Initiation by alkoxides

During the initiation of methyl methacrylate polymerization by alkyl-lithium, lithium alkoxide is formed [see eqn. (37)].This compound directly affects the subsequent course of the reaction. It has aroused the interest of scientists who started to used various lithium alkoxides directly as initiators 

In solvents of various solvating strength, the /m.-alkoxides of alkali metals are associated to various degrees. For initiation, the aggregated alkoxide has to be activated by the monomer or by a saturated ester of suitable structure 

Only about 10% of /er/.-ROLi molecules yield polymer chains. The remaining alkoxide, coordinated to the growth centres, forms a protective envelope. It suppresses the basicity of the centres and thus limits their ability to react with the ester group of the monomer. When the reaction conditions are selected so that almost no tert-ROLi is left for the formation of the protective envelope, propagation occurs simultaneously with termination. Polymerization ceases prior to the consumption of all monomer and a strongly branched polymer results. 

The polymerizations of many other monomers are initiated by alkoxide ions. 

The anionic polymejization of / -nitrostyrenes may serve as an example. It is initiated by alkoxide ions in solvents able to loose a proton (alcohols in this case) 

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The ring-opening polymerization of cyclic monomers can be performed by ionic chain polymerization, as is the case of epoxy monomers. Anionic polymerization of ethylene oxide propylene oxide, and caprolactone can be initiated by alkoxides ... 

This equation assumes each initiator and alcohol molecule to be a potential polymer chain. Alcohol or other protonic substances can thus be used to control polymer molecular weight. The molecular weight limitation due to exchange reactions, as represented by Eq. (10.11), does not, however, apply to polymerizations initiated by alkoxides and hydroxides in aptotic polar solvents, nor does it apply to polymerizations initiated by other initiators such as metal alkyls and aryls and the various coordination initiators, since the latter initiators are dissolved in aprotic solvents such as benzene or tetrahydrofuran (Odian, 1991). 

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 ... 

The domino reaction is initiated by the chemoselective attack of the carbanion 2-458 on the terminal ring carbon atom of epoxyhomoallyl tosylate 2-459 to give the alkoxides 2-460 after a 1,4-carbon-oxygen shift of the silyl group. The final step to give the cyclopentane derivates 2-461 is a nucleophilic substitution. In some cases, using the TBS group and primary tosylates, oxetanes are formed as byproducts. 

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... 

Improved control was observed, however, upon addition of benzyl alcohol to the dinuclear complexes.887 X-ray crystallography revealed that whereas (296) simply binds the alcohol, (297) reacts to form a trinuclear species bearing four terminal alkoxides. The resultant cluster, (298), polymerizes rac-LA in a relatively controlled manner (Mw/Mn=1.15) up to 70% conversion thereafter GPC traces become bimodal as transesterification becomes increasingly prevalent. NMR spectroscopy demonstrates that the PLA bears BnO end-groups and the number of active sites was determined to be 2.5 0.2. When CL is initiated by (298) only 1.5 alkoxides are active and kinetic analysis suggests that the propagation mechanisms for the two monomers are different, the rate law being first order in LA, but zero order in CL. 

The reaction is initiated by mixing a precursor with water. Solvent molecules attack the alkoxide leading to a nucleophilic substitution of the alkoxy group (—OR) by a hydroxyl group (—OH). This can be presented in the general form by ... 

It is postulated that the mechanism of the silane-mediated reaction involves silane oxidative addition to nickel(O) followed by diene hydrometallation to afford the nickel -jr-allyl complex A-16. Insertion of the appendant aldehyde provides the nickel alkoxide B-12, which upon oxygen-silicon reductive elimination affords the silyl protected product 71c along with nickel(O). Silane oxidative addition to nickel(O) closes the catalytic cycle. In contrast, the Bu 2Al(acac)-mediated reaction is believed to involve a pathway initiated by oxidative coupling of the diene and... 

Interestingly, the ROP of GA by living PCL macroinitiators as initiated by aluminum alkoxides has led to PGA chains of unprecedented molecular weights (Mj as high as 70,000). Note also that symmetrical triblock copolyesters which present interesting elastomeric properties (see Sect. 4.1), have also been prepared by sequential addition of the sCL and DXO comonomers as will be discussed in Sect. 4.1. 

Apart from the alkoxides aggregation, the different factors which affect the kinetics of the (di)lactones ROP initiated by aluminum alkoxides might be summarized as follows. 

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]. 

In a very similar way, hydroxy functionalized ATRP initiators such as 2,2,2-tribromoethanol can be used for the simultaneous polymerization of eCL and MMA (Scheme 25) [83]. Purposely, the ROP of eCL is promoted by Al(OfPr)3 added in catalytic amount so that the rapid alcohol-alkoxide exchange reaction (see Sect. 2.4) activates all the hydroxyl functions. In order to avoid initiation by the isopropoxy groups of Al(0/Pr)3. The in-situ formed zPrOH is removed by distillation of the zPrOH/toluene azeotrope. On the other hand, the ATRP of MMA is catalyzed by NiBr2(PPh3)3. The two aforementioned one-step methods provide block copolymers with controlled composition and molecular weights, but with a slightly broad MWD (PDI=1.5-2). 

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