Contents Hydrocarbon Monomers

Commercial polystyrenes are normally rather pure polymers. The amount of styrene, ethylbenzene, styrene dimers and trimers, and other hydrocarbons is minimized by effective devolatilization or by the use of chemical initiators (33). Polystyrenes with low overall volatiles content have relatively high heat-deformation temperatures. The very low content of monomer and other solvents, eg, ethylbenzene, in PS is desirable in the packaging of food. The negligible level of extraction of organic materials from PS is of cmcial importance in this appHcation. 

The gas has a high content of monomers (ethylene and propylene) and other useful hydrocarbons with only some 15% being methane. The feedstock is collected in two stages since the heavy fraction is a wax below about 60 °C. The heavy fraction is typically 60% by weight of the product with the light fraction being 40% by weight. 

Strictly, there is no direct evidence concerning the presence of ions for any aliphatic hydrocarbon monomers. The spectroscopic studies which are of such great diagnostic value for aromatic systems are at present useless for those involving aliphatic monomers and therefore such information as we have for these (Table 3) consists of measurements of electrical conductivity and other, more circumstantial, evidence. It is not claimed that the evidence assembled in these Tables is complete and as far as Table 3, especially, is concerned, its content depends obviously on what one considers to be satisfactory evidence for the participation of ions. 

An alternative approach to the use of partially fluorinated systems to reduce the cost of fluorinated PEMs has been developed by DeSimone et al. a perfluo-rinated vinyl ether is copolymerized with a hydrocarbon monomer (styrene), sulfonated, and then subsequently fluorinated to replace existing C-H bonds with C-E bonds (Eigure 3.18). Thus yields the perfluorinated, cross-linked sul-fonyl fluoride membrane that can then be hydrolyzed to give the PEM (7). Because the membranes are cross-linked, considerably higher acid contents (up to 1.82 meq/g) are possible for these materials in comparison to Nafion, leading also to higher proton conductivity values. 

Eor most hydrocarbon resins where numerous stmcturaHy different monomers are polymerized, nmr is typically used as a general tool to quantify the aromatic and/or olefinic content of a resin. In conjunction with gpc and ftir, nmr measurements are used to identify and quantify particular functionaHties or monomers present in hydrocarbon resins. 

EPM and EPDM mbbers are produced in continuous processes. Most widely used are solution processes, in which the polymer produced is in the dissolved state in a hydrocarbon solvent (eg, hexane). These processes can be grouped into those in which the reactor is completely filled with the Hquid phase, and those in which the reactor contents consist pardy of gas and pardy of a Hquid phase. In the first case the heat of reaction, ca 2500 kJ (598 kcal)/kg EPDM, is removed by means of cooling systems, either external cooling of the reactor wall or deep-cooling of the reactor feed. In the second case the evaporation heat from unreacted monomers also removes most of the heat of reaction. In other processes using Hquid propylene as a dispersing agent, the polymer is present in the reactor as a suspension. In this case the heat of polymerisation is removed mainly by monomer evaporation. 

The Ticona materials are prepared by continuous polymerisation in solution using metallocene catalysts and a co-catalyst. The ethylene is dissolved in a solvent which may be the comonomer 2-norbomene itself or another hydrocarbon solvent. The comonomer ratio in the reactor is kept constant by continuous feeding of both monomers. After polymerisation the catalyst is deactivated and separated to give polymers of a low residual ash content and the filtration is followed by several degassing steps with monomers and solvents being recycled. 

The microstructure of polyisoprene prepared by lithium initiation in hydrocarbons is 95% 1,4 under all conditions. The trans 1,4 content however falls from about 20% to zero as the monomer/initiator ratio increases leading finally to a 95% cis 1,4 polymer. This variation can be explained with the following scheme. 

This study was began with hexamethylcyclotrlslloxane (D3) as monomer since It has been shown that under certain conditions It polymerizes to give linear polymer of negligible cycloslloxane content at a much more rapid rate than In the case of octamethyl-cyclotetraslloxane (D4) (29,32). A suitable Initiator Is butyl-lithium which can be mixed with D3 In hydrocarbon solvents to form Bu-Sl(CH3)20Li (33). No polymerization occurs even In the presence of excess D3 until a donor solvent such as THF (29,32), triglyme (29), DME (34), HMPA (34) or DMSO (32) Is added then a reasonably rapid polymerization starts to give near monodlsperse polymers. 

However, the regio-random distribution of functional groups can be avoided by an acyclic diene metathesis (ADMET) polymerization technique using symmetric monomers (33). The molecular weights of these polymers are restricted to < 3 x 104 Dalton by ADMET. Due to their rich hydrocarbon content, the barrier properties in final ethylene vinyl alcohol copolymers are reduced. 

Figure 3.10—Formation of bonded organosilanes at the interface of silica gel. Representation of organic monomers and polymers at the surface of silica gel. The arrangement Si-O-Si C is more stable than Si O C. This reaction leads to a carbon content of 4 or 5%. Other reactions can also be used (hydrosilylation in particular). When a monolayer of hydrocarbons is bonded to the surface of silica, they orient in a particular manner at the interface due to their lipophilic and hydrophilic character.

Copolymerizations initiated by lithium metal should give the same product as produced from lithium alkyls. Usually the radical ends produced by electron transfer initiation have so short a lifetime they can have no influence on the copolymerization. This is true for instance in the copolymerization of isoprene and styrene (50). The product is identical if initiated by lithium metal or by butyllithium. With the styrene-methylmethacrylate system, however, differences are observed (79,80,82). Whereas the butyllithium initiated copolymer contains no styrene at low conversions, the one initiated by lithium metal has a high styrene content if the reaction is carried out in bulk and a moderate one even in tetrahydrofuran. These facts led O Driscoll and Tobolsky (80) to suggest that initiation with lithium occurs by electron exchange and that in this case the radical ends are sufficiently long-lived to produce simultaneous radical and anionic reactions at opposite ends of the chain. Only in certain rather exceptional circumstances would the free radical reaction be of importance. Some of the conditions required have been discussed by Tobolsky and Hartley (111). The anionic reaction should be slow. This is normally true for lithium based catalysts in hydrocarbon solvents. No evidence of appreciable radical participation is observed for initiation by sodium and potassium. The monomers should show a fast radical reaction. If styrene is replaced by isoprene, no isoprene is found in the copolymer for isoprene polymerizes slowly by free radical initiation. Most important of all, initiation should be slow to produce a low steady concentration of radical-anions. An initiator which produces an almost instantaneous and complete electron transfer to monomer produces a high radical concentration which will ensure their rapid mutual termination. 

This structure is similar to that of the copolymer TFE and ethylene, except that the random onentation of the methyl group from nonstereospeciftc free radical copo-lymenzation of propylene affords a noncrystalline structure [35] The relatively low fluorine content (54%) of these elastomers compared with VDF-based elas tomers (66-69 5%) makes them significantly less resistant to swelling by hydrocarbons Because of strict alternation, these elastomers have a relatively high glass transition temperature ( 2 °CJ and consequently limited low temperature properties Furthermore, they must be polymerized with a cure site monomer or receive a postpolymerization treatment to adequately activate them for vulcanization [36] To counteract the limited cure response, low-temperature flexibility, and hydrocarbon resistance, these polymers have also been modified with substantial amounts (ca 35 wt %) of VDF [37, 38] This provides some improvements but inevitably decreases the resistance to bases and polar solvents... 

Several conclusions have to be drawn. The first is related to the obvious gap between the empiricism and even archaism of most of industrial cationic polymerization processes and the level of fundamental science devoted for decades to these reactions. Previous chapters in this volume clearly illustrate the situation. This feature was pointed out in the early book of Kennedy and Marechal [1], and the explanation based on the very favorable price/performances characteristics of the products is still realistic. Nevertheless it is noteworthy that recent improvements or new processes based on more scientific approaches led to a better control of the polymerization, of polymer structure, and to high-performance commercial products which will increasingly occupy the market. This is the case for the recently marketed reactive BF3-based polybutenes with high content of exomethylenic chain ends, for the strongly developing pure monomer hydrocarbon resins ( + 8% in 1994), or for the new benzyl halide-based halobutyl rubber, and it is revealing that these products represent the three families of cationically prepared industrial polymers... 

Recent developments in ADMET polymerization and its use in materials preparation have been presented. Due to the mild nature of the polymerization and the ease of monomer synthesis, ADMET polymers have been incorporated into various materials and functionaUzed hydrocarbon polymers. Modeling industrial polymers has proven successful, and continues to be appUed in order to study polyethylene structure-property relationships. Ethylene copolymers have also been modeled with a wide range of comonomer contents and absolutely no branching. Increased metathesis catalyst activity and functional group tolerance has allowed polymer chemists to incorporate amino acids, peptides, and various chiral materials into metathesis polymers. Sihcon incorporation into hydrocarbon-based polymers has been achieved, and work continues toward the application of latent reactive ADMET polymers in low-temperature resistant coatings. 

The BP process [7] is based on a sand fluidized-bed pyrolysis reactor. The cracking temperature is kept at 400-600°C. Low-molecular hydrocarbons can be obtained. The process mainly involves converting waste plastics into normal linear hydrocarbons, the average molecular weight of which is 300-500. Most plastics can be treated by this process. Polyolefins are decomposed into small molecules with the same linear structure. PS is converted into styrene monomers and PET into mixture of hydrocarbons, carbon monoxide and carbon dioxide. A maximum of 2% PVC is allowed in this process, and the content of chlorine in the products is lower than 5 ppm. The distribution of alkene products in this process is like that in petroleum pyrolysis. The BP process was industrialized in 1997. 

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