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Chemicals Based on Benzene, Toluene, and Xylenes 297
Liquid-phase oxidation of o-xylene also works at approximately 150°C.
Cobalt or manganese acetate in acetic acid medium serves as a catalyst.
The major by-products of this process are maleic anhydride, benzoic
acid, and citraconic anhydride (methylmaleic anhydride). Maleic anhy-
dride could be recovered economically.
34
Phthalic anhydride’s main use is for producing plasticizers by reac-
tions with C
4
–C
10
alcohols. The most important polyvinyl chloride plas-
ticizer is formed by the reaction of 2-ethylhexanol (produced via
butyraldehyde, Chapter 8) and phthalic anhydride:
Phthalic anhydride is also used to make polyester and alkyd resins. It is
a precursor for phthalonitrile by an ammoxidation route used to produce
phthalamide and phathilimide. The reaction scheme for producing
phthalonitrile, phthalamide, and phathilimide is shown in Figure 10-17.
34
The oxidation of m-xylene produces isophthalic acid. The reaction occurs
in the liquid-phase in presence of ammonium sulfite:
Chapter 10 1/22/01 11:08 AM Page 297
Isophthalic acid’s main use is for producing polyesters that are character-
ized by a higher abrasion resistance than those using other phthalic acids.
Polyesters from isophthalic acid are used for pressure molding applications.
Ammoxidation of isophthalic acid produces isophthalonitrile. The
reaction resembles the one used for ammoxidation of phthalic anhydride:
298 Chemistry of Petrochemical Processes
Figure 10-17. The reaction scheme for o-xylene to phthalonitrile.
34
Chapter 10 1/22/01 11:08 AM Page 298
Isophthalonitrile serves as a precursor for agricultural chemicals. It is
readily hydrogenated to the corresponding diamine, which can form
polyamides or be converted to isocyanates for polyurethanes.
REFERENCES
1. Chemical and Engineering News, April 10, 1995, p. 17.
2. Roberts, R. and Khalaf, A., Friedel-Crafts Alkylation Chemistry,
Marcel Dekker, Inc. New York, 1984, Chapter 2.
3. “Petrochemical Handbook,” Hydrocarbon Processing, Vol. 70, No. 3,
1991, p. 154.
4. Lewis, P. J. and Dwyer, F. G., Oil and Gas Journal, Sept. 26, 1977,
pp. 55–58.
5. Dwyer, F. G., Lewis, P. J., and Schneider, F. H., Chemical
Engineering, Jan. 5, 1976, pp. 90–91.
6. Hummel, H. K., DeWit G. B., and Maarleveld, A., “The Optimization
of EB Plant by Constraint Control,” Hydrocarbon Processing, Vol.
70, No. 3, 1991, pp. 67–71.
7. Sundaram, K. M. et al., “Styrene Plant Simulation and Optimization,”
Hydrocarbon Processing, Vol. 70, No. 1, 1991, pp. 93–97.
8. CHEMTECH, Vol. 7, No. 6, 1977, pp. 334–451.
9. Chemical and Engineering News, June 20, 1994, p. 31.
10. “Petrochemical Handbook,” Hydrocarbon Processing, Vol. 70, No. 3,
1991, p. 152.
11. Illman, D. “Environmentally Benign Chemistry Aims for Processes
That Don’t Pollute,” Chemical and Engineering News, Sept. 5, 1994,
p. 26.
12. “Petrochemical Handbook,” Hydrocarbon Processing, Vol. 70, No. 3,
1991, p. 168.
13. Fulmer, J. W. and Graf, K. C. “Distill Acetone in Tower Packing,”
Hydrocarbon Processing, Vol. 70. No. 10, 1991, pp. 87–91.
14. Platkin, J. and Fitzgerald, M. “Patent Watch,” CHEMTECH, June
1999, p. 39. U.S. patent 5874646, Feb. 23, 1999.
15. “Petrochemical Handbook,” Hydrocarbon Processing, Vol. 78, No. 3,
1999, p. 98.
16. Vora, B. V. et al., “Latest LAB Developments,” Hydrocarbon
Processing, Vol. 63, No.11, 1984, pp. 86–90.
17. “Petrochemical Handbook,” Hydrocarbon Processing, Vol. 70, No. 3,
1991, p. 130.
Chemicals Based on Benzene, Toluene, and Xylenes 299
Chapter 10 1/22/01 11:08 AM Page 299
18. Frontier Chemical Co., U.S. Patent 3, 148, 222 (1964).
19. “Petrochemical Handbook,” Hydrocarbon Processing, Vol. 70, No. 3,
1991, p. 136.
20. Matar, S., Mirbach, M., and Tayim, H., Catalysis in Petrochemical
Processes, Kluwer Academic Publishers, The Netherlands, 1989,
pp. 84–108.
21. Abraham, O. C. and Chapman, G. L. “Hydrogenate benzene,” Hydro-
carbon Processing, Vol. 70, No. 10, 1991, pp. 95–97.
22. Ohsumi, Y. and Komatsuzaki, Y., U.S. Patent 3, 903, 186, Sept. 2,
1975 to Mitsubishi Chemical Industries, Ltd. and Asis Oil Co. Ltd.
Japan.
23. “Petrochemical Handbook,” Hydrocarbon Processing, Vol. 78, No. 3,
1999, p. 122.
24. Vora, B. V., Jensen, R. H., and Rockett, K. W., Paper No. 20 (P-I)
Second Arab Conference on Petrochemicals, Abu Dhabi, March
15–22, 1976.
25. Hasting, S. H. and Nicholson D. E., J. Chem. Eng. Data, Vol. 6,
1961, p. l.
26. Kaeding, W., Chu, C., Young, L. and Butter, S. “Selective Dispropor-
tionation of Toluene to Produce Benzene and p-Xylene,” Journal of
Catalysis, Vol. 69, No. 2, 1981, pp. 392–398.
27. Meisel, S. L. “Catalysis Research Bears Fruit,” CHEMTECH, January
1988, pp. 32–37.
28. “Petrochemical Handbook,” Hydrocarbon Processing, Vol. 70, No. 3,
1991, p. 150.
29. Gelbein, A. D. and Nislick, A. S., Hydrocarbon Processing, Vol. 57,
No. 11, 1978, pp. 125–128.
30. Cines, M. R., U.S. Patent 3, 746, 754, July 17, 1973 to Phillips
Petroleum Co.; U.S. Patent 2, 905, 709 and 2, 794, 830.
31. Sittig, M., Aromatic Hydrocarbons, Manufacture and Technology,
Park Ridge, N.J.: Noyes Data Corp., 1976, pp. 303–306.
32. Masseling, J. H., CHEMTECH, Vol. 6, No. 11, 1976, p. 714.
33. Braggiato, C. and Gualy, R., “Improve DMT Production,” Hydrocar-
bon Processing, Vol. 77, No. 6, 1998, pp. 61–65.
34. Sze, M. C. and Gelbein, A. P., Hydrocarbon Processing, Vol. 55,
No. 2, 1976, pp. 103–106.
300 Chemistry of Petrochemical Processes
Chapter 10 1/22/01 11:08 AM Page 300
CHAPTER ELEVEN
Polymerization
INTRODUCTION
Polymerization is a reaction in which chain-like macromolecules are
formed by combining small molecules (monomers).
Monomers are the building blocks of these large molecules called
polymers. One natural polymer is cellulose (the most abundant organic
compound on earth), a molecule made of many simple glucose units
(monomers) joined together through a glycoside linkage.
1
Proteins, the
material of life, are polypeptides made of α-amino acids attached by
an amide
301
linkage.
The polymer industry dates back to the 19th century, when natural
polymers, such as cotton, were modified by chemical treatment to pro-
duce artificial silk (rayon). Work on synthetic polymers did not start until
the beginning of the 20th century. In 1909, L. H. Baekeland prepared the
first synthetic polymeric material using a condensation reaction between
formaldehyde and phenol. Currently, these polymers serve as important
thermosetting plastics (phenol formaldehyde resins). Since Baekeland’s
discovery, many polymers have been synthesized and marketed. Many
modern commercial products (plastics, fibers, rubber) derive from poly-
mers. The huge polymer market directly results from extensive work in
synthetic organic compounds and catalysts. Ziegler’s discovery of a
coordination catalyst in the titanium family paved the road for synthe-
sizing many stereoregular polymers with improved properties. This
chapter reviews the chemistry involved in the synthesis of polymers.
Chapter 11 1/22/01 11:10 AM Page 301
MONOMERS, POLYMERS, AND COPOLYMERS
A monomer is a reactive molecule that has at least one functional
group (e.g. -OH, -COOH, -NH
2
, -C=C-). Monomers may add to them-
selves as in the case of ethylene or may react with other monomers hav-
ing different functionalities. A monomer initiated or catalyzed with a
specific catalyst polymerizes and forms a macromolecule—a polymer.
For example, ethylene polymerized in presence of a coordination catalyst
produces a linear homopolymer (linear polyethylene):
n CH
2
=CH
2
r
[–CH
2
–CH
2
]–
n
(Linear polyethylene)
A copolymer, on the other hand, results from two different monomers
by addition polymerization. For example, a thermoplastic polymer with
better properties than an ethylene homopolymer comes from copolymer-
izing ethylene and propylene:
302 Chemistry of Petrochemical Processes
Block copolymers are formed by reacting two different prepolymers,
which are obtained by polymerizing the molecules of each monomer
separately. A block copolymer made of styrene and butadiene is an
important synthetic rubber:
Alternating copolymers have the monomers of one type alternating in
a regular manner with the monomers of the other, regardless of the com-
position of the reactants. For example, an alternate copolymer of vinyl
acetate and vinyl chloride could be represented as:
Chapter 11 1/22/01 11:10 AM Page 302
Random copolymers have the different monomer molecules distrib-
uted randomly along the polymer chain.
A polymer molecule may have just a linear chain or one or more
branches protruding from the polymer backbone. Branching results
mainly from chain transfer reactions (see “Chain Transfer Reactions”
later in this chapter) and affects the polymer’s physical and mechanical
properties. Branched polyethylene usually has a few long branches and
many more short branches
Polymerization 303
Intentional branching may improve the properties of the product poly-
mer through grafting. A graft copolymer can be obtained by creating
active sites on the polymer backbone. The addition of a different
monomer then reacts at the active site and forms a branch. For example,
polyethylene irradiated with gamma rays and then exposed to a reactive
monomer, such as acrylonitrile, produces a polyethylene-polymer with
acrylonitrile branches:
2,3
Crosslinked polymers have two or more polymer chains linked
together at one or more points other than their ends. The network formed
improves the mechanical and physical properties of the polymer.
Crosslinking may occur during the polymerization reaction when multi-
functional groups are present (as in phenol-formaldehyde resins) or through
outside linking agents (as in the vulcanization of rubber with sulfur).
POLYMERIZATION REACTIONS
Two general reactions form synthetic polymers: chain addition
and condensation.
Chapter 11 1/22/01 11:10 AM Page 303
Addition polymerization requires a chain reaction in which one
monomer molecule adds to a second, then a third and so on to form a
macromolecule. Addition polymerization monomers are mainly low
molecular-weight olefinic compounds (e.g., ethylene or styrene) or con-
jugated diolefins (e.g., butadiene or isoprene).
Condensation polymerization can occur by reacting either two similar
or two different monomers to form a long polymer. This reaction usually
releases a small molecule like water, as in the case of the esterification of
a diol and a diacid. In condensation polymerization where ring opening
occurs, no small molecule is released (see “Condensation Polymeri-
zation” later in this chapter).
ADDITION POLYMERIZATION
Addition polymerization is employed primarily with substituted or
unsubstituted olefins and conjugated diolefins. Addition polymerization
initiators are free radicals, anions, cations, and coordination compounds.
In addition polymerization, a chain grows simply by adding monomer
molecules to a propagating chain. The first step is to add a free radical, a
cationic or an anionic initiator (I
Z
) to the monomer. For example, in eth-
ylene polymerization (with a special catalyst), the chain grows by attach-
ing the ethylene units one after another until the polymer terminates. This
type of addition produces a linear polymer:
I
Z
+ CH
2
=CH
2
r
I[–CH
2
–CH
2
]–
z
Branching occurs especially when free radical initiators are used due
to chain transfer reactions (see following section, “Free Radical
Polymerizations”). For a substituted olefin (such as vinyl chloride), the
addition primarily produces the most stable intermediate (I). Inter-
mediate (II) does not form to any appreciable extent:
304 Chemistry of Petrochemical Processes
I
z
= a free radical I
•
, cation I
+
, or an anion I
–
R = alkyl, phenyl, Cl, etc.
Propagation then occurs by successive monomer molecules additions to
the intermediates. Three addition modes are possible: (a) Head to tail; (b)
Head to head, and (c) tail to tail.
Chapter 11 1/22/01 11:10 AM Page 304
The head-to-tail addition mode produces the most stable intermediate.
For example, styrene polymerization mainly produces the head-to-
tail intermediate:
Polymerization 305
Head-to-head or tail-to-tail modes of addition are less likely because the
intermediates are generally unstable:
Chain growth continues until the propagating polymer chain terminates.
Free Radical Polymerization
Free radical initiators can polymerize olefinic compounds. These chem-
ical compounds have a weak covalent bond that breaks easily into two
free radicals when subjected to heat. Peroxides, hydroperoxides and azo
compounds are commonly used. For example, heating peroxybenzoic acid
forms two free radicals, which can initiate the polymerization reaction:
Free radicals are highly reactive, short lived, and therefore not selective.
Chain transfer reactions often occur and result in a highly branched prod-
uct polymer. For example, the polymerization of ethylene using an
organic peroxide initiator produces highly branched polyethylene. The
branches result from the abstraction of a hydrogen atom by a propagat-
ing polymer intermediate, which creates a new active center. The new
center can add more ethylene molecules, forming a long branch:
Head to-tail mode
Chapter 11 1/22/01 11:10 AM Page 305
Intermolecular chain transfer reactions may occur between two propa-
gating polymer chains and result in the termination of one of the chains.
Alternatively, these reactions take place by an intramolecular reaction by
the coiling of a long chain. Intramolecular chain transfer normally results
in short branches:
4
306 Chemistry of Petrochemical Processes
Free radical polymers may terminate when two propagating chains com-
bine. In this case, the tail-to-tail addition mode is most likely.
Polymer propagation stops with the addition of a chain transfer agent.
For example, carbon tetrachloride can serve as a chain transfer agent:
—CH
2
—C
˙
H
2
+ CCl
4
r —CH
2
—CH
2
Cl + C
˙
Cl
3
The C
˙
Cl
3
free radical formed can initiate a new polymerization reaction.
Cationic Polymerization
Strong protonic acids can affect the polymerization of olefins (Chapter
3). Lewis acids, such as AlCl
3
or BF
3
, can also initiate polymerization. In
this case, a trace amount of a proton donor (cocatalyst), such as water or
methanol, is normally required. For example, water combined with BF
3
forms a complex that provides the protons for the polymerization reaction.
An important difference between free radical and ionic polymeriza-
tion is that a counter ion only appears in the latter case. For example, the
intermediate formed from the initiation of propene with BF
3
-H
2
O could
be represented as
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