Polybutadiene high molecular weight

High molecular weight polybutadiene homopolymers are prepared conunercially with anionic catalysts and with coordinated anionic ones. Polybutadiene formed with sodium dispersions was prepared industrially in the former USSR, and perhaps might still be produced in that area today. This sodium-catalyzed polybutadiene contains 65% of 1,2-adducts. Most of the preparations by others, however, utilize either alkyllithium or Ziegler-Natta-type catalysts prepared with titanium tetraiodide or preferably containing cobalt. 

When butadiene is polymerized with lithium metal or with alkyllithium catalysts, inert solvents like hexane or heptane must be used to obtain high cw-1,4 placement (see Chapter 3). Based on NMR spectra, 1,4-polybutadiene formed with n-butyllithium consists of blocks of cw-1,4 units and trans-XA units that are separated by isolated vinyl structures  

The quantity of such units in the above polybutadienes is approximately 48-58% transAA, 33-45% cis-XA, and 7-10% 1,2 units. There is little effect of the reaction temperatures upon this composition. As described in Chapter 3, however, addition of Lewis bases has a profound effect. Reactions in tetrahydrofiiran solvent result in 1,2 placement that can be as high as 87%. 

The microstructures of polybutadienes prepared withZiegler-Natta catalysts vary with catalyst composition. It is possible to form polymers that are high either in 1,2 placement or in 1,4 units. The catalysts and the type of placement are summarized in Table 5.7. 

Butadiene can be polymerized with chromium oxide catalyst on support to form solid homopolymers. The products, however, tend to coat the catalyst within a few hours after the start of the reaction and interfere with further polymerization. Polybutadiene can also be prepared in the presence of molybdena catalyst promoted by calcium hydride. The product contains 80% of 1,4 units and 20% of 1,2 units. Of the 1,4 units, 62.5% are cis and 37.5% are transP 

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The stress relaxation properties of a high molecular weight polybutadiene with a narrow molecular weight distribution are shown in Figure 1. The behavior is shown in terms of the apparent rubber elasticity stress relaxation modulus for three differrent extension ratios and the experiment is carried on until rupture in all three cases. A very wide rubber plateau extending over nearly 6 decades in time is observed for the smallest extension ratio. However, the plateau is observed to become narrower with increasing extension... 

No enhancement of concentrated solution viscosity was observed in high molecular weight polybutadienes with branching as might have been expected from the bulk viscosities measured by Kraus and Gruver (9). 

How are high molecular weight polybutadienes prepared and used ... 

Polydiene microstructure is also dependent on the ratio of concentrations of monomer to organolithium. This is true for both non-polar and polar media. As catalyst levels are reduced in a non-polar system, cis-l,4-values as high as 86% have been observed in high molecular weight polybutadienes prepared in the absence of any solvents. This compares with 96% cw-l,4-content in similarly prepared polyisoprenes. In the case of high-vinyl BRs (HVBRs) prepared with a polar modifier, such as DIPIP, a decrease in alkyllithium (RLi) concentration at constant ratio of DIPIP/RLi produces smaller amounts of vinyl structure. 

Table 2 Commercial sources and properties of high molecular weight polybutadienes... 

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