Butadieno

Solo disponible en BuenasTareas
  • Páginas : 54 (13315 palabras )
  • Descarga(s) : 0
  • Publicado : 10 de enero de 2011
Leer documento completo
Vista previa del texto
Vol. 5

BUTADIENE POLYMERS

317

BUTADIENE POLYMERS
Introduction
1,3-Butadiene was first prepared in 1863, and its conjugated structure was proposed in 1886 (1,2). However, the ability of butadiene to polymerize was not recognized until almost 50 years later when, in 1909, a rubbery polymer was first reported as being prepared from butadiene via thermal polymerization (3). Shortlythereafter, the more controlled polymerization of butadiene initiated by sodium metal was reported in 1911 (4). During this time period, a sharp rise in natural rubber prices prompted the Bayer Corp. to develop methyl rubber from 2,3-dimethylbutadiene. Though interest in synthetic rubber faded after World War I, in 1926 a rise in price of natural rubber prompted the German company I. G. Farbenindustrie toresume research on the sodium-initiated polymerization of butadiene. This work eventually led to the German commercialization of two synthetic rubbers: Buna 32 and Buna 115 (from butadiene and natrium). Concurrently, in the 1920s research on the emulsion polymerization of butadiene was being carried out in Germany and the United States. The first butadiene–styrene copolymer prepared from anemulsion polymerization (Buna S) at I. G. Farbenindustrie proved to be superior to polybutadiene (5). From this work, Buna-N, a copolymer of butadiene and acrylonitrile, was developed for its solvent and oil resistance. Although the products of this work were inferior to natural rubber, their technology, with considerable modification and improvement, formed the basis for synthetic rubber production (GR-Sand GR-N) in the United States. Under the government-established Rubber Reserve in World War II, GR-S and SBR became a general-purpose rubber with an annual production of ca 717,700 t in 1945.

Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

318

BUTADIENE POLYMERS

Vol. 5

Butadiene Monomer
1,3-Butadiene, CH2 CH CH CH2 , is thesimplest conjugated diene, and its structure has received much theoretical attention because of its symmetry and simplicity (6). Heats of hydrogenation and combustion reveal that the stabilization contributed by the conjugation of the double bonds is 12.5–14.6 kJ/mol (3–3.5 kcal/mol) (7). Electron diffraction reveals a planar molecule with bond lengths C C, 0.1337 nm; C C, 0.1483 nm; and C H,0.1082 nm; the bond angles are C C C 122.4◦ and C C H 119.8◦ (8). The values for the C C bond lengths predicted by valence bond approximations are in close agreement with these observed figures (9). The C C single bond between the two double bonds is shorter than the usual 0.154 nm of an isolated single C C bond. Some of the proposed explanations for this shortened bond, which indicates some doublebond character, can be expressed in polar resonance or terminal diradical structures with some long bond or interaction between the end carbons. However, because the energies of these structures are high, molecular orbital calculations indicate that there is little resonance in the ground state and that the bond lengths are determined by the state of hybridization of carbon (10). The resistance torotation about the central bond is attributed to π-conjugation and leads to two conformers, the nonpolar s-trans and polar s-cis form as seen in the conformational equation (1).

(1) Although at dry ice temperature the s-cis form predominates (11), chemical (12) and spectroscopic (13,14) evidence suggests that s-cis-butadiene is present to the extent of only 3% at room temperature. The energydifference between the two forms has been variously determined as 7.1 ± 2.1 kJ/mol (1.7 ± 0.5 kcal/mol) (15) and 9.6 kJ/mol (2.3 kcal/mol) (16). The ultraviolet spectrum of gaseous butadiene is highly complex, but the origin of each of the transitions in the 230–135 nm region has been identified (17). In hexane solution butadiene absorbs at λmax 217 nm, = 21,000 (18). The bathochromic effect of...
tracking img