Molecular structure polyisoprene

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 standard molecular structural parameters that one would like to control in block copolymer structures, especially in the context of polymeric nanostructures, are the relative size and nature of the blocks. The relative size implies the length of the block (or degree of polymerization, i.e., the number of monomer units contained within the block), while the nature of the block requires a slightly more elaborate description that includes its solubility characteristics, glass transition temperature (Tg), relative chain stiffness, etc. Using standard living polymerization methods, the size of the blocks is readily controlled by the ratio of the monomer concentration to that of the initiator. The relative sizes of the blocks can thus be easily fine-tuned very precisely to date the best control of these parameters in block copolymers is achieved using anionic polymerization. The nature of each block, on the other hand, is controlled by the selection of the monomer for instance, styrene would provide a relatively stiff (hard) block while isoprene would provide a soft one. This is a consequence of the very low Tg of polyisoprene compared to that of polystyrene, which in simplistic terms reflects the relative conformational stiffness of the polymer chain. 

In summary, cationic polymerization gives polymers of structure P8a Ziegler-Natta complexes mainly lead to blocks of structure P8b. This study gives further evidence for cyclization reactions of high-molecular-weight polyisoprenes. This point of view has been confirmed by the study of the cyclization of model polyisoprene molecules with two, three, or four monomer units toward the catalysts able to initiate such a reaction (24). 

A comparison of polychloroprene and natural rubber or polyisoprene molecular structures shows close similarities. However, while the methyl groups activates the double bond in the polyisoprene molecule, the chlorine atom has the opposite effect in polychloroprene. Thus polychloroprene is less prone to oxygen and ozone attack than natural rubber is. At the same time accelerated sulfur vulcanization is also not a feasible proposition, and alternative vulcanization or curing systems are necessary. 

The earliest emulsion polymers are those found in nature. Natural rubber (NR) latexes have been extracted from the rubber tree (Hevea brasiliensis) (286) for hundreds of years. The latexes are comprised of dispersions of high molecular weight, linear cis-1,4-polyisoprene (210) particles ranging in diameter from 10 nm to several microns. Because the natural mbber particles are in a colloidal form, they must first be separated from the aqueous phase by coagulation before processing. Since the molecular structure of natural rubber is stereoregular, it has excellent mechanical properties that have not been duplicated by modem synthetic mbbers. 

The polymerization of butadiene monomer proceeds with chain propagation via 1,2-,, A-trans- or 1,4-cw-additions. If the polymerization is controlled to form mostly the 1,2-addition product, the polymer has a — CH2— chain with a terminal vinyl, — CH=CH2, substituent, at alternating carbon atoms. However, if 1,4-addition dominates the polymerization proceeds to form a polymer chain with a molecular structure of — (CH2 —CH=CH—CH2) —, normally with a trans configuration at the double bond. 2-Chloro-1,3-butadiene (CH2=CC1—CH=CH2 chloroprene) and 2-methyl-1,3-butadiene (isoprene) are polymerized in a similar manner. With these compounds, the asymmetry of the carbon atoms at positions 1 and 4 produces a variety of addition products with 1,2-, 1,4-cw,, A-trans, and 3,4-configurations. In the case of polyisoprene, which in nature occurs as natural rubber, the 1,4-cis configuration is the dominant structure. A summary of the polymerization products of butadiene, isoprene, and chloroprene is provided in Fig. 31. 

The studies performed on the vulcanised or non-vulcanised natural and synthetic elastomers, of polyisoprene and polybutadiene type, allowed the elucidation of the molecular-structural transformations, recorded in different time-temperature conditions. On their... 

Chemically the NR produced by the Hevea brasiliensis species is almost pure cw-l,4-polyisoprene. So far none of the manufacturers of synthetic cis-, A-polyisoprene is able to achieve more than 95% of the cis isomer in their commercial products. Jitladda and coworkers have conducted extensive studies on the molecular structures of the NR molecules found in Hevea and correlated these to the biosynthesis of this in the trees. This is intrinsically linked to the end groups of the rubber molecules involving the phospholipids and proteins that are linked to the charging mechanism at the latex particle surface and hence has a direct impact on the stability of the latex. 

Poddubnyj, I. Ya., Grechanovskii, V. A., Ivanova, L. S. (1972). Molecular structure and microscopic properties of synthetic cis-polyisoprene. Report at the international symposium on isoprene rubber. Moscow, 20-24/IX, 1972. M. TsNIITEneftekhim, 19. 

FIGURE 13.3 Molecular structures of (a) fluorinated polyisoprene and (b) fluorinated polybutadiene. 

Figure 5.1.35 Molecular structure of natural rubber (c/s-1,4-polyisoprene) and of gutta-percha (trans-1,4-polyisoprene), which has no importance for technical applications.

Oligomer nature as well as molecular structure of a second component determines whether solubility depends on n or not. A good graphical presentation of this effect for two of above mentioned homologous rows and three polymer matrices is shown in Figure 3.11, where UCST values as well as its relative variation y = (UCST)n / (UCST)n i are analyzed as a function of n. As we can see, increase of n for n-oxyethylene-bimetacrylic oligomers in mixtures with cis-polyisoprene or polybutadienenitrile or polyvynilchloride always reduces compatibility, while increase of n for n-methylene-bimetacrylic oligomers (at least, for ones used in experiments) leads to expected reduction of compatibility only in mixtures with... 

The main objective of this chapter is to inform rubber technologists how and when to use synthetic high cw-polyisoprene and to attempt, where possible, to link certain observed behaviour with studies made by rubber scientists concerning molecular structure. Emphasis is placed on situations where technical advantage is established for the synthetic polymer, since it is almost invariably sold at a premium to natural rubber and its value is appreciated by rubber manufacturers. Selective usage is the pattern that has developed and correctly so, since synthetic polyisoprene is unlikely to be a cheap replacement for natural rubber in the foreseeable future. 

The hard thermoplastic hydrocarbon obtained from the latex of Mimusops globosa. Balata has the same molecular formula as natural rubber (C5H8)n but has the tram polyisoprene structure whereas natural rubber has c/ s-structure. 

Fig. 4.1 a Typical time evolution of a given correlation function in a glass-forming system for different temperatures (T >T2>...>T ), b Molecular dynamics simulation results [105] for the time decay of different correlation functions in polyisoprene at 363 K normalized dynamic structure factor at the first static structure factor maximum solid thick line)y intermediate incoherent scattering function of the hydrogens solid thin line), dipole-dipole correlation function dashed line) and second order orientational correlation function of three different C-H bonds measurable by NMR dashed-dotted lines)... 

Several studies have been published which utilize size exclusion chromatography (SEC) for characterization of the molecular weight distribution of multi-arm structures of polystyrene, polyisoprene, and block copolymers of styrene/ butadiene and styrene/isoprene (1, 2, 8, 17, 25-26). An... 

The general correlations of structure and properties of homopolymers are summarized in Table 2.13. Some experiments which demonstrate the influence of the molecular weight or the structure on selected properties of polymers are described in Examples 3-6 (degree of polymerization of polystyrene and solution viscosity), 3-15, 3-21, 3-31 (stereoregularity of polyisoprene resp. polystyrene), 4-7 and 5-11 (influence of crosslinking) or Sects. 4.1.1 and 4.1.2 (stiffness of the main chain of aliphatic and aromatic polyesters and polyamides). 

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 pseudo solid-like behavior of the T2 relaxation is also observed in i) high Mn fractionated linear polydimethylsiloxanes (PDMS), ii) crosslinked PDMS networks, with a single FID and the line shape follows the Weibull function (p = 1.5)88> and iii) in uncrosslinked c/.s-polyisoprenes with Mn > 30000, when the presence of entanglements produces a transient network structure. Irradiation crosslinking of polyisoprenes having smaller Mn leads to a similar effect91 . The non-Lorentzian free-induction decay can be a consequence of a) anisotropic molecular motion or b) residual dipolar interactions in the viscoelastic state. 

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