professor Binghe Wang, she produced the amide with a primary or secondary amine with the aid of DCC and DMAP. On ultraviolet irradiation, the amide cyclizes to coumarin with expulsion of the amine. In another approach, chemist Mikhail Lebedev of SynPhar has developed methoxymethyl methyl sulfate as a convenient reagent to apply a methoxymethyl group to alcohols or carboxylic acids. That reagent avoids toxic halomethyl methyl ethers that are often used now. Lebedev made the reagent by treating commercially available dimethoxymethane (DMM) with sulfur trioxide at -30 °C in either methylene chloride or excess DMM as solvent. The sulfate ester is stable enough to distill at 79 to 81 °C at 1 torr. He demonstrated u$e of the reagent by making methoxymethyl ethers of ethyl and isopropyl alcohols and the ester of ampicillin by treating the substrate with the sulfate ester and triethylamine in methylene chloride or dimethylformamide solvent. Stephen Stinson
Polyesterification mechanism study
Polyesterffication mechanism starts as acetolysis . . . ,C0 2 H
^^^i^N^C°2H
+ CH3C02H
. . . then changes via decarboxylation . . .
M
„C02H
+ co2
* V
O
. . . and pyrolysis8 . . . 0
A, H,cr ^o
-
0=C=CH2
HOXO:
. . . to phenolysisb
From the ACS meeting The mechanism of a polyesterification reaction used commercially turns out to be more complicated than it appears. Chemists at the University of Arizona, Tucson, and Hoechst Celanese report that the mechanism of copolyesterification of 4-hydroxybenzoic acid and 6-hydroxy-2naphthoic acid changes over the course of the reaction. Hoechst Celanese markets this liquid-crystal polyester under the trade name Vectra, and new information about mechanisms involved may change plastics technologists' thinking about the resin. One result of the mechanistic change is evolution of phenol, which may serve as a high-boiling plasticizer in the resin. And because the mechanistic change involves the 4-hydroxybenzoic acid monomer whenever it is an end group, the sequence distribution of hydroxy acid units may be affected. Chemistry professor Hank K. Hall Jr. described the effort to the Division of Polymer Chemistry. The team—which includes postdoctoral fellows Xinghua Han and Paul A. Williams and laboratory coordinator Anne Buyle Padias at Arizona, and research chemists Clay C. Linstid,
n
HO
o
- ^V • 6 o
a Not shown is pyrolysis of acetoxynaphthoyi units to naphthois. b Naphthois can also transesterify with phenyl esters.
Ho-Nan Sung, and Cherylyn C. Lee at Hoechst Celanese—has been studying reaction of 4-acetoxybenzoic acid and 6-acetoxy-2-naphthoic acid with removal of acetic acid. In commercial production, the acetoxy acids are gradually heated from 195 to 325 °C under nitrogen. A vacuum is applied when the temperature reaches 325 °C. In the current research, the investigators pass nitrogen through the reaction vessel and through a series of three traps. They keep the first trap at -10 °C to capture such products as acetic acid, phenol, and acetic anhydride. The second trap contains aniline to catch ketene in the form of solid acetanilide, which can be weighed. And the third trap contains barium hydroxide solution to collect carbon dioxide. As a result of quantitative determinations of the trapped compounds and studies on model compounds like 4-benzoyloxybenzoic acid and 6-benzoyloxynaph-
thoic acid, the researchers come to a better understanding of the course of the polymerization. Until the polymerization is about 70% complete, it proceeds by acetolysis of the acetoxy acids. This accounts for acetic acid and acetic anhydride in the one trap. At higher temperatures, pyrolysis of acetoxy groups produces ketene, freeing hydroxyl end groups. And 4-oxybenzoic acid end groups—but not 6-oxy-2naphthoic acid groups—undergo decarboxylation. This results in an end group that is a phenyl ester of the monomer second from the end. These phenyl ester end groups can condense with hydroxyl end groups of other chains to lengthen the polymer with expulsion of phenol. Thus at the end of the polymerization, the kinetics and mechanism change from faster acetolysis to slower phenolysis for final formation of high polymer resin. Inclusion of phenol in the resin explains colored impurities. Stephen Stinson APRIL 28, 1997 C&EN 29
