may influence starting reagent selection. Representative structures of these classes of precursors are illustrated in Fig. 2.2.8... 

The mechanism for cyclic formation via depolymerization is the same type of transesterification which occurs on polymerization, as outlined in Scheme 3.3. Metal alkoxides such as tetraalkyl titanates or dibutyl tin alkoxides have proven... 

Tetraalkyl titanates are the most commonly used catalysts for PBT polymerization [8], The varieties of titanates include tetraisopropyl titanate (TPT), tetrabutyl titanate (TBT) and tetra(2-ethylhexyl) titanate (TOT). Titanates effectively speed the reaction rate with few detrimental effects on the resin. Alkoxy zirconium and tin compounds, as well as other metal alkoxides, may also be used in PBT polymerization. 

Two different mechanisms have been proposed for the ROP of (di)lactones depending on the nature of the organometalhc derivatives. Metal halides, oxides, and carboxylates would act as Lewis acid catalysts in an ROP actually initiated with a hydroxyl-containing compound, such as water, alcohol, or co-hydroxy acid the later would result more hkely from the in-situ hydrolysis of the (di)lac-tone [11]. Polymerization is assumed to proceed through an insertion mechanism, the details of which depends on the metal compound (Scheme la). The most frequently encountered Lewis acid catalyst is undoubtedly the stannous 2-ethylhexanoate, currently referred to as stannous octoate (Sn(Oct)2). On the other hand, when metal alkoxides containing free p-, d-, or f- orbitals of a favo-... 

The polymerization of lactones is initiated by nucleophilic metal alkoxides. It is worth noting that bulky alkoxides are not nucleophilic enough and react as bases. For example, potassium ferf-butoxide deprotonates (3-propionolactone and sCL into new anionic species, which are anionic initiators for the polymerization of lactones [8] (Fig. 4). As a rule, carboxylic salts are less nucleophilic than the corresponding alkoxides and are, in general, not efficient initiators for the polymerization of lactones. Nevertheless, (3-lactones are exceptions because their polymerization can be initiated by carboxylic salts [8]. 

Later, Kricheldorf and coworkers extended the concept of the aluminum alkox-ide-initiated ROP of lactones to a set of other metal alkoxides such as tin(lV) [23-25], titanium, and zirconium alkoxides. As a rule, the polymerization takes place according to the same coordination-insertion mechanism shown in Fig. 12. 

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