Polybutadiene structural units

Chemical methods for structure determination in diene pol3 mers have in large measure been superseded by infrared absorption techniques. By comparing the infrared absorption spectra of polybutadiene and of the olefins chosen as models whose ethylenic structures correspond to the respective structural units, it has been possible to show that the bands occurring at 910.5, 966.5, and 724 cm. are characteristic of the 1,2, the mns-1,4, and the m-1,4 units, respectively. Moreover, the proportion of each unit may be determined within 1 or 2 percent from measurements of the absorption intensity in each band. The extinction coefficients characteristic of each structure must, of course, be known these may be assigned from intensity measurements on model compounds. Since the proportions of the various units depend on the rates of competitive reactions, their percentages may be expected to vary with the polymerization temperature. The 1,2 unit occurs to the extent of 18 to 22 percent of the total, almost independent of the temperature, in free-radical-polymerized (emulsion or mass) poly butadiene. The ratio of trans-1,4 to cfs-1,4, however,... 

Similarly Stable and Posnjak (87) proposed cyclic formulas of four, five, or possibly more structural units for polystyrene. Lebedev (88) first assigned the cyclooctadiene structure to polybutadiene shown below before expanding the concept of the ring to include several isoprene units. 

The new absorptions in the spectra of crosslinked rubber are assigned on the basis of 13C solution NMR chemical shifts for a variety of model compounds, such as pentenes and mono-, di- and tri-sulfidic compounds, by using the 13C chemical shift substituent effect. From the calculated values for particular structural units, the experimental spectra of a sulfur vulcanized natural rubber 194,195,106), natural rubber cured by accelerated sulfur vulcanization 197 y-irradiation crosslinked natural rubber198 and peroxide crosslinked natural rubber and cis-polybutadiene 193 1991 are assigned. 

Polybutadiene formed by high-temperature, free-radical addition polymerization is a copolymer of these three kinds of structural units. With isoprene (2-methyIbutadlene), the number of ways the unit can enter the polymer chain is still larger for example, the... 

Table 2 gives the structural composition of polybutadiene and polyisoprene obtained by free-radical polymerization [46-48]. The fact that the content of the vinyl structure units in polydienes is independent of the polymerization temperature over a wide range and that their content is much higher than the... 

Metal Structure of the valence shell of the metal n-allyl complex Polybutadiene structure, % of units ... 

In polybutadiene, a substituted cyclopentane structural unit has been identified for polymerization carried out at low concentration. It was suggested that this unit is due to an intramolecular cyclization involving a penultimate 1,2-unit. 

For all catalysts, the cw-1,4 structure units of the polybutadiene range between a content of 74 and 85.8%, the trans-, A between 0.5 and 4.2%, and the 1,2-units between 13.7 and 22.6% (Table 7). The most active systems generate the polymer with the highest content of cis-, A and the lowest content of trans-, A and 1,2-units. The fluorinated compounds show a similar behavior. A mechanism for the formation of these microstructures is published by Porri [192]. 

Typical of epoxidized diene polymers are the products obtained by treatment of polybutadiene (Chapter 18) with peracetic acid. Such materials contain a variety of structural units as illustrated in the following representation of a polymer segment CHj... 

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. 

Butadiene can form three repeat units as described in structure 5.47 1,2 cw-1,4 and trans-, A. Commercial polybutadiene is mainly composed of, A-cis isomer and known as butadiene rubber (BR). In general, butadiene is polymerized using stereoregulating catalysts. The composition of the resulting polybutadiene is quite dependent on the nature of the catalyst such that almost total trans-, A, cis-, A, or 1,2 units can be formed as well as almost any combination of these units. The most important single application of polybutadiene polymers is its use in automotive tires where over 10 t are used yearly in the U.S. manufacture of automobile tires. BR is usually blended with NR or SBR to improve tire tread performance, particularly wear resistance. 

Polyisoprene is composed of four structures as shown in Equation 5.48. As in the case of polybutadiene, it is the cis-, A structure that is emphasized commercially. The cA-1,4-polyisoprene is similar to the cw-l,4-polybutadiene material except it is lighter in color, more uniform, and less expensive to process. Polyisoprene is composition-wise analogous to NR. The complete cw-1,4 product has a Tg of about —71°C. Interestingly, isomer mixtures generally have higher Tg values. Thus, an equal molar product containing cA-1,4 trans-, A, and 3,4 units has a Tg of about —40°C. 

Charge distribution on butadiene one unit model and % 1,2 structure in polybutadiene. 19... 

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