Polychloroprene structure

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. 

The monomer, 2-chlorobuta-1,3-diene, better known as chloroprene, is polymerised by free-radical emulsion methods to give a polymer which is predominantly (-85%) fr<2 s-l, 4-polychloroprene but which also contains about 10% cii-1,4- 1.5%, 1,2- and 1% of 3,4-structures (Figure 11.17). The commercial polymers have a Tg of about -A3°C and a of about 45°C so that at usual ambient temperatures the rubber exhibits a measure of crystallinity. 

The close structural similarities between polychloroprene and the natural rubber molecule will be noted. However, whilst the methyl group activates the double bond in the polyisoprene molecule the chlorine atom has the opposite effect in polychloroprene. Thus the polymer is less liable to oxygen and ozone attack. At the same time the a-methylene groups are also deactivated so that accelerated sulphur vulcanisation is not a feasible proposition and alternative curing systems, often involving the pendant vinyl groups arising from 1,2-polymerisation modes, are necessary. 

Structural applications of rubber base adhesives were also obtained using rubber-thermosetting resin blends, which provided high strength and low creep. The most common formulations contain phenolic resins and polychloroprene or nitrile rubber, and always need vulcanization. 

Both side groups and carbon-carbon double bonds can be incorporated into the polymer structure to produce highly resilient rubbers. Two typical examples are polyisoprene and polychloroprene rubbers. On the other hand, the incorporation of polar side groups into the rubber structure imparts a dipolar nature which provides oil resistance to these rubbers. Oil resistance is not found in rubber containing only carbon and hydrogen atoms (e.g. natural rubber). Increasing the number of polar substituents in the rubber usually increases density, reduces gas permeability, increases oil resistance and gives poorer low-temperature properties. 

In some cases, diene polymers (for instance polychloroprene rubbers) can add to the growing polymer chain by 1,2 addition (also called vinyl addition). This creates labile hydrogen or reactive halogen on tertiary carbon atoms. A few percent of this type of structure in the rubber will assist cross-linking reactions. 

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. 

At least 90 percent of free-radical-polymerized 2,3-dimethylbutadiene consists of 1,4 units according to ozone degradation experiments. Successive substitution of the methyl groups on carbons 2 and 3 of butadiene is seen to increase the proportion of 1,4 units formed. In polychloroprene no less than 97 percent of the structure consists of 1,4 Cl... 

In a combined elemental microanalysis (to determine the C, H, N and Cl contents of char), TGA, DSC, mid-infrared and NMR study of the char forming process in polychloroprene, CPMAS solid-state 13C NMR was used to probe for structural changes that occurred during the degradation steps [88]. The NMR study supplied both valuable extra detail and confirmatory and complementary information. It was observed that while the dehydrochlorination of polychlo-prene proceeded, there was loss of sp3-hybridised carbon and commensurate... 

Yet as the many sided debate went on, Wallace Carothers started a series of investigations in 1928 which would eventually establish the macromolecular concept. His objective from the beginning was to prepare polymers of known structure through the use of established reactions of organic chemistry (85). In the brilliant years before his untimely death in 1937, he studied the preparation and properties of polyesters, polyanhydrides, polyamides, and polychloroprene (28). 

Irradiation of polychloroprene latexes of two different structures, one containing some sulfur and having a lower degree of branching and the other a highly branched polymer, made by mercaptane modification, showed a... 

The oxidation of other rubbers has been studied by FT-IR including polychloro-prenes >. These results suggest that the thermal oxidation of polychloroprenes involves the 1,2 and 3,4-structural irregularities in the initial stage. In particular, it is felt that the initial step is the abstraction of a tertiary allylic chlorine or hydrogen from the 1,2 or 3,4 units yielding a tertiary carbon radical. 

The value of these charts, in showing what adjustments of atomic parameters will increase or decrease the structure amplitude, has already been mentioned. To this little need be added except a suggestion on procedure in dealing with several independent reference atoms simultaneously. This is best explained by an actual example. In attempting to find the x and y coordinates of the atoms in the hkO projection of polychloroprene (Bunn, 1942 a) a particular set of postulated coordinates... 

The results of stereochemical interest which came out of this work may be indicated (Bunn, 1942 a-c). It paved the way to a solution of the crystal structure of rubber itself (the cis isomer of poly-isoprene) and of the synthetic rubber-like substance polychloroprene... 

It has been shown that the dihalocarbenes (CX2) react with macro-molecular polyenes to give polydihalocyclopropane type products (17, 18). The transformations effected may be either partial or total on poly-isoprenes and polybutadienes. The reactivity of the polychloroprenes is slighter, and they undergo marked conversion only when they react with CClo. A study was made of the structures obtained and in particular their reduction to polycyclopropane hydrocarbons. 

As the polymeric halide for our initial investigation we have chosen polychloroprene (Neoprene), which has the following structure ... 

Wallace Carothers will be the subject of one of our Polymer Milestones when we discuss nylon in Chapter 3. Among his many accomplishments in the late 1920s and early 1930s, Carothers and his coworkers made a major contribution to the discovery and eventual production of the synthetic rubber, polychloroprene. It was synthesized from the diene monomer, chloroprene, CH2=CCI-CH=CHr Chloroprene, which is a very reactive monomer—it spontaneously polymerizes in the absence of inhibitors— was a product of some classic studies on acetylene chemistry performed by Carothers and coworkers at that time. In common with butadiene and iso-prene, in free radical polymerization chloroprene is incorporated into the growing chain as a number of different structural isomers. Elastomeric materials having very different physical and mechanical properties can be made by simply varying the polym-... 

Petcavich et al. (1978) employed IR subtraction techniques to elucidate the mechanism of the oxidative degradation of polychloroprenes at 60 °C. The spectra were taken at 60 2°C. The results lead to the conclusion that 1,2- and 3,4-structural irregularities are involved in the initial stage of the thermal oxidation of these compounds at 60 °C. In addition, a simple free radical mechanism seems to be consistent with the experimental results. The observed results suggest that polychloroprenes may be stabilized towards oxidative degradation by eliminating the 1,2- and 3,4-structures by chemical modification of the polymer after synthesis. 

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).

Polybutadiene with no substituent groups larger than hydrogen has greater resilience than natural rubber, in which a methyl group is contained in each iso-prcnc repeating unit. Polychloroprenes (neoprenes) have superior oil resistance but lose their elasticity more readily at low temperatures since the substituent is a bulky, polar chlorine atom. (The structures of these monomers are given in Fig. 1-4.)... 

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 ... 

Neoprene, or polychloroprene, is a synthetic rubber discovered by the Du Pont Company in 1931. It is an organic polymer composed of carbon, hydrogen, and chlorine in the ratio of 55 5 40. Its relatively high chlorine content was responsible for the early recognized resistance of the polymer to burning. Practical use of this property was not developed until procedures for making foam structures from neoprene latex were developed in the 1940 s. The U.S. Navy adapted the material to make neoprene foam mattresses that reduced the fire hazards in the crews quarters of naval vessels. For many years neoprene has been the only material to meet Navy specifications for this application. 

Carothers, Wallace H. (1896-1937). Born in Iowa, Carothers obtained his doctorate in chemistry at the University of Illinois. He joined the research staff of Du Pont in 1928, where he undertook the development of polychloroprene (later called neoprene) that had been initiated by Nieuland s research on acetylene polymers. Carothers s crowning achievement was the synthesis of nylon, the reaction product of hexamethylenetetramine and adipic acid. Carothers s work in the polymerization mechanisms of fiber like synthetics of cyclic organic structures was brilliant and productive, and he is regarded as... 

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