Thermal polymerization process

A combination of the thermal polymerization process and the U.O.P. catalytic process was introduced in 1937 at the Shamrock Oil and Gas Co., Sunray, Tex. (28). In 1934 the Shell Development Co. introduced the cold acid process (18), which selectively polymerizes isobutene, using sulfuric acid as catalyst. The hot acid process was also developed by them and differed from the cold acid process in polymerizing all C4 olefins. Both products are predominantly the dimer. The cold acid process produces a large pro-... 

Similar ions were recently observed in the spectrum of Cp2TiNCO (78). Ion association was assumed but probably some thermal polymerization process is responsible (cf. [CpFe(CO)2]2, Section C, 2). 

The hydrocarbon resins can be produced by a simple thermal polymerization process (48-50) or by Lewis acid catalyzed reaction (51). The thermal process is carried out at a high temperature in the range of 200-280°C and a reactor pressure above 300 psig. At temperatures below 200°C, the Diels-Alder polymers are formed. They are not desirable in most resins because they are insoluble in aromatic solvents. If reaction temperature exceeds 280°C, decomposition of the resins would occur. 

Both defect absorption and electroabsorption grow during the thermal polymerization process though differently. The absorption band saturates after about 6 h at 80°C. The electroabsorption peak increases rapidly when the autocatalytic range is reached, together with a red shift by about 70 meV similar as the excitons (Fig. 2, PTS-7). As shown in Fig. 3 the growth of Aa follows closely the polymerization curve (14). When most of the material is polymerized, however, Aa decreases again to about half its peak value quite in contrast to the absorption spectrum. 

Because of their high technical properties, polyamide-based thermoplastic elastomers have attracted a lot of interest and their synthesis has been attempted via various polymerization techniques. On the industrial scale, the major companies are producing TPE-A via one- and two-step thermal polymerization processes. Many parameters have to be adjusted to ensure optimal reaction efficiency, including catalyst nature and content, temperature, vacuum level and stirring rate. Obviously, all these parameters are also dependent on the nature of the raw materials used, since some polyamide/polyether pairs, depending on their structure and/or the nature of their end-groups, are easier to prepare than others. 

Feed Preparation. Although propane, butane, etc., may be decomposed or dehydrogenated to produce propylene and butenes (pages 644 to 652), this method is not followed except in thermal polymerization processes... 

Aqueous Dispersions. The dispersion is made by the polymerization process used to produce fine powders of different average particle sizes (58). The most common dispersion has an average particle size of about 0.2 p.m, probably the optimum particle size for most appHcations. The raw dispersion is stabilized with a nonionic or anionic surfactant and concentrated to 60—65 wt % soHds by electrodecantation, evaporation, or thermal concentration (59). The concentrated dispersion can be modified further with chemical additives. The fabrication characteristics of these dispersions depend on polymerization conditions and additives. 

The majority of thermal polymerizations are carried out as a batch process, which requires a heat-up and a cool down stage. Typical conditions are 250—300°C for 0.5—4 h in an oxygen-free atmosphere (typically nitrogen) at approximately 1.4 MPa (200 psi). A continuous thermal polymerization has been reported which utilizes a tubular flow reactor having three temperature zones and recycle capabiHty (62). The advantages of this process are reduced residence time, increased production, and improved molecular weight control. Molecular weight may be controlled with temperature, residence time, feed composition, and polymerizate recycle. 

In the process of thermal dimerization at elevated temperatures, significant polymer is formed resulting in seriously decreased yields of dimer. Dinitrocresol has been shown to be one of the few effective inhibitors of this thermal polymerization. In the processing of streams, thermal dimerization to convert 1,3-cyclopentadiene to dicyclopentadiene is a common step. Isoprene undergoes significant dimerization and codimerization under the process conditions. 

Thermal polymerization is not as effective as catalytic polymerization but has the advantage that it can be used to polymerize saturated materials that caimot be induced to react by catalysts. The process consists of the vapor-phase cracking of, for example, propane and butane, followed by prolonged periods at high temperature (510—595°C) for the reactions to proceed to near completion. Olefins can also be conveniendy polymerized by means of an acid catalyst. Thus, the treated olefin-rich feed stream is contacted with a catalyst, such as sulfuric acid, copper pyrophosphate, or phosphoric acid, at 150—220°C and 1035—8275 kPa (150—1200 psi), depending on feedstock and product requirement. 

When initiators are decomposed thermally, the rates of initiator disappearance (/rj) show marked temperature dependence. Since most conventional polymerization processes require that kj should lie in the range 10 6-1 O 5 s 1 (half-life ca 10 h), individual initiators typically have acceptable >fcd only within a relatively narrow temperature range (ca 20-30 °C). For this reason initiators are often categorized purely according to their half-life at a given temperature or vice For initiators which undergo unimolecular decomposition, the half-life is... 

The thermal polymerization of S has a long history.310 The process was first reported in 1839, though the involvement of radicals was only proved in the 1930s. Carefully purified S undergoes spontaneous polymerization at a rate of ca 0.1% per hour at 60 C and 2% per hour at 100 °C. At 180 aC, 80% conversion of monomer to polymer occurs in approximately 40 minutes. Polymer production is accompanied by the formation of S dimers and trimers which comprise ca 2% by weight of total products. The dimer fraction consists largely of cis- and trans-1,2-diphenylcyclobutanes (90 and 91) while the stereoisomeric tetrahydronaphthalenes (92 a nd 93) are the main constituents of the trinier fraction.313... 

There are additional factors that may reduce functionality which are specific to the various polymerization processes and the particular chemistries used for end group transformation. These are mentioned in the following sections. This section also details methods for removing dormant chain ends from polymers formed by NMP, ATRP and RAFT. This is sometimes necessary since the dormant chain-end often constitutes a weak link that can lead to impaired thermal or photochemical stability (Sections 8.2.1 and 8.2.2). Block copolymers, which may be considered as a form of end-functional polymer, and the use of end-functional polymers in the synthesis of block copolymers are considered in Section 9.8. The use of end functional polymers in forming star and graft polymers is dealt with in Sections 9.9.2 and 9.10.3 respectively. 

PA-6,6 is made from the relatively expensive materials hexamethylene diamine and adipic acid. An alternative synthesis of PA-6,6 from adiponitrile and hexamethylene diamine utilizing water is under investigation.16 PA-6 can be synthesized in a continuous process at atmospheric pressure, but reaction times are very long as the ring-opening initiation step is particularly slow. The reaction time can be shortened considerably by carrying out prepolymerization in the presence of excess water at pressure however, this makes the continuous polymerization process more complex. Copolymers with amide units of uniform length (diamides) are relatively new the diamide units are able to crystallize easily and have a thermally stable crystalline structure. 

Consider alternative polymerization processes in solid state, inducing the polymerization reaction of N3P3CI6 thermally [40-42],photochemically [61, 67,68],y-radiolytically [66,210], using X-rays [74,75,90] or electron irra-... 

The solution polymerization process for hexachlorocyclophosphazene to poly-dichlorophosphazene is an interesting and attractive alternative to the classic bulk thermal polymerization reaction of this trimer. 

Differential scanning calorimetry (DSC) experiments on the various dimeric carbocycles indicated that, depending on the length of the alkyl groups, thermal polymerization had occurred between 100 and 125°C as an abrupt, exothermic process. The narrow temperature range for each exotherm was suggestive of a chain reaction however, IR spectroscopy revealed the absence of acetylene functionalities in the polymerized material. Consequently, none of the substi-... 

Analysis of DSC experiments on various alkyl-substituted trimers gave even more disappointing results. Although more thermally resilient, these macrocycles polymerized with very broad exotherms. For the hexyl-substituted trimer, melting occurred around 150 °C, while polymerization extended from ca. 170 to 230°C. This pattern was thought to be indicative of a random polymerization process. Overall, polymerization of trimeric macrocycles occurred at sufficiently high temperatures that the resultant materials were intractable brown tars. 

---