Alkoxide initiation

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. 

Prior to the development of the aluminum initiators described above, a series of bimetallic /x-oxo alkoxides, such as (269) and (270) were examined as lactone polymerization initiators.811 812 At 10 °C, (269) polymerizes CL in a moderately controlled manner (60 equivalents, t1/2 = 23min), as shown by a linear DP vs. [M]0/[I]o plot and Mw/Mn values between 1.3 and 1.5. Mn data is consistent with only one of the terminal alkoxides initiating the ring opening, although in the presence of "BuOH, which is known to dissociate (269), all four alkoxides are active. 

NMR studies on the alkoxide initiators confirm that all the lactones polymerize via an acyl— oxygen scission, including /3-PL (which, by contrast, opens at the alkyl—oxygen bond with (251)). Monomer coordination and subsequent ring opening may be observed by 111 NMR spectroscopy. Coordination is also observed with 7-BL and 7-VL, although these adducts are stable to insertion and polymerization does not proceed. 

Bis(ethylacetoacetonate)-lanthanide(III) alkoxides, represented by structure (314), also initiate the well-controlled ROP of CL.895 Mn increases linearly with conversion (with Mw/Mn<1.10 throughout), and increasing [M]0/[I]o- Kinetic analysis implies a first order dependence on the lanthanide initiator, consistent with a non-aggregated active site. Block copolymers with moderately narrow polydispersities (1.25-1.45) have also been prepared using these initiators. NMR spectroscopy confirms well-controlled block sequences suggesting that these initiators are less susceptible to transesteriflcation than other lanthanide alkoxides. Initiation occurs exclusively at the alkoxide bond, and the tris(ethylacetoacetonate) analogs are inactive under the same conditions. 

Alkoxide gels, 23 60 Alkoxide gels, in optical fiber manufacturing, 11 145 Alkoxide initiators, 14 259 Alkoxide ligands, thorium, 24 770 Alkoxides, 12 190 25 72-86 controlled hydrolysis of, 23 56 iron, 14 533 mixed-metal, 25 100 titanium, 25 82 uranium complexation with,... 

Fig. 14 Series of well-defined lanthanide alkoxide initiators...

Several different nucleophilic substitution reactions have been observed in the polymerization of methyl methacrylate. Attack of initiator on monomer converts the active alkyl-lithium to the less active alkoxide initiator (Eq. 5-75). Further, methyl methacrylate (MMA) is converted to isopropenyl alkyl ketone to the extent that this reaction occurs. 

In conclusion, we would like to mention that, in addition to this new direction, a large consumer of metal alkoxides (initially aluminium and titanium) is by tradition the technology of materials, where the alkoxides are used for hy-drophobization and for cross-linking of the polyhydroxocompounds, epoxides and polyester resins, and organosilicon polymers. The products of the partial hydrolysis and pyrolysis of alkoxides — polyorganometalloxanes — are applied as components of the thermally stable coatings [48J. 

Aldehydes and ketones (RxCOR2) undergo an unusual tandem alkynylation and franx-hydrosilylation with alkynylsilanes (e.g. Ph-C=C-SiHR2) to give oxasilacy-clopentenes (54).210 A mild alkoxide initiator is required. 

Parameters that influence the number of transesterifications are temperature, reaction time, and type and concentration of catalyst or initiator [44]. Depending on the metal used, the initiator is more or less active towards side-reactions such as transesterification reactions [44,45]. The relative reactivity of different metal alkoxide initiators towards chains already formed has been reported to be Bu2Sn(OR)2>Bu3SnOR>Ti(OR)4>Zn(OR)2>Al(OR)3 [44]. 

ROP reactions initiated with a metal alkoxide initiator are generally characterized by an equilibrium between the free and the aggregated metal alkoxide [10, 101,102] ... 

In 2001, Chisholm s group reported a (3-diiminate (BDI) gallium-OSiMe3 complex which was unreactive for LA ROP the authors cited substrate binding as the limiting step [76]. Recently, a gallium alkoxide initiator (11) has been reported as an efficient polymerization initiator [72]. The dimeric complex was formed by reaction of GaMe3 with (A)-methyl lactate (Fig. 15). 

The present study on Ti02 powder formation from Ti(0-iC3H7>4 in supercritical isopropanol has allowed the determination of reaction kinetic constants and activation energy in a temperature range from 531 to 568 K at 10 MPa. The proposed mechanism is based on a hydrolytic decomposition of the alkoxide initiated by water formed in alcohol dehydration catalysed by reactor walls. The derived reaction kinetic order is unity in accordance with experimental results. Such a mechanism also explains that special cares must be taken about the internal surface state of the reactor in order to obtain reproducible results. 

In this case, the silylation of the metal alkoxide initially formed represents the key step of the overall process which releases the chromium salt from the organic product. The other crucial parameter is the use of the stoichiometric reducing agent for the regeneration of the active Cr" species. Commercial Mn turned out to be particularly well suited, as it is very cheap, its salts are essentially non-toxic and rather weak Lewis acids, and the electrochemical data suggest that it will form an efficient redox couple with Cr . Moreover, the very low propensity of commercial Mn to insert on its own into organic halides guarantees that the system does not deviate from the desired chemo- and diastereoselective chromium path. Thus, a mixture of CrX ( = 2, 3) cat., TMSCl and Mn accounts for the first Nozaki reactions catalytic in chromium [13]. 

Figure 2 Coordination-insertion mechanism for the polymerization of lactide by a metal complex with an alkoxide initiator...

Complexes ( )- and (R,R)-2 are highly active and controlled for the polymerization of rac-LA yielding a linear relationship between the observed PLA molecular weights and the added monomer, assuming both alkoxides initiate the polymerization, with low molecular weight distributions (Figure 36)." The rate of polymerization with these complexes is first order in lactide concentration with values comparable with the tri-dentate diaminophenolate indium complexes desaibed in Section 2. " -The rates of polymerization are much faster than any of the known chiral aluminum salen systems, which require elevated temperatures and days to reach full conversion." -... 

Meerwein-Ponndorf-Verley-Oppenauer (MPVO) reactions are usually mediated by metal alkoxides such as Al(0/-Pr)3. The activity of these catalysts is related to their Lewis-acidic character in combination with ligand exchangeability. The mechanism of these homogeneous MPVO reactions proceeds via a cyclic six-membered transition state in which both the reductant and the oxidant are co-ordinated to the metal center of the metal alkoxide catalyst (Scheme 1). The alcohol reactant is co-ordinated as alkoxide. Activation of the carbonyl by co-ordination to Al(III)-alkoxide initiates the hydride-transfer reaction from the alcoho-late to the carbonyl. The alkoxide formed leaves the catalyst via an alcoholysis reaction with another alcohol molecule, usually present in excess [Ij. 

Table 2. Chiral Alkoxides, Initiators, Monomers, and Properties of the Resulting Polymers "...

The anionic polymerization of propylene oxide initiated by potassium alkoxide or hydroxide occurs predominantly (95%) by cleavage of the O-CH2 bond. For bulk polymerization at 80 °C, approximately 4% head-to-head placements occur. However, there is no stereocontrol in this alkoxide-initiated ring opening and the resulting polymer is nontactic [140]. 

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