Nature of the active centers and the propagation mechanism of the polymerization of .beta.-propiolactones initiated by potassium anions

Abstract

15) Feijoo, J. L.; Ungar, G.; Owen, A. J.; Keller, A.; Percec, V. Mol. (16) Percec, V.; Asami, K., unpublished data. (17) Keller, A.; Ungar, G.; Yourd, R.; Percec, V., unpublished data.ABSTRACT The chemical structure of the active centers formed during the polymerization of P-propiolactones initiated by potassium anions has been elucidated on the basis of the results of spectroscopic ('H NMR, IR), chemical, and elemental analyses. In the initial step of propagation, two types of active species, carboxylate and alkoxide anions, are formed. The alkoxide anions disappear during the polymerization, and carboxylate anions are eventually the main propagation species.ABSTRACT: The changes in morphology due to the formation of polyethylene in the pores of three silica-supported, chromium oxide catalysts were followed by using mercury porosimetry and electron microscopy. Ethylene polymerization from 0.1 to 20 g of polymer/g of catalyst was carried out from the gas phase in a fluid bed reactor a t 1-atm total pressure with a nitrogen diluent. A catalyst with 1.7 cm3/g pore volume fragmented due to the formation of polymer in the pores and thereby maintained an open structure. Catalysts containing 1.1 and 2.3 cm3/g pore volume did not fragment extensively, and the product polymer congested the pores and impeded the continued polymerization. Total pore volume and pore size are not the only controlling factors in the fracturing process. Mercury porosimetry showed that fracture of the 1.7 cm3/g catalyst started after a polymer yield of just 0.4 gpE/geat, maintaining monomer access to the active sites. The O.l-l-pm catalyst fragments contained a pore microstructure much like that of the starting material, thus demonstrating how the pore structure of the original catalyst particles may influence the polymerization process after fragmentation is complete.

Structure and morphology of poly(?-hydroxybutyrate) synthesized by ring-opening polymerization of racemic (R,S)-?-butyrolactone with distannoxane derivatives

“…The synthesis of poly(β‐hydroxybutyrate) (PHB) from ( R,S )‐β‐butyrolactone (β‐BL) is used as a route to the preparation of stereochemical and structural isomers of this polyester, which will be studied for their biodegradation behaviour. Synthetic analogues of material‐origin PHB can be prepared by ring‐opening polymerization of β‐BL using various organometallic catalysts 4–12. Ring‐opening polymerization of the β‐BL ring for the preparation of poly(( R,S )‐β‐hydroxybutyrate) allows modulation of the polymer chain stereochemistry, which is not possible through the biosynthetic route where only R stereochemistry has been observed.…”

Syndiospecific ring-opening polymerization of .beta.-butyrolactone to form predominantly syndiotactic poly(.beta.-hydroxybutyrate) using tin(IV) catalysts

“…An excellent example of such work is that which has been carried out for the preparation of the poly(¡3-hydroxybutyrate) (PHB) from butyrolactone (BL) using various organometallic species to catalyze the ring-opening polymerization. [1][2][3][4][5][6][7][8][9] The corresponding natural polymer (fi)-PHB is a member of a family of poly(jS-hydroxyalkanoates) (PHAs) that are formed by a wide variety of bacteria.10 An important consideration for the purposes of this paper is that the natural polymer has to date been found exclusively in the enantiomerically pure form where the stereocenters are in the (fi) configuration. Clearly, an important advantage associated with the ring-opening route to PHB and other PHAs is the possibility of preparing these polymers in a variety of stereochemical forms having tailored physical and biological characteristics.…”

Biomimetic polyesters and their role in ion transport across cell membranes.

“…The strong tendency to two electron transfer is due to the unusual oxidation state of potassium anion bearing on its outer s orbital two labile electrons shielded from the positive potassium nucleus by inner orbitals. Using S-enantiomer of b-butyrolactone as a monomer and potassium supramolecular complex as catalyst, enolate carbanion is formed as the first reactive intermediate which induces polymerization, yielding poly-(R)-3-hydroxybutanoate [15,16]. Direct evidence for two electron transfer from the supramolecular complex to the monomer is provided by 39 K NMR (Fig.…”