Polybutadiene monomeric units

Butadiene and isoprene have two double bonds, and they polymerize to polymers with one double bond per monomeric unit. Hence, these polymers have a high degree of unsaturation. Natural rubber is a linear cis-polyisoprene from 1,4-addition. The corresponding trans structure is that of gutta-percha. Synthetic polybutadienes and polyisoprenes and their copolymers usually contain numerous short-chain side branches, resulting from 1,2-additions during the polymerization. Polymers and copolymers of butadiene and isoprene as well as copolymers of butadiene with styrene (GR-S or Buna-S) and copolymers of butadiene with acrylonitrile (GR-N, Buna-N or Perbunan) have been found to cross-link under irradiation. 

Catalysts based on 7r-allylic derivatives of transition metals supported on alumina, silica or silica-alumina gels exhibit generally enhanced activity by comparison with their unsupported counterparts, while the stereospecificity depends on the nature of the catalyst carrier. For instance, Cr(All)3, which predominantly produces 1,2-polybutadiene [137], becomes a stereospecific catalyst for the formation of trans- 1,4-polybutadiene when supported on silica or silica-alumina gel and for the formation of cis- 1,4-polybutadiene when supported on alumina [148]. However, an increase in the content of cis-1,4 monomeric units in polybutadiene with increasing silica concentration in n-allylnickel-alumina-silica catalysts has been observed [149]. 

The anti > syn isomerisation is also responsible for a decrease in the content of cis-1,4 monomeric units and the simultaneous increase in the trans-1,4 content of polybutadienes derived from polymerisation with several Ti-, Co- and Ni-based... 

Catalyst complexation with a Lewis base or other electron donor may affect the polymer microstructure in different ways. If the added component occupies one coordination site, a monomer coordinates to another site of the active species with one double bond, i.e. as an s-trans-rf ligand, which gives rise to the formation of trans-1,4 monomeric units via the pathway (a)-(b) [scheme (10)]. Depending on the lifetimes of metal species complexed with the monomer and with the Lewis base or the other donor [scheme (11)], mixed cis-1,4/trans- 1,4-polybutadienes or an eb-czs-1, 1 A trans-1,4-polymer can be formed. One should mention in this connection that equibinary cis-l,A/trans- 1,4-butadiene polymers can also be formed in systems without the addition of a Lewis base or other electron donor in such cases, the equilibrium of the anti-syn isomerisation is not shifted and there are equal probabilities for the reaction pathways involving coordination of a transoid monomer and a cisoid monomer [7]. 

Explain why polybutadienes obtained with rare-earth metal-based catalysts exhibit a higher degree of stereoregularity (higher contents of cis-1,4 monomeric units) than those derived from polymerisations in the presence of transition metal-based catalysts. 

Characteristic pseudo-solid spin-echoes recorded from polybutadiene are shown in Fig.2 the polymer concentration in deuterated toluene was 0.88 (w/w). The fraction of monomeric units in the ds conformation was 0.96 (M =110,000 g/mol.). 

It is of interest to point out that polypentenamer (obtained by ring opening polymerization of cyclopentene) that possesses a structure similar to that of 1,4-polybutadiene, with an extra methylene group in each monomeric unit, also undergoes cis—trans isomerization [47]. However, vinyl group formation according to... 

The monomeric unit in polybutadiene can exist in three distinct forms 1,4-cis, 1,4-trans, and 1,2-vinyl (see Fig. 4). The relative portions of each one of these... 

Besides the positional isomerism described above, one can encounter another form of isomerism in homopolymers as illustrated by the polymerization of butadiene. Either 1,2 or 1,4 monomeric units may be incorporated in the polybutadiene chain, depending upon the experimental conditions used ... 

Depending on the selected method, the polymerization of conjugated dienes can lead to various isomers of the monomeric units shown hereafter depending on whether the residual double bonds are localized either laterally or in the main chain, one can obtain polyvinyl (iso- and syndio-) tacticity or geometrical type isomerisms, respectively, that are presented hereafter. They exhibit different chemical and physicochemical properties. Actually, polybutadienes contain variable proportions of each one of these structures and can thus be assimilated to statistical copolymers whose properties vary continuously with respect to their composition. 

Butadiene is used primarily in the production of synthetic rubbers, including styrene-butadiene rubber (SBR), polybutadiene nibber (BR), styrene-butadiene latex (SBL), chloroprene rubber (CR) and nitrile rubber (NR). Important plastics containing butadiene as a monomeric component are shock-resistant polystyrene, a two-phase system consisting of polystyrene and polybutadiene ABS polymers consisting of acrylonitrile, butadiene and styrene and a copolymer of methyl methacrylate, butadiene and styrene (MBS), which is used as a modifier for poly(vinyl chloride). It is also used as an intermediate in the production of chloroprene, adiponitrile and other basic petrochemicals. The worldwide use pattern for butadiene in 1981 was as follows (%) SBR + SBL, 56 BR, 22 CR, 6 NR, 4 ABS, 4 hexamethylenediamine, 4 other, 4. The use pattern for butadiene in the United States in 1995 was (%) SBR, 31 BR, 24 SBL, 13 CR, 4 ABS, 5 NR, 2 adiponitrile, 12 and other, 9 (Anon., 1996b). 

---