Potassium -butoxide, isotactic

Some of the polymers slowly change their helicity in solution. A chiral crown ether-potassium ferf-butoxide combined system was reported to cause polymerization of methyl, tert-butyl, and benzyl methacrylate to form isotactic polymers that had high rotation values (164). Detailed scrutiny, however, raised questions about the result (135, 165). At first, in the presence of the initiator, the oligomers exhibit considerable activity, but after removal of the catalyst, the optical activity decreases. This decrease may be attributed to unwinding of the helixes in the chain the helicity could be caused by the anchored catalyst. 

In a companion paper Price and Akkapeddi [22] report the kinetics of base initiated polymerization of epoxides in DMSO and hexamethyl-phosphoramide (HMPT). The initiator is potassium t-butoxide. Second order rate coefficients for (R,S)—PO were about double those for (+)—(R) or (—)—(S) monomer. They conclude that the steric factor favouring alternation of isotactic and syndiotactic placement of the t-BuEO also influences PO. Chain transfer to solvent (DMSO) was also studied. For PO polymerization in DMSO they obtain k = 1.5 x 10 exp(—17,200/RT). However, due to some erratic results they are not very confident about the accuracy. In HMPT rates are about three fold faster than in DMSO k = 7.3 X 10 exp(—16,300/RT). Three other epoxides were also studied in HMPT EO, k = 2.75 x 10 exp(-13,300/RT) t-BuEO, fe = 2.0 x 10 exp(-17,100/RT) phenylglycidyl ether (PGE), fc = 5.4 x 10" ... 

A further significant factor which comes to the fore in coordinate catalysis is stereoregular polymerization. A number of the catalyst systems are capable of producing isotactic, optically active and/or crystalline polymers. Except for Price s potassium t-butoxide system [21], this has not been observed in anionic or cationic polymerization. Thus, in addition... 

The stereoregularity of polystyrenes prepared by anionic polymerization is predominantly syndiotactic (racemic diad fraction P = 0.53-0.74) and the stereoregularity is surprisingly independent of the nature of the cation, the solvent, and the temperature, in contrast to the sensitivity of diene stereochemistry to these variables [3, 156]. The homogeneous alkyllithium-initiated polymerization of styrene in hydrocarbon media produces polystyrene with an almost random (i.e., atactic) microstructure for example, was 0.53 for the butyllithium/toluene system [3, 191, 192]. A report on the effect of added alkali metal alkoxides showed that polystyrene stereochemistry can be varied from 64% syndiotactic triads with lithium f-butoxide to 58% isotactic triads with potassium f-butoxide [193]. 

---