Trans -1, 4-Polychloroprene

The polymerization produces primarily trans-1,4-polychloroprene. The trans content can be increased somewhat by lowering the polymerization temperature 62,63... 

Historically, spectral subtraction led to one of the first applications of FTIR spectroscopy to polymers, that is, the study of defects introduced during polymerization at different temperatures [79]. In Fig. 3.24, the FTIR spectra at 70 C in the frequency range of 500-3200 cm is shown for trans-1,4-polychloroprene polymerized at -20 C (spectrum a) and at -40°C (spectrum b). The b — a difference spectrum, which is also shown, reflects the increased presence of defects in the polychloroprene polymerized at —40 C relative to the sample polymerized at —20 C. The bands exhibited in the difference spectrum (but not visible in the normal spectra) correspond to the ds-l,4-polychloroprene structure and reflects the increased amount of these defects. 

Since it is not possible to commercially produce a polymer that is based on the cis 1,4 form, commercial polymers are based on the Irons 1,4 form which has a crystalline melting point, Tm, of +75 °C and a Tg of -45 °C. Pure 1,4 trans polychloroprene thus crystallises readily and would normally be considered to be of limited use for a rubber. Such a polymer, however, does not crystallise when dissolved in a solvent, but will do so when the solvent evaporates. This feature is used to good effect in the production of contact adhesives. 

Figure 5.1. Molecular structures of die chemical repeat units for common polymers. Shown are (a) polyethylene (PE), (b) poly(vinyl chloride) (PVC), (c) polytetrafluoroethylene (FI FE), (d) polypropylene (PP), (e) polyisobutylene (PIB), (f) polybutadiene (PBD), (g) cw-polyisoprene (natural rubber), (h) trans-polychloroprene (Neoprene rubber), (i) polystyrene (PS), (j) poly(vinyl acetate) (PVAc), (k) poly(methyl methaciylate) (PMMA), (1) polycaprolactam (polyamide - nylon 6), (m) nylon 6,6, (n) poly(ethylene teraphthalate), (o) poly(dimethyl siloxane) (PDMS).

Chloroprene Elastomers. Polychloroprene is a polymer of 2-chloro-l,3-butadiene. The elastomer is largely composed of the trans isomer. There are two basic polymer types the W-type and the G-type. G-types are made by using a sulfur-modified process W-types use no sulfur modification. As a result, G-types possess excellent processing and dynamic properties, and tend to be used in V-belts. However, they have poorer aging properties than W-types. The W-types tend to be used in appHcations requiring better aging, such as roUs and mechanical goods (see Elastomers, SYNTHETIC-POLYCm.OROPRENE). 

Increase in the trans content of the polychloroprene, increasing its crystallization tendency. 

Neoprene AC (1947). This polychloroprene was developed to provide better viscosity stability and resistance to discolouration, but it cures much more slowly at room temperature. It is a fast crystallizing grade and contains about 90% trans-, 4 structure. 

Neoprene AD ( 95 ). This is more stable than Neoprene AC. This polychloroprene does not change colour in contact with iron and viscosities of the solutions are maintained stable over long periods of time. They are fast ciystallizing grades and contain about 90% trans-1,4 structure. 

Neoprene WHV-A. It is a non-peptizable and mercaptan-modified polychloro-prene elastomer. It is a slow-crystallizing, high molecular weight type and contains only 85% trans-, 4 structure. It is generally used in blends with low molecular weight crystallizing polychloroprene types to increase solution viscosity. 

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. 

Polychloroprene is the polymer of 2-chloro-l,3 butadiene. Emulsion polymerization produces an almost entirely trans-1,4 polymer, which is highly crystalline. Less crystalline polychloroprenes are produced by incorporating several wt.% of 2,3-dichloro-l,3 butadiene into the polymer to break up crystalline sequences. 

The microstructure of polychloroprene, polymerised at +12 to +70 °C, was analysed using H- and 13C-NMR [32], Signal assignments were made for head-and-tail arrangements of trans-1,4 units, which was the major component of polychloroprene, and for other isomeric units. Polymerisation at high temperature resulted in a slight increase in head-to-head and tail-to-tail linkage of trans-1,4 units as well as the increase in cis-1,4 units. 

PVDF is just one polymer where sequence isomerism has been obseved using NMR spectroscopy. The 13C NMR spectrum of polychloroprene, for example, has also been analyzed by one of your authors (in collaboration with his coworkers) and the olefinic region is shown in Figure 7-28. It proved possible to assigu lines to triad sequence isomers of the trans-1,4 units (TH and HT) and also triad sequences containing the cis-lA... 

Often, however, you are trying to determine how many c/s-1,4- 1,2- 3,4- etc. units there are in a sample that may largely consist of trans-1,4- units, as in polychloroprene, for example. Consider the spectra marked A and B shown in Figure 7-31. Sample A was polymerized at -40° C, while sample E... 

Polychloroprene (Neoprene) Rubbers. Polychloroprene or neoprene rubbers (CR) are polymers of 2-chloro-1,3-butadiene. The stereochemical structure of these rubbers is fundamentally the trans configuration (80%), with the rest being predominantly cis-1,4, though small quantities of 1,2 (1.5%) and 3,4 (1%) are also present. A scheme is given below ... 

Flash pyrolysis in He at 600° C of polychloroprene is dominated by the presence of the dimer of 2-chloro-1,3-butadiene and that of monomer as shown in the pyrogram given in Figure 7.3.1. Table 7.3.1 shows the peak identification obtained by MS library searches. The pyrogram was obtained in the same experimental conditions as those used for other examples in this book (see Table 4.2.2). The sample was made of polychloroprene 10% cis, 85% trans. 

Oxidation Mechanism of CR. A chlorine substituent on the double bond Introduces considerable complexity In the degradation of trans-l,4-polychloroprene (CR). CR Is more resistant to oxidation than BR and IR, but Is subject to dehydrochlorlnatlon (32. 33, 34). 

Detailed studies of the autoxidation of polychloroprene and of trans-4-chloro-4-octene, a model comparison, have been reported by Bailey [177]. The major part of the hydrogen chloride evolved on heating polychloroprene was confirmed to be associated with oxidative degradation and the kinetics of HQ formation from pre-oxidised polymer heated under nitrogen was investigated. 

In some cases, however, steric hindrance causes the main chain to assume a minimum energy conformation other than the trans form. Some of these variations may be mere distortions of the fully extended planar zigzag conformation, as in most polyesters, polyisoprenes, and polychloroprene. In other... 

The effect of polymerization temperature upon the microstmcture of poly-chloroprenes produced by emulsion polymerization is illustrated by the results, reported by I ynard and Mochel [25], shown in Table 15.6. The chloroprene units are present mainly as trans- A structures, irrespective of the polymerization temperature. However, the distribution of the microstructures does depend somewhat upon polymerization temperature. The ratio cw-l,4/tra 5-l,4 units decreases as the polymerization temperature is reduced, but the overall content of 1,4 units increases. The balance comprises 1,2 and 3,4 units in approximately equal proportions, except for polymers produced at very low temperatures, where the 1,2 units predominate over the 3,4 units. A consequence of the overall content of 1,4 units increasing with decreasing polymerization temperature is that the sum of the contents of 1,2 and 3,4 units decreases. As will be seen in Section 15.4.4, although 1,2 units are present in relatively low concentration, their presence is very important for the technology of polychloroprene rubbers. 

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