Aluminum-based Initiators

The most widely studied aluminum-based initiator for the ROP of lactones is Al(0 Pr)3, (246), which has been used to initiate the living polymerization of a variety of cyclic esters including CL 747-749 LA 750,751 GA 752 . 745 and cyclic anhydrides.753 754 Polymerizations initiated by 

The relative rates of polymerization of a series of substituted e-caprolactones initiated by (246) demonstrate that methyl groups, particularly adjacent to the acyl oxygen, retard polymerization.757 In addition, the rate of polymerization of the parent unsubstituted CL at 25 °C was found to be 4 x 102 times greater than L-LA at 70 °C. The slower propagation of LA is usually attributed to coordination of the nearest inserted carbonyl of the polymer chain to the A1 center, leading to formation of a stable 5-membered chelate, which hinders monomer uptake.758 

When a mixture of LA and CL is copolymerized using Al(0 Pr)3, LA is incorporated preferentially (rLA=17.9, rCL = 0.58).757 The propagating species derived from LA are 2.5 x 105 less reactive to CL than CL-derived species are towards LA.758,771 In line with these observations, the corresponding diblock copolymer has only been prepared by polymerizing CL first. 

In order to effect stereocontrol over the ROP of lactones, several research groups have examined monoalkoxide aluminum initiators of the general formula LnM(OR). Preliminary studies indicated that dialkylaluminum alkoxides, e.g., Et2Al(OR), initiate the controlled ROP of lactones.772-775 

These initiators may be used in the presence of protic sources such as MeOH.780 The rapid and reversible exchange of propagating alkoxides with alcohol leads to a narrow molecular weight distribution (for example, Mw/Mn 1.15 for oligomeric PCL), with the number of polymer molecules closely approaching the sum of the number of molecules of initiator and alcohol. In the absence of alcohol, the polymerization of CL is less well behaved with Mw/Mn= 1.5, presumably a consequence of unfavorable initiation and propagation rates. 

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With appropriately substituted oxetanes, aluminum-based initiators (321) impose a degree of microstmctural control on the substituted polyoxetane stmcture that is not obtainable with a pure cationic system. A polymer having largely the stmcture of poly(3-hydroxyoxetane) has been obtained from an anionic rearrangement polymerisation of glycidol or its trimethylsilyl ether, both oxirane monomers (322). Polymerisation-induced epitaxy can produce ultrathin films of highly oriented POX molecules on, for instance, graphite (323). Theoretical studies on the cationic polymerisation mechanism of oxetanes have been made (324—326). 

The dramatic acceleration observed for the ROP of epoxides when (255) is present has also been observed with several other aluminum-based initiators, including salen- (325)-(327), tetraazaan-nulene- (328) and phthalocyanine- (329) ligated systems.946 For example, (325) polymerizes PO extremely slowly at room temperature (4% conversion of 50 equivalents over 8 days), but in the presence of MAD (255), 200 equivalents require just 70min to attain 43% con version.794,795,947 Complex (328) shows a similar increase in activity upon addition of MAD 200 equivalents PO reach 74% conversion within 90 seconds in the absence of MAD such a yield requires several days. 

RarG Earth Metals. A large variety of rare earth derivatives have been used to initiate ROP of lactones and lactides (46). Their usually high reactivity must be emphasized, as exemplified by the polymerization of a low-strained lactone, )-pentadecalactone, by Y(0-j-C3H7)3 (47) Similarly, /3-BL is polymerized by Y(0CH2CH20Me) at room temperature (48), in contrast to the long reaction time and high temperature required when aluminum-based initiators are used. 

With propylene oxide and aluminum-based initiators, such as the Vandenberg Catalyst, the mechanism proposed can be rep-... 

Scheme 16 Aluminum-based initiators for the stereoselective ROP of rac-lactide...

ASA (Azide-Styphnate-Aluminum). British initiating mixts of LA, LSt Al (flake powder) for detonators using tetryl as a base charge. Brit "Service ASA" contains the crystalline "Service Azide," while commercial ASA contains dextrin at ed LA... 

The activity of an initiating system is also affected by the nature of the Friedel-Crafts acid. The following Friedel-Crafts acidity scale can he established BF, < AlCI, < TiCI4 < BCI, - SbFx < SbCL . BBr,. The advantage of the TiCL and the aluminum-based systems is their relative insensivity toward solvent polarity. The activity or the BCIt- or BBrt-based system is greatly solvent-dependent, i.e.. sufficient activity only occurs in polar solvent. 

Hyper-branched polymers are prepared in a single-step polymerization from ABX monomers. Thus, a perfectly branched structure is present in dendrimers, whereas irregular branching is present in hyper-branched polymers. Aluminum alkoxide-based initiators or tin-based catalysts have been successfully used for the preparation of, hyper-branched [160-162, 166-168], dendrimer-like star polymers [160], and star-shaped polymers. The first and second generations of the benzyl ester of 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) are effective initiators for the ROP of lactones (e-CL) in the presence of Sn(Oct)2. The... 

ROP of lactones and lactides using lanthanide alkoxide-based initiators is a relatively recent discovery. The first example of lactone polymerization by lanthanide alkoxide complexes was reported in a DuPont patent written by McLain and Drysdale in 1991 [89]. In general, the activity of these catalysts is much higher than that determined for aluminum alkoxides, especially in lactide polymerization [90-92]. Polymers of relatively high molecular weight and narrow MWD are formed. The negative side-reactions such as macrocycle formation, transesterification, and racemization are absent. 

Aldehydes and ketones can be reduced using sodium borohydride or lithium aluminum hydride. The reaction is base-initiated with hydride ion as the nucleophile. One mole of sodium borohydride or lithium aluminum hydride reduces four moles of aldehyde or ketone the reaction mixture is then acidified to produce the neutral alcohol. 

Concomitant with these efforts, Jacobsen was extending the utility of his salen catalysts.36 Initially, the aluminum-based salen catalyst 68 was found to afford a-aminonitriles 67 from imines 66 with excellent enantioselectivities. 

Cameron, P. A. Gibson, V. C. Irvine, D. J. Nickel-catalyzed generation of Schiff base aluminum enolate initiators for controlled methacrylate polymerization. Angew. Chem., Int. Ed Engl. 2000, 39, 2141-2144. 

Spassky, N. Wisniewski, M. Pluta, C. Le Borgne, A. Highly stereoelective polymerization of roc-(D,L)-lactide with a chiral Schiff s base/aluminum alkoxide initiator. Macromol. Chem. Phys. 1996, 197, 2627-2637. 

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