3.4- Polyisoprene

Polyisoprenes occur in nature. They are also prepared synthetically. Most commercial processes try to duplicate the naturally occurring material. 

Rubber hydrocarbon is the principle component of raw rabber. The subject is discussed in greater detail in Chap. 7. Natural rubber is 97% c -l,4 polyisoprene. It is obtained by tapping the bark of rubber trees (Hevea brasiliensis) and collecting the exudates, a latex consisting of about 32-35% rubber. A similar material can also be found in the sap of many other plants and shrubs. The structure of natural rubber has been investigated over 100 years, but it was only after 1920, however, that the chemical structure was elucidated. It was shown to be a linear polymer consisting of head to tail links of isoprene units, 98% bonded 1,4. 

Alkyllithium-initiated polymerizations of isoprene yield polymers with 92-93% cis-1,4 content. One industrial process uses butyllithium in a continuous reaction in two lines each consisting of four reaction kettles. The heat of the reaction is removed by vaporization of the solvent and the monomer. The catalyst solution is added to the solvent stream just before it is intensively mixed with the isoprene monomer stream and fed to the first reactor. After the stream leaves each reactor, small quantities of methanol are injected between stages into the reaction mixture. This limits the molecular weight by stopping the reaction. Fresh butyllithium catalyst is added again at the next stage in the next reactor to initiate new polymer growth [117-119]. 

As is described in Chaps. 3 and 4, the monomer placement into the polyisoprene chain can occur potentially in nine different ways. These are the three tactic forms of the 1,2 adducts, two 1,4 adducts, cis and tram, and three tactic forms of 3,4-adducts. In addition, there is some possibility of head to head and tail to tail insertion, though the common addition is head to tail. Table 6.8 presents the various microstructures that can be obtained in polymerizations of isoprene with different catalysts. 

Cationic polymerizations of isoprene proceed more readily than those of butadiene, though both yield low molecular weight liquid polymers. AICI3 and stannic chloride can be used in chlorinated solvents at temperatures below 0°C. Without chlorinated solvents, however, polymerizations of isoprene require temperatures above 0°C. At high conversions, cationic polymerizations of isoprene result in formations of some cross-linked material [120]. The soluble portions of the polymers are high in trans-, A structures. Alfin catalysts yield polymers that are higher in trans-l,A structures than free-radical emulsion polymerizations [121]. 

Polyisoprene (hydrogenated natural rubber) is a completely alternating etbylene propylene copolymer (i.e., does not have ethylene or propylene blocking) and is therefore an interesting substance for Py-GC studies. Tbe surface area of tbe main peaks up to C13 obtained by van Schooten and Evenhuis [13, 14] indicate that the unzipping reaction which would yield equal amounts of ethylene and propylene in the hydrogenated pyrolysate takes place to some extent, but is less important than the hydrogen transfer reactions  

The large numbered peaks produced upon Py-GC reflect the many possible transfer reactions for this polymer, some of which are illustrated below. Hydrogen transfer 

Hydrogen transfer also occurs from the ninth carbon atom, which is shown by the size of the 3-methyl nonane, 3,7-dimethyldecane and the 2,6-dimethylydecane peaks  

Polyisoprene is a thermoset elastomer and has high tensile properties and its main advantage over natural rubber is that it is generally easier to process. It is used for the manufacture of rubber bands, baby milk bottle teats, sporting goods and engine mounts. 

Cyanoacrylates show good adhesion to polyisoprene and primers are not usually required (Table 4.10). 

Cyclic peroxides are formed preferably in polyisoprene rather than in polybutadiene, because the mesomeric stabilization of allylic radicals is much less enhanced in polybutadienoid than in polyisoprenic sequences, on account of the lack of tertiary carbon in the former. 

4-polyisoprene (5.//6) can form two different allyl radicals 3.117) and (3.118) by a mechanism involving hydrogen abstraction (because of the asymmetry of the isoprene units [26, 237-250]  

In the presence of oxygen both allyl radicals form polymer peroxy radicals (POO ) and finally polymer hydroperoxides (POOH)  

Tertiary hydroperoxides are further decomposed to polymer oxy radicals and hydroxyl radicals  

The tertiary alkoxy radicals can be transformed into hydroxyl groups by a hydrogen abstraction reaction  

Due to the methyl group, opportunities for regioisomerism exists for all four types of unsaturation present in polyisoprene  

Head to head Tail to tail Head to tail  

Natural rubber is a stereoregular polymer composed of isoprene units attached in a cis configuration. This arrangement gives the rubber high resilience and strength. 

Isoprene can be polymerized using free radical initiators, but a random polymer is obtained. As with butadiene, polymerization of isoprene can produce a mixture of isomers. However, because the isoprene molecule is asymmetrical, the addition can occur in 1,2-, 1,4- and 3,4- positions. Six tactic forms are possible from both 1,2- and 3,4- addition and two geometrical isomers from 1,4- addition (cis and trans)  

Stereoregular polyisoprene is obtained when Zieglar-Natta catalysts or anionic initiators are used. The most important coordination catalyst is a-TiCls cocatalyzed with aluminum alkyls. The polymerization rate and cis 

BLEND STATION DRYING COLUMN REACTOR (SEVERAL) 

Polyisoprene is a synthetic polymer (elastomer) that can he vulcanized hy the addition of sulfur. cis-Polyisoprene has properties similar to that of natural ruhher. It is characterized hy high tensile strength and insensitivity to temperature changes, hut it has low abrasion resistance. It is attacked hy oxygen and hydrocarbons. 

Major polymer applications pressure-sensitive adhesives, ablatives 

Important processing methods mixing, vulcanization, extrusion, calendering, molding 

Typical fillers carbon black, zinc oxide, kaolin, calcium carbonate, silicates, titanium dioxide 

Special considerations zinc oxide is a curing agent 

4 TYPICAL FORMULATIONS Sealant with damping propeiTies  

Reactions of MA with both natural and synthetic rubber has had substan-tial study. Maleinated rubbers may be prepared by three techniques  

Thermal initiation for the rubber maleination reaction requires a temperature 160°C and preferably about 220-240°C, along with a considerable excess of MA. Maleates and maleimides also undergo thermal addition to rubber under the same conditions. The maleination reaction is promoted by the addition of a variety of initiators, such as BPO, AIBN, dibenzothlazil disulfide, and ascaridol however, oxygen inhibits the reaction. The 

Addition of monomer such as methyl methacrylate and styrene during mechanochemical modification helps retard and/or regulate gel formation. The additional vinyl monomer improves grafting of MA to the rubber. Styrene addition may be used to prepare rubber with pendent styrene-co-MA grafts, The reactions of rubber with MA under mechanical stress were shown to be independent of the oxygen content in the atmosphere from 20 to below 5% oxygen. 

It has been shown that natural rubber and MA reactions are best run in the presence of compounds such as 2,6-di-ter/-butyl-p-cresol, 6-phenyl-2,2,4-trimethyl-1,2-dihydroquinoline, and -dicarbonyl compounds as antioxidants.Both for the mass and solution production procedures at 180°C, these reagents inhibit crosslinking. Recent patents claim that triazine derivatives, such as 2,4,6-trichloro-5-triazine, and phosphorus compounds, such as trioctadecylphosphite, are also very effective for preventing crosslinking during treatment of liquid rubbers with MA. 

Natural rubber, due to the double bonds and 2-methylenic carbons, is highly sensitive to oxygen. It has been shown that the oxidizability of rubber 

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Figure C2.1.11. Morjrhologies of a microphase-separated di-block copolymer as function of tire volume fraction of one component. The values here refer to a polystyrene-polyisoprene di-block copolymer and ( )pg is tire volume fraction of the polystyrene blocks. OBDD denotes tire ordered bicontinuous double diamond stmcture. (Figure from [78], reprinted by pemrission of Annual Reviews.)...

Polymerization of isoprene by 1,4-addition produces polyisoprene that has a cis (or Z) configuration. 

As examples of natural polymers, we consider polysaccharides, proteins, and nucleic acids. Another important natural polymer, polyisoprene, will be considered in Sec. 1.6. 

Figure 1.3 shows several repeat units of cis-l,4-polyisoprene and trans-1,4-polyisoprene. Natural rubber is the cis isomer of 1,4-polyisoprene, and gutta-percha is the trans isomer. 

Figure 1.3 1,4-polyisoprene with R=CH3 (a) cis isomer natural rubber (b) trans... 

In the two polyisoprene isomers, the length of the repeat unit and the steric hindrance factor vary oppositely for the two isomers. The greater end-to-end distance in the trans isomer is the dominant influence on the order of 1q values. 

Figure 2.15 Log of viscosity enhancement factor versus parameter measuring branch length for polyisoprene, [Data from W. W. Graessley, T. Masuda, J. E. I. Roovers, and N. Hadjichristidis, Afacromo/ecu/ej 9 127 (1976).]...

The following are approximate (in dyne cm ) versus y datat for three different samples of polyisoprene in tetradecane solutions of approximately the same concentration ... 

Our time has seen The synthesis of polyisoprene And many cross-linked helixes unknown To Robert Hooke but each primoridal bean Knew cellulose by heart. . . ... 

Natural rubber, cis-1,4-polyisoprene, cross-linked with sulfur. This reaction was discovered by Goodyear in 1839, making it both historically and commercially the most important process of this type. This reaction in particular and crosslinking in general are also called vulcanization. 

Figure 3.12 Log-log plots of compliance versus time for polystyrene at 100 C and cis-polyisoprene at -30°C. (Data of D. J. Plazek and V. M. O Rourke and of N. Nemoto, M. Moriwaki, H. Odani, and M. Kurata from Ref. 4.)...

A diblock copolymer, 71% polyisoprene (1) by weight and 29% polybutadiene (B), was blended in different proportions into a 71%-29% mixture of the individual homopolymers. The loss tangent was measured as a function of temperature for various proportions of copolymer. Two peaks are observed ... 

Increasing the copolymer content of the system lowers the Tg of both peaks, but the effect is far more pronounced for the peak associated with I. The latter is essentially constant in height, while the B peak diminishes in height with increasing copolymer. This suggests that the copolymer dissolves preferentially in the polyisoprene. 

The following data were obtained on the same system described in Example 3.6. This time the copolymer (C) concentration is fixed at 25% by weight and the proportions of poly butadiene (B) and polyisoprene (I) are varied ... 

NBRin pLASTOMERS, SYNTHETIC - NITRILE RUBBER] (Vol 8) -cis-l,4-polyisoprene in pLASTOMERS SYNTHETIC - POLYISOPRENE] (Vol 9)... 

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