Poly dienes isomerization

The mechanism is complicated by the possibility of anti-syn-isomerization and by n - a-rearrangements (it - r 3-allyl Act - r 1 -allyl). In the case of C2-unsubstituted dienes such as BD the syn-form is thermodynamically favored [646,647] whereas the anti-isomer is kinetically favored [648]. If monomer insertion is faster than the anti-syn-rearrangement the formation of the czs- 1,4-polymer is favored. A higher trans- 1,4-content is obtained if monomer insertion is slow compared to anti-syn-isomerization. Thus, the microstructure of the polymer (czs-1,4- and frazzs-1,4-structures) is a result of the ratio of the relative rates of monomer insertion and anti-syn-isomerization. As a consequence of these considerations an influence of monomer concentration on cis/trans-content of BR can be predicted as demonstrated by Sabirov et al. [649]. A reduction of monomer concentration results in a lower rate of monomer insertion and yields a higher trans-1,4-content. On the other hand the czs-1,4-content increases with increasing monomer concentration. These theoretical considerations were experimentally verified by Dolgoplosk et al. and Iovu et al. [133,650,651]. Furthermore, an increase of the polymerization temperature favors the formation of the kinetically controlled product and results in a higher cis- 1,4-content [486]. l,2-poly(butadiene) can be formed from the anti- as well as from the syn-isomer. In both cases 2,1-insertion occurs [486]. By the addition of electron donors the number of vacant coordination sites at the metal center is reduced. The reduction of coordination sites for BD results in the formation of the 1,2-polymer. In summary, the microstructure of poly(diene) depends on steric factors on the metal site, monomer concentration and temperature. 

Experimentally, the chemical shifts of both [68] and [69, 70] nuclei in poly (1,4-butadiene) are found to depend almost entirely only on the structure at the monomer level. In the CH2 region of the spectrum, cis units appear at 27.4ppm and trans units at 32.8 ppm [69]. In the olefinic region, there is a slight dependence on sequence structure [70] peaks from cis units at 128.8 ppm and trans units at 129.35 ppm are each split into a doublet of about 0.1 ppm due to a slight sensitivity to dyad structure. It should be noted that analysis of the spectra of poly(dienes) in terms of cis/trans isomerism is severely complicated by the occurrence of 1,2- as well as 1,4-addition. 

The all-trans-all-isotactic and all-trans-all-syndiotactic structures for the 1,4-polymerization of 1,3-pentadiene are shown in Fig. 8-6. In naming polymers with both types of stereoisomerism, that due to cis-trans isomerism is named first unless it is indicated after the prefix poly. Thus, the all-trans-all-isotactic polymer is named as transisotactic l,4-poly(l,3-penta-diene) or isotactic poly( -3-methylbut-l-ene-l,4-diyl). 

A novel cure chemistry employed for addition poly(imides) has recently been published. The successful preparation of 4-aminobenzocyclobutene allowed the synthesis of benzocyclobutene-terminated imide oligomers and bisfbenzocylobutenes) (17). The benzocyclobutene group is a latent diene which isomerizes to o-guinodimethane at temperatures of about 200 °C and may homo- and/or co-polymerize for example with bismaleimide (83). Details on the benzocyclobutene chemistry are described in chapter I of this book. 

Kennedy, J. P. and Chou, T. Poly(isobutylcnc-co-(J-Pinenc) A New Sulfur Vulcanizable, Ozone Resistant Elastomer by Cationic Isomerization Copolymerization. Vol. 21, pp. 1-39. Kennedy, J. P. and Delvaux, J. M. Synthesis, Characterization and Morphology of Poly(buta-diene-g-Styrene). Vol. 38, pp. 141-163. 

The reader should note that stereoisomerism does not exist if the substituents X and Y in the monomer 4-14 are identical. Thus there are no configurational isomers of polyethylene, polyisobutene, or polyfvinylidene chloride). It should also be clear that 1,2-poly-butadiene (reaction 4-3) and the 1,2- and 3,4-isomers of polyisoprene can exist as isotactic, syndiotactic. and atactic configurational isomers. The number of possible structures of polymers of conjugated dienes can be seen to be quite large when the possibility of head-to-head and head-to-tail isomerism is also taken into account. 

Cyclooctadiene (1,5-COD) is converted selectively to cyclooctene (97.8% on the addition of 1 mol of H2 per mol of diene) over a colloidal Pd supported on poly(N-vinyl-2-pyrrolidone) in methanol. The reaction is in part direct and in part proceeds through isomerization to the 1,4-COD, which is observed, and, presumably, through 1,3-COD, which is not observed. 1,3-COD is the most reactive and selective of the three dienes at its complete conversion it yields 99.9% cyclooctene. Nishimura discovered that the reduction of 1,5-COD is highly selective over Pd/CaCOa or Pd black in the presence of phenylacetalde-hyde with almost complete suppression of the reduction of cyclooctene. The inhibition is probably due to the decarbonylation of the aldehyde the adsorbed CO competes with the cyclooctene for surface sites. ... 

It thus appears that the principal structural features found In poly(1,4-dimethylenecyclohexane) can be explained by conventional carbonium ion chemistry. There is no indication that cyclopolymerization occurs in these polymerizations and there is much evidence to indicate that the double bonds present in this diene react independently. Some of them are involved in polymerization reactions, but a large proportion isomerize to relatively stable endocyclic double bonds. Under polymerization conditions where isomerization is favorable, soluble, unsaturated polymers having complex structures are obtained. When isomerization reactions are not favorable (low temperatures, use of Ziegler-Natta catalysts), the double bonds polymerize independently and crosslinked products are obtained. 

Hydrocarbon Solvents One of the most important synthetic and commercial aspects of anionic polymerization is the ability to prepare polydienes [poly(l,3-dienes)] with high 1,4-microstructure using lithium as the counterion in hydrocarbon solutions [3, 156]. The key discovery was reported in 1956 by scientists at the Firestone Tire and Rubber Company that polyisoprene produced by lithium metal-initiated anionic polymerization had a high (>90%) cm-1,4-microstructure similar to natural rubber [47], In general, conjugated 1,3-dienes [CH2=C(R)-CH=CH2] can polymerize to form four constimtional isomeric microstructures as shown below. The stereochemistry of the anionic polymerization of isoprene and... 

The thermal reaction of cis or trans 1,4-poly(butadiene) with iron carbonyls results in geometrical isomerization of the alkenyl moieties and formation of polymers containing conjugated diene) iron tricarbonyl units. In the course of our continuing studies of the formation of stable colloidal iron dispersions by thermal decomposition of metal carbonyls in the presence of functional polymers, some aspects of this work have been repeated. Our objective was to isolate the soluble organometallic polymer which was intermediate to particle nucleation and independently examine the intramolecular condensation of metal atoms to yield metal clusters and metal particles. In this paper, the structure of the intermediate obtained on thermolysis of an excess of FeCCO) in a dilute xylene solution of c/5-poly(butadiene) has been described. 

C/5-2,6 octadiene would be the most appropriate small molecular model for the repeat unit in m-poly(butadiene). However, in light of the fact that FeCCO) catalyzes double bond migration and isomerization any isomeric octadiene monomer should serve as an appropriate model for the polymer reaction. We reacted 1,7-octadiene with FeCCO) and obtained a simple infrared spectrum which was nearly identical to that for the reaction product of c/5-poly (butadiene) with Fe(C0)5, i.e., both materials displayed two discrete peaks in the carbonyl region, a sharp peak at 2050 cm and a broad absorption ( "30 cm HWHH) centered at 198O cm" (See Fig. 2). The broad unresolved character of the 1980 cm" band, as compared to that in rj -butadiene iron tricarbonyl which shows discrete sharp peaks at 1980 and 1990 cm in addition to the sharp peak at 2056 cm", can be understood in terms of the variety of isomeric diene iron tricarbonyls which can be formed in the reaction of FeCCO) with either c/5-poly(butadiene) or 1,7-octadiene. 

Diene type polymers, prepared by either free radical or anionic methods, contain chain units that although chemically identical are isomeric to one another. Hence, from a crystallization point of view this class of polymers behave as copolymers. For example, polymers prepared from the 1,3-dienes are subject to several different kinds of chain irregularities. For poly (butadiene), the following structures are known to exist ... 

In this example 2-vinylbutadiene acts as both the diene and the dienophile to give a poly cyclohexene. (For clarity, only one of the possible isomeric structural units is shown.)... 

Since 1,4-polyisoprene has a secondary carbon atom at the double bond it follows that it is generally more reactive to both free radicals and to carbonium ions than 1,4-poly butadiene. The typical addition reactions associated with the double bond suggest that the ultimate hydrogenated, halogenated, hydrohalogenated and isomerized diene polymers would have the same structure irrespective of the initial cis-ltrans- ratio. 

The importance of repeat unit isomerism in poly (1,3-dienes) is very clearly demonstrated by the naturally-occurring polyisoprenes. Gutta... 

Multi-component reactions open a direct access to poly-functional derivatives/ For instance, reactions of unsaturated substrates and aldehydes in the presence of EtaSiH opened a rapid access to allyllic, homo-allylic or co-unsatu-rated alcohols, and NHC-Ni complexes were recently reported to be efficient catalysts for such oxidative coupling. Mori and co-workers described the first example of such condensation in 2001. They showed that an in situ generated IPr/Ni 1 1 complex led to the stereoselective formation of (Z)-homoal-lylic silyl alcohols [eqn (10.33)], while classical Ni -PPhj catalyst produced only (Ej-coupling products. Of note, IPr/Ni 1 2 and IMes/Ni 1 2 were less efficient in this reaction. These first results were extended to silylated dienes. Interestingly, it was shown that a mixed complex [(IPr)Ni(PPh3)] stereoselectively produced (Z)-allylsilanes while [Ni(PPh3)4] led only to (Ej-isomeric products. 

In most cases irradiations constitute only another way of access to modifications which can be carried out chemically. Thus cis-trans isomerization of poly-1,4-dienes (see Section 1.3.1) can be obtained by UV irradiation in the presence of photoinitiators. 

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