Initiators, metal alkoxides

The kinetics of nonhydrolytic sol gel processes depend on the nature of the metal, the nature of the oxygen donor, electronic effects of the R CToup, and the composition of the initial metal alkoxide/metal chloride (carboxylate) mixture. ... 

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). 

Enolate Initiators. In principle, ester enolate anions should represent the ideal initiators for anionic polymeri2ation of alkyl methacrylates. Although general procedures have been developed for the preparation of a variety of alkaU metal enolate salts, many of these compounds are unstable except at low temperatures (67,102,103). Usehil initiating systems for acrylate polymeri2ation have been prepared from complexes of ester enolates with alkak metal alkoxides (104,105). 

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 ... 

Obviously, racemization occurred during the reduction step. For this benzylic alkoxyamine. racemization may occur by initial elimination of a metal alkoxide followed by reduction of the resulting achiral imine. 

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. 

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... 

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. 

Multifunctional initiators made up of metal alkoxides rather than alcohols have been less used for the synthesis of star-shaped polyesters than have the tin (II) bis-(2-ethylhexanoate)/alcohol system. Nevertheless, Kricheldorf initiated the polymerization of sCL using a spiro-cyclic tin(IV) aUcoxide to obtain a tin-containing height-shaped polyester whose final hydrolysis resulted in the formation of a star-shaped polyester (Fig. 35) [25, 159-161]. 

Application of metal salts and well-defined metal complexes in ROP has enabled the exploitation of a three-step coordination-insertion mechanism, first formulated in 1971 by Dittrich and Schulz [17]. This proceeds through coordination of lactide by the carbonyl oxygen to the Lewis acidic metal center, leading to the initiation and subsequent propagation by a metal alkoxide species. This species can be either isolated or generated in situ by addition of an alcohol to a suitable metal precursor to result in the formation of a new chain-extended metal alkoxide, as shown in Scheme 3 [16]. 

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... 

Strong bases such as alkali metals, metal hydrides, metal amides, metal alkoxides, and organometallic compounds initiate the polymerization of a lactam by forming the lactam anion XXXIV [Hashimoto, 2000 Sebenda, 1989 Sekiguchi, 1984], for example, for e-caprolactam with a metal... 

A variety of anionic initiators, both ionic and covalent, have been used to polymerize lactones [Duda and Penczek, 2001 Jedlinski, 2002 Jerome and Teyssie, 1989 Penczek and Duda, 1993]. Much of the more recent activity involves the use of anionic covalent (coordination) initiators such as alkylmetal alkoxides and metal alkoxides such as R2A OR and Al(OR)3, metal carboxylates such as tin(II) 2-ethylhexanoate, metalloporpyrins (VI), and aluminox-anes such as oligomeric [A1(CH3)0] [Biela et al., 2002 Duda et al., 1990 Endo et al., 1987a,b Gross et al., 1988 Kricheldorf et al., 1990 Penczek et al., 2000a,b Sugimotoa and Inoue, 1999]. 

Tin(II) 2-ethylhexanoate is an important industrial initiator for cyclic ester polymerization [Duda and Penczek, 2001 Kricheldorf et al., 2001 Storey and Sherman, 2002]. Metal car-boxylates are useful initiators only in the presence of alcohols. The polymerization rate is very slow in the absence of alcohol, less than 1% of the rate in the presence of alcohol. The actual initiator is the metal alkoxide formed by the reaction between metal carboxylate and alcohol. 

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... 

There are many liquids capable of rapidly generating a solid product, when their droplets dispersed in an inert gas are contacted with a vapor coreactant. Such compounds include metal alkoxides, halides, and oxyhalides that react with water, hydrogen sulfide, etc., and organic monomers that can undergo either addition or condensation polymerization in the presence of appropriate initiators. The resulting powders consist of spherical particles, the size distribution of which depends on the method and conditions of droplet generation. 

Polymerization of butadiene with lithium morpholinide, an initiator with a built-in microstructure modifier, has been carried out in hexane. In general, the vinyl content of the polymers prepared with this initiator is dependent on the initiator concentrations and on the polymerization temperatures. This dependence is identical to that observed in a THF-modified lithium diethylamide polymerization initiator system. A comparison of these initiator systems for polymerization of butadiene is presented. In addition, a study of the effect of metal alkoxides on the vinyl content of lithium morpholinide initiated butadiene polymerization is included. 

The effect of the addition of alkali metal b-butoxides on the lithium morpholinide-initiated heterogeneous butadiene polymerization was also studied at 30°C in hexane. The data in Table III show that the 1,2 content of the polybutadiene is sensitive to the type of alkali metal used. For example, by changing metal alkoxides from Li to Na to K, the 1,2 content of polybutadiene changes from 46.0% to 58.2% to 55.4%, respectively, at an initiator concentration of about 10 mmoles. However, the 1,2 content of the polybutadienes is unaffected by the metal alkoxide concentration (1,2 content, 42-46% for LiOtBu, 55-58% for NaOtBu, 53-55% for KOtBu) when the alkoxide/ morpholinide ratio is greater than 1 1. 

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