Preparation of Monomers

By the mid-1990s capacity for polyethylene production was about 50 000 000 t.p.a, much greater than for any other type of plastics material. Of this capacity about 40% was for HDPE, 36% for LDPE and about 24% for LLDPE. Since then considerable extra capacity has been or is in the course of being built but at the time of writing financial and economic problems around the world make an accurate assessment of effective capacity both difficult and academic. It is, however, appeirent that the capacity data above is not reflected in consumption of the three main types of material where usage of LLDPE is now of the same order as the other two materials. Some 75% of the HDPE and LLDPE produced is used for film applications and about 60% of HDPE for injection and blow moulding. 

Polymers of low molecultu weight and of very high molecular weight are also available but since they are somewhat atypical in their behaviour they will be considered separately. 

Since impurities can affect both the polymerisation reaction and the properties of the finished product (particularly electrical insulation properties and resistance to heat aging) they must be rigorously removed. In particular, carbon monoxide, acetylene, oxygen and moisture must be at a very low level. A number of patents require that the carbon monoxide content be less than 0.02%. 

It was estimated in 1997 that by the turn of the century 185 million tonnes of ethylene would be consumed annually on a global basis but that at the same time production of polyethylene would be about 46000000t.p.a., i.e. about 25% of the total. This emphasises the fact that although polyethylene manufacture is a large outlet for ethylene the latter is widely used for other purposes. 

There are five quite distinct routes to the preparation of high polymers of ethylene  

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The stmcture of DPXN was determined in 1953 from x-ray diffraction studies (22). There is considerable strain energy in the buckled aromatic rings and distorted bond angles. The strain has been experimentally quantified at 130 kj/mol (31 kcal/mol) by careful determination of the formation enthalpy through heat of combustion measurements (23). The release of this strain energy is doubtiess the principal reason for success in the particularly convenient preparation of monomer in the parylene process. 

The protection-deprotection reaction sequences constitute an integral part of organic syntheses such as the preparation of monomers, fine chemicals, and reaction intermediates or precursors for pharmaceuticals. These reactions often involve the use of acidic, basic or hazardous reagents and toxic metal salts [30], The solvent-free MW-accelerated protection/deprotection of functional groups, developed during the last decade, provides an attractive alternative to the conventional cleavage reactions. 

The dimerization of propene has been extensively studied because the propene dimers are of considerable interest as fuel additives and as starting materials for the preparation of monomers (4, 48, 49, 101). The reaction course can be controlled to give methylpentenes, 2,3-dimethyl-butenes (2, 4, 7, 47, 51), or hexenes (44-46) as the main products. 

The simplest model compound is cyclohexene oxide III. Monomers IV, V and VII represent different aspects of the ester portion of I, while monomers VII and VIII reflect aspects of both the monomer I and the polymer which is formed by cationic ring-opening polymerization. Monomers IV-VII were prepared using a phase transfer catalyzed epoxidation based on the method of Venturello and D Aloisio (6) and employed previously in this laboratory (7). This method was not effective for the preparation of monomer VIII. In this specific case (equation 4), epoxidation using Oxone (potassium monoperoxysulfate) was employed. 

The monomers described so far have all been prepared by starting with 4-bromobenzocyclobutene, 2. A different approach to the preparation of monomers begins with the parent hydrocarbon benzocyclobutene 1 by carrying out electrophilic aromatic substitution reactions [36]. Benzocyclobutene readily undergoes a Friedel-Crafts benzoylation reaction with a variety of substituted acid chlorides (Fig. 7). 

The Friedel-Crafts type of technology can also be used for the preparation of monomers that contain one benzocyclobutene and a second functional group which can react with the benzocyclobutene. These types of molecules are commonly called AB monomers. An example of this class of monomer is shown in Fig. 8. 

The preparation of monomer 119 required a different approach. 4-Ben-zocyclobutenyl-3-hydroxyphenyl ether 115 was reacted with 4-nitro-chloro-benzene 116 under basic conditions to afford the nitro adduct 117 [114]. 

Scheme 4 Preparation of Monomer 3-[4-(/V-Benzyloxycarbonyl)aminobutyl]-6-methylmorpholine-2,5-dioneI271...

Enzymatic hydrolysis is a useful method for the preparation of monomers from chitin and chitosan because the yield of monomers is greater by enzymatic hydrolysis than by acid hydrolysis. The enzyme chitin deacetylase hydrolyzes the acetamido group in the N-acetylglucosamine units of chitin and chitosan, thus generating glucosamine imits and acetic acid (Fig. 2.5). 

Preparation of Monomers. Methyl vinyl ketone (MVK) was obtained from Pfizer Chemical Division, New York, and distilled to remove the inhibitor. Methyl isopropenyl ketone (MIPK) was prepared by the aldol condensation of methyl ethyl ketone and formaldehyde, according to the method of Landau and Irany 0. The major impurity in this monomer is ethyl vinyl ketone (5. The monomer was redistilled before use. 3 Ethyl 3 buten 2 one (EB) was prepared by the aldol condensation of methyl propyl ketone and formaldehyde. Ethyl vinyl ketone (EVK) was prepared by a Grignard synthesis of the alcohol, followed by oxidation to the ketone. t-Butyl vinyl ketone (tBVK) was prepared from pinacolone and formaldehyde by the method of Cologne (9). Phenyl vinyl ketone (PVK) was prepared fay the dehydrochlorlnatlon of 0 cbloro propiophenone (Eastman Kodak). Phenyl isopropenyl ketone (PPK) was prepared by the Mannich reaction using propiophenone, formaldehyde and dimethylamine HCl. 

Most of the work described above was carried out at Queen Mary College, London and the author is grateful to Professor D. Bloor and Dr. D. N. Batchelder for introducing him to polydiacetylenes and for their continued encouragement and help. He would also like to thank Mr. D. Ando and Mr. I. F. Chalmers for their help in the preparation of monomers. Finally, he must extend his gratitude to Dr. R. T. Read, Dr. C. Galiotis, Dr. P. H. J. Yeung and Mr. I. M. Robinson who performed the bulk of the experimental work described above. 

The next step of the investigation deals with the study of the behaviour of still larger molecules, namely mono and dimethyl naphthalenes. It was observed that the smaller naphthalenes could diffuse into the tube, while the larger naphthalenes is unable to enter the mouth of the tube. 2,6-dimethyl naphthalene (DMN) is a valuable intermediate for the preparation of monomer to produce thermotropic liquid crystalline polymers, altho-... 

Scheme 7 Preparation of monomers used in the Warren synthesis of HA.

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