Aluminum alkoxides, polymerizations

Aluminum alkoxides, polymerizations of poly[bis(methoxyethoxyethoxy)phos-phazene] molecular composites, 258-266 Aluminum nitride applications, 427... 

Sulfuric acid is also a very satisfactory catalyst aluminum alkoxides also are useful, especially when the alcohols would be adversely affected by strong acids. Sodium alkoxides produce undesirable side reactions and give lower yields. When alkaline catalysts are employed, an alkaline polymerization inhibitor, such as j j-phenylenediamine or phenyl-d-naphthylamine, should be used instead of hydroquinone. 

Novel catalytic systems, initially used for atom transfer radical additions in organic chemistry, have been employed in polymer science and referred to as atom transfer radical polymerization, ATRP [62-65]. Among the different systems developed, two have been widely used. The first involves the use of ruthenium catalysts [e.g. RuCl2(PPh3)2] in the presence of CC14 as the initiator and aluminum alkoxides as the activators. The second employs the catalytic system CuX/bpy (X = halogen) in the presence of alkyl halides as the initiators. Bpy is a 4,4/-dialkyl-substituted bipyridine, which acts as the catalyst s ligand. 

Anionically prepared hydroxy-terminated PBd was reacted with AlEt3 to form the corresponding aluminum alkoxide macroinitiator, capable of initiating the polymerization of L-lactide [117]. Using ratios [PBd-OH]/[AlEt3] between 1 and 6, reaction temperatures between 70 and 120 °C and maintaining the conversion of the lactide polymerization below 90%, products with narrow molecular weight distribution were obtained. 

A range of tetradentate Schiff-base ligands have also been employed to prepare discrete aluminum alkoxides. The most widely studied system is the unsubstituted parent system (256), which initiates the controlled ROP of rac-LA at 70 °C in toluene. The polymerization displays certain features characteristic of a living process (e.g., narrow Mw/M ), but is only well behaved to approximately 60-70% conversion thereafter transesterification causes the polydispersity to broaden.788 MALDI-TOF mass spectroscopy has been used to show that even at low conversions the polymer chains contain both even and odd numbers of lactic acid repeat units, implying that transesterification occurs in parallel with polymerization in this system.789... 

A series of bis(phenoxide) aluminum alkoxides have also been reported as lactone ROP initiators. Complexes (264)-(266) all initiate the well-controlled ROP of CL, NVL.806,807 and L-LA.808 Block copolymers have been prepared by sequential monomer addition, and resumption experiments (addition of a second aliquot of monomer to a living chain) support a living mechanism. The polymerizations are characterized by narrow polydispersities (1.20) and molecular weights close to calculated values. However, other researchers using closely related (267) have reported Mw/Mn values of 1.50 and proposed that an equilibrium between dimeric and monomeric initiator molecules was responsible for an efficiency of 0.36.809 In addition, the polymerization of LA using (268) only achieved a conversion of 15% after 5 days at 80 °C (Mn = 21,070, Mn calc 2,010, Mw/Mn = 1.46).810... 

The polarity of the Al-C bond allows easy derivatization of the five-coordinate aluminum alkyls by alkane elimination (Figure 9). For example, Salen aluminum alkyls LAlMe could be converted to dimeric or polymeric Salen aluminum phosphinates [LAl 02P(H)Ph ] 98 (n = 2 or oo, depending on the Salen ligand backbone)98,99 by reaction with phenyl phosphinic acid, Salen aluminum siloxides LA10SiPh3 by reaction with triphenyl silanol,96 or Salen aluminum alkoxides LAIOR by reaction with an alcohol.100... 

Aluminum Alkoxides Mediated Ring Opening Polymerization of Lactones and Lactides... 

Polymerization of e-CL and (L- or D,L-)LA is perfectly living when initiated with any of the aforementioned functional aluminum alkoxides in toluene at 25 and 70 °C, respectively. This is supported by the close agreement between the mean degree of polymerization (DP) at total monomer conversion and the mon-omer-to-initiator molar ratio, the narrow molecular weight distributions (PDI=... 

Considerable effort has been carried out by different groups in the preparation of amphiphihc block copolymers based on polyfethylene oxide) PEO and an ahphatic polyester. A common approach relies upon the use of preformed co- hydroxy PEO as macroinitiator precursors [51, 70]. Actually, the anionic ROP of ethylene oxide is readily initiated by alcohol molecules activated by potassium hydroxide in catalytic amounts. The equimolar reaction of the PEO hydroxy end group (s) with triethyl aluminum yields a macroinitiator that, according to the coordination-insertion mechanism previously discussed (see Sect. 2.1), is highly active in the eCL and LA polymerization. This strategy allows one to prepare di- or triblock copolymers depending on the functionality of the PEO macroinitiator (Scheme 13a,b). Diblock copolymers have also been successfully prepared by sequential addition of the cyclic ether (EO) and lactone monomers using tetraphenylporphynato aluminum alkoxides or chloride as the initiator [69]. 

Moreover, alcohol functionalities have been introduced into the polynor-bornene (PNB) backbone by copolymerization of norbornene with a few percent of 5-acetate norbornene and subsequent acetate reduction. After transformation of the pendant hydroxyl functions into diethyl aluminum alkoxides, sCL has been ring opening polymerized (Scheme 31). Owing to the controlled/ liv-ing character of both polymerization processes the isolated poly(NB- -CL) graft copolymers were characterized by well-defined composition, controlled molecular weight and branching density, and narrow MWD (PDI=1.2-1.4) [92]. 

Control of the ROP of lactones was improved by using aluminum alkoxides instead of their anionic counterpart, as witnessed by the increase of the selectivity factor p, as shown for instance in Table 3 in the case of the polymerization of sCL. The lower reactivity of aluminum alkoxides compared to their anionic counterparts is shown by the decrease in the rate of propagation of the polymerization (A p). 

Nevertheless, the use of enzymes presents some drawbacks. Enzymes are expensive and large amounts are needed for polymerization. Besides, it is stiU quite challenging to synthesize high molar mass polyesters. The control of polymerization remains less efficient with enzymes than with chemical initiators such as aluminum alkoxides. 

It is worth noting that 6,7-dihydro-2(3//)-oxepinone is an unusual lactone because it can be polymerized by two distinct mechanisms ROP of the cyclic esters by aluminum alkoxides, and the ring-opening metathesis polymerization (ROMP) of endocyclic olefins by the Schrock s catalyst (Fig. 28) [121]. 

The second approach is based on the polymerization of lactones bearing functionalized substituents. A set of lactones bearing chloride [124-126], bromide [127, 128], iodide [129], alkene [130-134], or alkyne [135] were synthesized and (co)polymerized by tin octoate, tin(IV), and aluminum alkoxides (Fig. 29). 

Ropson N, Dubois P, Jerome R, Teyssie P (1995) Macromolecular engineering of polylactones and polylactides. 20. Effect of monomer, solvent, and initiator on the ringopening polymerization as initiated with aluminum alkoxides. Macromolecules 28 7589-7598... 

Although anionic polymerization of cyclic ethers is generally limited to oxiranes, there are reports of successful oxetane and tetrahydrofuran polymerizations in the presence of a Lewis acid. Aluminum porphyrin alone does not polymerize oxetane, but polymerization proceeds in the presence of a Lewis acid [Sugimoto and Inoue, 1999]. Similarly, THF is polymerized by sodium triphenylmethyl in the presence of a Lewis acid such as aluminum alkoxide [Kubisa and Penczek, 1999]. The Lewis acid complexes at the ether oxygen, which weakens (polarizes) the carbon-oxygen bond and enhances nucleophilic attack. 

Polymerizations of six- and four-membered lactones initiated with aluminum porphyrins [(TPP)AlX, 1] proceed via (porphinato)aluminum alkoxide and car-boxylate as the growing species, respectively (Schemes 9 and 10) [74,75] to give polyesters with narrow MWD, although it is well known that the polymerization... 

Ring-opening polymerization of racemic iV-carboxyamino acid anhydrides can be achieved with a chirally modified aluminum alkoxide (180). 

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