SYNOPSISLithium diisopropylamide is an efficient initiator for the anionic polymerization of methyl methacrylate in polar solvents at -78°C. Predictable molecular weights based on the initiator concentration were obtained, yields were quantitative, and molecular weight distributions were relatively narrow ( <1.30). The presence of an amino end group and confirmation of the initiation mechanism were confirmed by potentiometric titration using HC104. Numberaverage molecular weights determined by titration agreed well with values determined by SEC. Other lithium dialkylamides that contain larger alkyl groups such as lithium diisobutylamide were less efficient initiators. This was attributed to the more nucleophilic anion due to less steric hindrance near the nitrogen atom. Molecular weight distributions were significantly broadened (>2.0), and molecular weight control was not achieved. However, polymer yields were quantitative. In all cases, PMMA stereochemistry was indicative of a solvent-separated lithium counterion, and triad compositions were identical to organolithium initiated homopolymers, i.e., 78% syndiotactic, 20% heterotactic, and 2% isotactic. 0 1994 John Wiley & Sons, Inc.

SYNOPSISWholly aromatic polyamides, including poly ( 1,4-phenyleneisophthalamide) and poly ( 1,4-phenyleneterephthalamide) are N-methylated to render them soluble in N,N-dimethylformamide, a common size-exclusion chromatography eluent. The procedure N-methylates 50-55% of the total amide linkages in these two examples, permitting reproducible measurement of their absolute molecular weight distributions using differential viscometry detection. There is no observable degradation in molecular weight resulting from the Nmethylation, provided the excess methylating reagent is destroyed shortly after completion of the derivatization by quenching with potassium acetate. The validity of the molecular weight data obtained by the method is confirmed by light-scattering measurements on derivatized polymer and by comparison to the molecular weight of underivatized polymer that can be approximated from the intrinsic viscosity in concentrated sulfuric acid. The method is applicable to a variety of wholly aromatic polyamide structures. Examples are given.

No abstract

No abstract