Polyisoprene repeating unit

An orthogonal set of approximations involves the structural representation of the polymer. As an example. Fig. 1 shows four different ways in which the structure of polyisoprene might be represented in an MD or BD simulation. The top structure (a) is an atomic representation of the polyisoprene repeat unit. The second structure (b) results from coUapsing all the hydrogens onto their parent carbons (the united atom approximation). The third structure (c) further collapses the three carbon centers in the fairly rigid double bond unit into a single pseudo-atom. Any of these three structures might be used in a simulation of local polymer dynamics. The fourth structure (d) shows a bead-... 

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. 

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. 

Polyisoprene (synthetic natural rubber) (IR) Designation in ISO 1629 - IR Repeat Unit... 

On the other hand, the fact that the methyl resonances appear as sharp singlets at 1.53 ppm for the hydrochlorinated product and at 1.69 ppm for the hydrobrominated product indicates the exclusiveness of Markownikoff s rule for the addition of either hydrogen halide to the repeating units of 1,4-polyisoprene. 

Figure 1.10. Repeating units of poly-1,3-butadiene and polyisoprene.

Natural rubber is a polymer of isoprene- most often cis-l,4-polyiso-prene - with a molecular weight of 100,000 to 1,000,000. Typically, a few percent of other materials, such as proteins, fatty acids, resins and inorganic materials is found in natural rubber. Polyisoprene is also created synthetically, producing what is sometimes referred to as "synthetic natural rubber". Owing to the presence of a double bond in each and every repeat unit, natural rubber is sensitive to ozone cracking. Some natural rubber sources called gutta percha are composed of trans-1,4-poly isoprene, a structural isomer which has similar, but not identical properties. Natural rubber is an elastomer and a thermoplastic. However, it should be noted that as the rubber is vulcanized it will turn into a thermoset. Most rubber in everyday use is vulcanized to a point where it shares properties of both, i.e., if it is heated and cooled, it is degraded but not destroyed. 

A number of plants and some trees contain a white, milky liquid that is released when the stem or bark is cut. The liquid is called a latex from the Latin meaning liquid. Common sources include dandelions, milkweed, goldenrod, and potted rubber plants. Rubber trees, from which substantial quantities of latex can be harvested, grow in some tropical areas of the world. A major constituent of this latex is a homopolymer of isoprene (2-methyl-1,3-butadiene), called polyisoprene. Polyisoprene, as well as a number of other elastomers, has a carbon-carbon double bond in every repeat unit. The properties of polyisoprene are the result of the presence of these double bonds. Just as stereochemistry plays a critical role in both proteins and polysaccharides, we will see its importance here. 

Figure 5.1. Molecular structures of the chemical repeat units for common polymers. Shown are (a) polyethylene (PE), (b) poly(vinyl chloride) (PVC), (c) polytetrafluoroethylene (PTFE), (d) polypropylene (PP), (e) polyisobutylene (PIB), (f) polybutadiene (PBD), (g) c/5-polyisoprene (natural rubber), (h) traw5-polychloroprene (Neoprene rubber), (i) polystyrene (PS), (j) poly(vinyl acetate) (PVAc), (k) poly(methyl methacrylate) (PMMA), ( ) polycaprolactam (polyamide - nylon 6), (m) nylon 6,6, (n) poly(ethylene teraphthalate), (o) poly(dimethyl siloxane) (PDMS).

The hydrogenation of polyisoprene [55] provides the equivalent of poly (ethylene-a//-propylene) or PEP, typically with low levels of a 3-methyl-1-butene repeat unit due to 3,4 incorporation of isoprene during the anionic polymerization (Scheme 23.5). Hydrogenated polyisoprene is amorphous regardless of the microstructure of the polymer prior to hydrogenation. 

Scheme 23.5 Repeat unit structures of polyisoprene and hydrogenated polyiso-prene...

The principles of nomenclature for copolymers are based on their structure and are given in Table 1.2, where A and B represent the names of repeating units. For example, a statistical copolymer of ethylene and propylene would be called poly(ethylene-stat-propylene), and a triblock copolymer of styrene (A) sand isoprene (B) would be called polystyrene-Z)/oeA -polyisoprene- /ocA -polystyrene. In some cases it is necessary to introduce square brackets in the nomenclature to clarify the notation. Let us see an example An alternating copolymer of styrene and maleic anhydride would be called poly[styrene-a/ (maleic anhydride)]. 

While Tm is a first order transition, Tg is a second order transition and this precludes the possibility of a simple relation between them. There is, however, a crude relation between Tm and Tg. Boyer [25] and Beamen [265] inspected data for a large number of semicrystalline polymers, some of which are shown in Table 2.4. They found that the ratio TgjTm ranged from 0.5 to 0.75 when the temperatures are expressed in degrees Kelvin. The ratio is closer to 0.5 for symmetrical polymers such as polyethylene and polybutadiene, but closer to 0.75 for unsymmterical polymers, such as polystyrene and polyisoprene. The difference in these values may be related to the fact that in unsymmterical chains with repeat units of the type -(CH2-CHX-)- an additional restriction to rotation is imposed by steric effects causing Tg to increase, and conversely, an increase in symmtery lowers Tg. 

From the data given in Exercise 2.4 calculate the values of a and Coo for cis- and trans-polyisoprene, given that the length of an isoprene repeating unit is 4.60x10 m. 

A third backbone index trans was defined by Equation 12.24, as the number of carbon-carbon double bonds along the chain backbone which are in a trans configuration divided by the number NBB of backbone atoms. If there is a mixture of cis and trans isomerization around the C=C bonds along the chain backbone, only the trans double bonds are counted. For example, all-trans polyisoprene has one trans C=C backbone bond per repeat unit with NBB=4, so that... 

A polyisoprene rubber has 2.5% of its repeating units crosslinked by sulfur vulcanization. Estimate the modulus of the sample at low extensions. (Density of vulcanizate = 0.94 g/cm at 25°C.)... 

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