Ethylene oxide lithium-based initiators

Reed 332) has reported that reaction of ethylene oxide with the a,(a-dilithiumpoly-butadiene in predominantly hydrocarbon media (some residual ether from the dilithium initiator preparation was present) produced telechelic polybutadienes with hydroxyl functionalities (determined by infrared spectroscopy) of 2.0 + 0.1 in most cases. A recent report by Morton, et al.146) confirms the efficiency of the ethylene oxide termination reaction for a,ta-dilithiumpolyisoprene functionalities of 1.99, 1.92 and 2.0j were reported (determined by titration using Method B of ASTM method E222-66). It should be noted, however, that term of a, co-dilithium-polymers with ethylene oxide resulted in gel formation which required 1-4 days for completion. In general, epoxides are not polymerized by lithium bases 333,334), presumably because of the unreactivity of the strongly associated lithium alkoxides641 which are formed. With counter ions such as sodium or potassium, reaction of the polymeric anions with ethylene oxide will effect polymerization to form block copolymers (Eq. (80) 334 336>). 

Ethylene Oxide Polymerization. The apparent inability of lithium bases to effect the anionic polymerization of ethylene oxide and Its homologues Is unique among the alkali metals. At least part of the unreactlvlty of lithium alkoxldes can be ascribed to their strong association In solution as shown In Table 1(18). However, It can be seen that the corresponding sodium and potassium alkoxldes are also highly associated In solution and yet they are active Initiators for the polymerization of ethylene oxide. 

Several factors influence the functionality and the solubility of the plurifunc-tional initiator, including the solvent and counter-ion. In polar solvents, the solubility of the polyfunctional initiator is greater, the ionic associations are minimized, and the rate of initiation is greater compared to nonpolar solvents. Divinylbenzene (DVB) was chosen as the bifunctional compound for the preparation of a plurifunctional initiator in nonpolar solvents [65]. Motivation for the development of the core-first method for the polymerization of oxi-ranes in polar solvents was driven by the need to rend available functional poly(ethylene oxide) star-shaped macromolecules. The initiator systems developed by Burchard are based on lithium as a counterion, which is why it could not be applied to the preparation of multifunctional PEOs under mild conditions. 

Quantitative Analysis of All llithium Initiator Solutions. Solutions of alkyUithium compounds frequentiy show turbidity associated with the formation of lithium alkoxides by oxidation reactions or lithium hydroxide by reaction with moisture. Although these species contribute to the total basicity of the solution as determined by simple acid titration, they do not react with allyhc and henzylic chlorides or ethylene dibromide rapidly in ether solvents. This difference is the basis for the double titration method of determining the amount of active carbon-bound lithium reagent in a given sample (55,56). Thus the amount of carbon-bound lithium is calculated from the difference between the total amount of base determined by acid titration and the amount of base remaining after the solution reacts with either benzyl chloride, allyl chloride, or ethylene dibromide. 

---