Diene Polymers Natural and Synthetic Rubber

Conjugated dienes can be polymerized just as simple alkenes can (Section 7.10). Diene polymers are structurally more complex than simple aikene polymers, though, because double bonds remain every four carbon atoms along the chain, leading to the possibility of cis-trans isomers. The initiator (In) for the reaction can be either a radical, as occurs in ethylene polymerization, or an acid. Note that the polymerization is a 1,4 addition of the growing chain to a conjugated diene monomer. 

As noted in Natural Rubber at the end of Chapter 7, rubber is a i urally occurring polymer of isoprene. The double bonds of rubber have stereochemistry, but gutta-percha, the E isomer of rubber, also occurs na urally. Harder and more brittle than rubber, gutta-percha has a variety ( minor applications, including occasional use as the covering on golf ba 

A number of different synthetic rubbers are produced commercially by diene polymerization. Both cis- and /rans-pol5dsoprene can be made, and the synthetic rubber thus produced is similar to the natural material. Chloroprene 2-chloro-l,3-butadiene) is polymerized to yield neoprene, an excellent, though expensive, synthetic rubber with good weather resistance. Neoprene is used in the production of industrial hoses and gloves, among other things. 

Sulfur cross-linked chains resulting from vulcanization of poiy-1,3-butadiene. 

Problem 14.7 Draw a segment of the polymer that might be prepared from 2-phenyl-1,3-buia-diene. 

Problem 14.12 Show the mechanism of the acid-catalyzed polymerization of 1,3-butadiene. 

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Classification of Polymers Free-Radical Chain-Growth Polymerization Cationic Chain-Growth Polymerization Anionic Chain-Growth Polymerization Stereoregular Polymers Ziegler-Natta Polymerization A WORD ABOUT... Polyacetylene and Conducting Polymers Diene Polymers Natural and Synthetic Rubber Copolymers... 

Rubber fibers from natural sources have been known for over 100 years. Natural rubber in commerce is derived from coagulation of Hevea brasilien-sis latex and is primarily cis-polyisoprene, a diene polymer. Most synthetic rubbers were developed during and following Worid War 11. They are crosslinked diene polymers, copolymers containing dienes, or amorphous polyolefins. Both the natural and synthetic rubbers must be crossl inked (vulcanized) with sulfur or other agents before true elastomeric properties are introduced. hi addition, accelerators, antioxidants, fillers, and other materials are added to the polymeric rabber prior to fiber formation. 

This discussion of the structures of diene polymers would be incomplete without reference to the important contributions which have accrued from applications of the ozone degradation method. An important feature of the structure which lies beyond the province of spectral measurements, namely, the orientation of successive units in the chain, is amenable to elucidation by identification of the products of ozone cleavage. The early experiments of Harries on the determination of the structures of natural rubber, gutta-percha, and synthetic diene polymers through the use of this method are classics in polymer structure determination. On hydrolysis of the ozonide of natural rubber, perferably in the presence of hydrogen peroxide, carbon atoms which were doubly bonded prior to formation of the ozonide... 

Diene polymers refer to polymers synthesized from monomers that contain two carbon-carbon double bonds (i.e., diene monomers). Butadiene and isoprene are typical diene monomers (see Scheme 19.1). Butadiene monomers can link to each other in three ways to produce ds-1,4-polybutadiene, trans-l,4-polybutadi-ene and 1,2-polybutadiene, while isoprene monomers can link to each other in four ways. These dienes are the fundamental monomers which are used to synthesize most synthetic rubbers. Typical diene polymers include polyisoprene, polybutadiene and polychloroprene. Diene-based polymers usually refer to diene polymers as well as to those copolymers of which at least one monomer is a diene. They include various copolymers of diene monomers with other monomers, such as poly(butadiene-styrene) and nitrile butadiene rubbers. Except for natural polyisoprene, which is derived from the sap of the rubber tree, Hevea brasiliensis, all other diene-based polymers are prepared synthetically by polymerization methods. 

Synthetic Rubber There are many different formulations for synthetic rubbers, but the simplest is a polymer of buta-1,3-diene. Specialized Ziegler-Natta catalysts can produce buta-1,3-diene polymers where 1,4-addition has occurred on each butadiene unit and the remaining double bonds are all cis. This polymer has properties similar to those of natural rubber, and it can be vulcanized in the same way. 

Though synthetic diene polymers have now replaced natural rubber in many applications, they too need to be cross-linked by vulcanization using essentially the same reactions, though the details vary from product to product and from company to company. 

We learned much from nature with these early attempts to produce useful polymer products based on modified, or reconstituted ( semisynthetic ) natural polymers, and many of these processes are still in use today. The first of the purely synthetic commercial polymers came with the small-scale introduction of Bakelite in 1907. This phenol-formaldehyde resin product was developed by Leon Baekeland. It rapidly became a commercial reality with the formation of The General Bakelite Company by Baekeland, and construction of a larger plant at Perth Amboy, New Jersey, in 1910. At about this time styrene was being combined with dienes in the early commercialization of processes to produce synthetic rubber. Polystyrene itself was not a commercial product in Germany until 1930 and in the U.S.A. in 1937. The only other purely synthetic polymers that made a commercial appearance during this early development period were polyvinyl chloride and polyvinyl acetate, both in the early 1920s. 

Both classes of polymers were attacked simultaneously, so that free-radical-initiated, self-propagating chain reactions and slow, endothermic step reactions were studied side by side. After the first results were attained, a grand strategy for practical applications developed quite naturally the vinyl- and diene-type addition polymers were pursued with the ultimate aim being the production of a synthetic rubber. The signals coming from the... 

Chain-growth polymerization exhibits a preference for head-to-tail addition. Branching affects the physical properties of the polymer because linear unbranched chains can pack together more closely than branched chains can. The substituents are on the same side of the carbon chain in an isotactic polymer, alternate on both sides of the chain in a syndiotactic polymer, and are randomly oriented in an atactic polymer. The structure of a polymer can be controlled with Ziegler-Natta catalysts. Natural rubber is a polymer of 2-methyl-l,3-butadiene. Synthetic rubbers have been made by polymerizing dienes other than isoprene. Heating mbber with sulfur to cross-link the chains is called vulcanization. 

The first attempts to prepare synthetic rubber were made ivith isoprene, which was known to be a building unit of the natural products, hevea, gutta percha, and others. The difficulty of producing isoprene economically, the poor properties of the early polyisoprene, and finally the realization that a successful synthetic rubber, unlike other natural substitutes, would not necessarily be an exact duplication of the natural product encouraged research with other monomers. Some of the first synthetic diene polymers produced commercially, the Neoprenes, a class of poly-2-chloro-butadienes, were superior to natural rubber in resistance to aging, chemical attack, and wear. ... 

Though natural rubber, SBR, and BR represent the largest consumption of elastomers, several additional polymers merit a brief discussion because of their economic significance—nitriles, polychloroprene, butyl, and ethylene-propylene-diene monomer (EPDM) elastomers (Datta, 2004 The Synthetic Rubber Manual, 1999). 

The miscibility of natural rubber (NR) blends is one of the most important factors when designing NR products. For instance, when the NR is miscible with a dissimilar polymer on a molecular level, we may improve the properties of NR as a function of the composition of the polymer. This is significantly different from the design for immiscible NR blends, whose properties are greatly dependent upon the morphology of the blend but less so on the composition. In most cases, NR is immiscible with non-polar synthetic rubbers, i.e. NR/butadiene rubber (BR) with high c -1,4-butadiene units, NR/styrene-butadiene rubber (SBR), NR/butyl rubber (IIR), NR/silicone rubber (q)13,i4 NR/ethylene-propylene-diene rubber (EPDM). This means it is important to find miscible NR blends and to control the morphology of the immiscible NR blends in a rational way. In this chapter, properties of NR blends are described from the viewpoint of miscibility, i.e. the miscible blend of NR/BR and the immiscible blend of NR/SBR. 

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