Vinyl content

Syndiotactic polybutadiene was fkst made by Natta in 1955 (28) with a melting point of 154°C. Syndiotactic polybutadiene [31567-90-5] can be prepared with various melting points depending on its vinyl content and degree of crystallinity. The physical, mechanical, and rheological properties of the polymer are gready affected by these parameters. 

Fig. 4. Effect of polymerisation temperature on vinyl content at a 2 1 modifier 1itbium ratio.

Rubbers containing traces of vinyl groups can be cross-linked by weaker peroxide catalysts, the reaction involving a vinyl group. It is, however, unlikely that vinyl-to-vinyl linking occurs. Where there is a high vinyl content (4-5% molar) it is possible to vulcanise with sulphur. 

SBS and SIS can be subsequently hydrogenated to form SEBS and SEPS, respectively. SEBS is obtained from SBS with a higher vinyl content (typically around 30%) in order to avoid crystallization of the mid-block. The properties of all four of these common styrenic block copolymers are displayed in Table 2. 

Polyisoprene can be UV or e-beam cured [43,44]. The 3,4 units are particularly prone to crosslinking at low dose [45]. SIS and SBS are also crosslinkable, even conventional linear materials with low vinyl content however, small amounts of liquid trithiol or triacrylate compounds speed cure dramatically [44]. Like UV, e-beam cure is strongly affected by tackifier choice. Hydrogenated, non-aromatic resins provide much less interference with cure [36,37]. 

By analogy to simple olefins, we propose that 0(3P) initially adds to the 1,4 or 1,2 double bonds in polybutadienes at ambient temperature. Since the rate constants for 0(3P) addition to cis-2-butene and 1-butene (as models for 1,4 and 1,2 double bonds, respectively) are in the ratio 4.2 1 at 298 K ( 6), preferential addition to the 1,4 double bonds is assumed to persist to very high vinyl contents (-8011). The biradical adducts then rearrange to epoxides and carbonyl compounds or give rise to chain rupture and/or crosslinking as a consequence of PIF, according to the scheme ... 

Ba-Li and Ba-Mg-Al, which provide unique steric control of the trans-1,4/cis-l,4 ratio of polybutadiene segments ( 6). Although Alfin rubbers ( 7) contain trans contents at 70-757., their vinyl contents are much greater (20-257.) than is characteristic of the new rubbers reported here. 

This barium salt contains about 9 mole 7, hydroxide ion. It is important to note that the vinyl content is not affected by an increase in the mole ratio of Ba/Li from 0 to 0.5. However, a further increase in the mole ratio above 0.5 results in a decrease in the trans-1,4 content and a corresponding increase in the vinyl content. 

Thus, this polymerization catalyst yields more random copolymers with styrene, while still maintaining a low vinyl content and high trans-1,4 content. 

The polybutadienes prepared with these barium t-butoxide-hydroxide/BuLi catalysts are sufficiently stereoregular to undergo crystallization, as measured by DTA ( 8). Since these polymers have a low vinyl content (7%), they also have a low gl ass transition temperature. At a trans-1,4 content of 79%, the Tg is -91°C and multiple endothermic transitions occur at 4°, 20°, and 35°C. However, in copolymers of butadiene (equivalent trans content) and styrene (9 wt.7. styrene), the endothermic transitions are decreased to -4° and 25°C. Relative to the polybutadiene, the glass transition temperature for the copolymer is increased to -82°C. The strain induced crystallization behavior for a SBR of similar structure will be discussed after the introduction of the following new and advanced synthetic rubber. 

Mole Ratio of Ba/Mg at Constant Mg/Al. Figure 10 shows the dependence of the microstructure of polybutadienes on the mole ratio of Ba/Mg with a Mg/Al ratio of 6/1 for polymerizations carried out in cyclohexane at 50°C. The amount of trans-1,4 structure is increased to a maximum of about 907. as the mole ratio of Ba/Mg is decreased from 1.0 to about 0.20. Concurrently, the vinyl content decreased from 77. to 27.. At a mole ratio of 0.05, polymerization was not observed in cyclohexane at 50°C after 3 days. 

Endlinked PDMS. Linear polydimethylsiloxanes with vinyl endgroups were supplied by Dow Corning Corporation. Three different molecular weight ranges were employed. Membrane osmometry yielded values for Mj, of 16,000, 24,000, and 37,000 g/g-mole. Endgroup analysis using mercuric acetate (5) gave vinyl contents of 0.47 0.03, 0.24 0.02, and 0.15 0.02 per cent, corres-... 

Polybutadiene is produced by solution polymerisation, and one important feature governing the performance of the resultant polymer is the cis 1,4, and 1, 2 vinyl contents. High cis 1,4 polymers (>90%) have a Tg around -90 °C, and hence exhibit excellent low temperature flexibility only exceeded by the phenyl silicones. They also exhibit excellent resilience and abrasion resistance unfortunately the high resilience gives poor wet grip in tyre treads, and hence this rubber finds limited use as the sole base for such compounds. 

As the cis 1,4 content decreases, and 1,2 vinyl content increases, the low temperature properties, abrasion resistance and resilience become inferior. 

Schwahn, D. Willner, L. Phase Behavior and Flory-Huggins Interaction Parameter of Binary Polybutadiene Copolymer Mixtures with Different Vinyl Content and Molar Volume. Macromolecules 2002,35, 239-247. 

Depending on the catalyst and process used, the microstructure of the polymer can be varied from a low to a high vinyl content (20-80% ). Increasing vinyl content raises viscosity and Tg and therefore should be kept low. Typical microstructure data for some prepolymer candidates are shown in Table I. 

Microstructure of Polybutadienes. Microstructure strongly influences the viscosity of the CTPB prepolymer. The viscosity of CTPB increases with increased vinyl content, but for CTPB prepolymers of the required molecular weight, an upper limit of 35% vinyl groups is satisfactory from the standpoint of propellant processing characteristics. It has also been found that the microstructure changes markedly with the synthesis process. Lithium-initiated polymerization yields prepolymers with slightly higher vinyl content than those produced by free radical initiation. 

It will be seen that there is an almost equal distribution of the charge between a and Y positions in THF for the heavier alkali metal counter-ions. If we suppose that increased charge produces an increased reactivity at a given position, then more vinyl unsaturation will be produced in THF than in hydrocarbon solvents and the highest vinyl content with heavier alkali metal counterions. The order in THF is however reversed, i.e. the highest, vinyl structures are produced by lithium catalysis (17) although microstructure determinations in this solvent normally apply to reactions with an appreciable free anion contribution and hence cannot be simply interpreted. In dioxane (18) and diethylether... 

Firestone and Shell started commercial production of cis-polyisoprene by the anionic process in the 1950 s but these plants are no longer in operation now. About the same time, Phillips started the manufacture of polybutadienes by the anionic route and ever since, there has been a steady growth in their use, particularly in the tire-tread as well as tire-carcass formulations. These solution polybutadienes, generally, have low vinyl contents but recently, Phillips has found some interesting applications for medium vinyl polybutadienes as well.14 Polybutadienes with 50-55 percent vinyl contents behave like emulsion polymerized SBR in tire tread formulations and exhibit very similar tread... 

The last property is related to the processing of the rubber in the tire making equipment. By using organo-lithium compound in this case, it was possible to maintain a vinyl content not greater than 18, but to produce a polybutadiene styrene copolymer that has random block styrene and without the use of polar modifiers, which normally will increase the 1,2 content. This copolymer, when compounded in the tread recipe, as shown in the Table XVI, gave properties that are actually equivalent to that of emulsion SBR and in some cases even better. This is particularly true in the properties of the Young modulus index, which showed between -38 to -54 C the Stanley London Skid Resistant, in which the control is 100, shows that 110-115 was obtained. 

Polymerization of butadiene with lithium morpholinide, an initiator with a built-in microstructure modifier, has been carried out in hexane. In general, the vinyl content of the polymers prepared with this initiator is dependent on the initiator concentrations and on the polymerization temperatures. This dependence is identical to that observed in a THF-modified lithium diethylamide polymerization initiator system. A comparison of these initiator systems for polymerization of butadiene is presented. In addition, a study of the effect of metal alkoxides on the vinyl content of lithium morpholinide initiated butadiene polymerization is included. 

To determine the concentration of cis-vinylene groups in irradiated specimens, the original cw-polybutadiene itself was used as a standard, and a value for A (assumed independent of concentration) was determined from Av and logr (I0/I)vo of unirradiated specimens. The total concentration of unsaturation was accepted (15) as 100% of theoretical and the density as 0.90 gram per cc. The cis concentration was obtained by subtracting from the total unsaturation the trans-vinylene and vinyl contents determined from the calibration curves developed as described below. The value of A so determined was 2.67 X 103 liters/mole-sq. cm. 

During the polymerization of the styrene, the poly(butadiene) or butadiene copolymer is grafted onto the styrene polymer chain. To increase the grafting efficiency of the poly(butadiene), it is desirable for the poly(butadiene) to have end segments having a high vinyl content rather than cis- or trans-configurations. 

The vinyl content refers to alkenyl groups configured pendant to the polymer backbone, as opposed to cis- or trans-configurations, which contain the alkenyl groups within the polymer backbone (12). 

In other words, the vinyl modifier increases the number of vinyl configured bonds that are formed during polymerization, hence leading to an increased vinyl content. The amount of 1,2-addition product can be increased from about the 5-15% up to 90-100%. 

However, if both ends of the polymer, or the ends of a star branched polymer should have a high vinyl content and the remaining segments should have a low vinyl content, this method will not work because coupling reactions result in high vinyl in the center of the polymer chain or star branched polymer. 

To produce linear or branched polymers with all ends having a high vinyl content, an organo lithium initiator can be used to polymerize a low vinyl middle segment. Subsequently, additional monomer and a vinyl modifier are added to produce ends having a high vinyl content. 

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