Addition and Condensation Polymerization Processes

Downloaded by 80.82.77.83 on September 30, 2017 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0091.pr001

PREFACE " p o l y m e r science has advanced to the point where macromolecules can be built at will. They can be designed with flexible or stiff chains, they can be linear or branched, and they can be tailored in length. They can be attached to each other by van der Waals' forces, dipole interaction, hydrogen or ionic bonding, or crosslinking. Amorphous and crystalline polymers may be made from established or new monomers by addition polymerization triggered by free radicals or ions or by condensation, coupling, ring-opening, or ring-closing reactions. The understanding of the relationship between the molecular structure, the kinetics of the process, and the performance characteristics of the product enables us to design better polymers with improved physical properties at reduced cost. Different macromolecules are being designed for different end uses which include fabrics, surface coating or adhesives, structural, automotive, packaging, electrical,, glazing, photographic, thermal, medicinal, and other applications. The first 26 chapters of this volume are devoted to process improvements and the kinetics of addition and condensation polymerization of common monomers. The remaining 22 chapters are concerned with new and better polymers designed for a specific end use or physical property. Process

Improvements

and

Polymerization

Kinetics

Process Technology. In commercial addition and condensation polymerization processes reactor design is an important factor for the quality and economics of the polymer. Combining macromolecular kinetics with reactor and process design has led to a new concept called reaction engineering. D . C. Chappelear and R. H . M . Simon review this novel concept in Chapter 1. Free-Radical Polymerization in Bulk, Solution, and Suspension. In free-radical polymerization in bulk, solution, and aqueous suspension the initiator is dissolved in the monomer. Aqueous suspension polymerization is considered as bulk polymerization in droplets. Some polymers, like poly (vinyl chloride) or polyacrylonitrile, are insoluble in the monomers and precipitate during bulk polymerization. The growth of the precipitated chain, which depends on the number of trapped radicals, is xi

Platzer; Addition and Condensation Polymerization Processes Advances in Chemistry; American Chemical Society: Washington, DC, 1969.


described by O. G . Lewis and R. M . King. Other commercial polymers, like polystyrene or poly (vinyl acetate), remain soluble i n their monomers. Glass-reinforced polyester is manufactured by copolymerizing styrene with unsaturated polyesters between glass fibers. For this copolymerization, enolizable phenylketo compounds may be used as initiator, according to H . Hopff and co-workers. The initiation mechanism is explained by an autoxidation to an active ketohydroperoxide form. The copolymerization proceeds at approximately the same rate as with the usual peroxide initiators, but the reaction is less exothermic, thus preventing crazing, bubble formation, and volume contraction. In the commercial terpolymerization of styrene with 2-ethylhexyl acrylate and glycidyl acrylate high conversion and narrow molecular weight distribution are desirable. The effects of polymerization temperatures and initiation rate on molecular weight, molecular weight distribution, and composition have been studied by A . Ravve and J. T. Khamis. The kinetics of free-radical polymerization of methyl methacrylate and other monomers with small amounts of S 0 2 and tert-butyl hydroperoxide in bulk and solution have been investigated by P. Ghosh and F. W . Billmeyer. The free-radical polymerization of methyl methacrylate, acrylonitrile, and other polymer monomers can be accelerated by adding Lewis acids, like zinc chloride or alkylaluminum chloride. The polar monomer forms a complex with the Lewis acid and becomes more electron accepting. In the presence of a nonpolar olefin or conjugated diene, the complexed polar monomer transfers its charge and copolymerizes readily, as described by N . G . Gaylord and A . Takahashi. E S R studies of free radical polymerization had been limited to solid state systems. Recently, B. Ranby and K. Takakura were able to explore vinyl polymerization in aqueous solution, using a redox system of T i C l * with HoOo. Free-Radical Polymerization in Emulsion. In suspension polymerization, the particle size is fixed by the size of the monomer droplet which contains the initiator. Emulsion polymerization differs from suspension polymerization in that the initiator is dissolved in the aqueous phase and the polymer particle grows during polymerization. Free radicals are generated in the water and diffuse to the monomer-water interface. The length of the polymer chain formed, or equivalently the molecular weight, depends on the rate of free radical arrival and termination. S. Katz, G. M . Saidel, and R. Shinnar present a computation method to calculate the molecular weight distribution for arbitrary rates of free radical arrival and termination. V i n y l chloride is polymerized in bulk or aqueous suspension to yield a dry-blend resin of porous particle surface. In aqueous emulsion it xii



polymerizes into particles of closed surface, used as paste resin. In continuous emulsion polymerization, a broad distribution of particle sizes is obtained, depending on the residence time in the reactor. In the batch emulsion polymerization of styrene, it is possible to obtain a latex of uniform large particle size by seeding the emulsion with particles and controlling the emulsifier concentration. In the batch emulsion polymerization of vinyl chloride this has not been possible. A latex of two particle sizes is obtainable—one about 1/x which is grown on 0.4-/* seeds, and one about 0.3//, grown as new particle from a micelle. G . Gatta and co-workers discovered that it is possible to produce also a poly (vinyl chloride) of a single large particle size by seeding. This requires not only control of the emulsifier concentration but also a definite ratio of seeded particle surface to the water volume. V i n y l acetate is polymerized in aqueous emulsion and used widely in surface coating and in adhesives. Copolymerized with vinyl esters of branched carboxylic acids and small quantities of acrylic acid, it gives paint latices of excellent performance characteristics. G . C. Vegter found that a coagulum-free latex of very low residual monomer content can be produced from a mixture of an anionic and a nonionic emulsifier according to a specific operating procedure. The freeze/thaw stability of polymeric latices has been investigated by H . Naidus and R. Hanzes. Radiation-Induced Polymerization. Polymerization induced by irradiation is initiated by free radicals and by ionic species. O n very pure vinyl monomers, D . J. Metz demonstrated that ionic polymerization can become the dominating process. In Chapter 12 he postulates a kinetic scheme starting with the formation of ions, followed by a propagation step via carbonium ions and chain transfer to the vinyl monomer. C. Schneider studied the polymerization of styrene and a-methylstyrene by pulse radiolysis in aqueous medium and found results similar to those obtained i n conventional free-radical polymerization. She attributes this to a growing polymeric benzyl type radical which is formed partially through electron capture by the styrene molecule, followed by rapid protonation in the side chain and partially by the addition of H and O H to the double vinyl bond. A . S. Chawla and L . E . St. Pierre report on the solid state polymerization of hexamethylcyclotrisiloxane by high energy radiation of the monomer crystals. Anionic Polymerization. Anionic polymerization is limited to nonpolar monomers with carbon-to-carbon double bonds. It takes place i n the presence of catalysts capable of generating carbanions—e.g., alkali metals, metal hydrides, metal alkyls, amides, and Grignard reagents. The xiii


initiating step may occur i n two principal ways, as follows: M° + C H 2 = C H -> M + . . . [ C H 2 — C H ?=* C H 2 — C H ] R

R

R anion radical

X — M + C H , = C H -> X — C H 2 — C H

.. M

R

R


anion

M + represents the positive counterion, also called "gegenion," which, in many cases, accompanies the growing chain. Propagation occurs through successive addition of monomer molecules to the charged or "living" ends of the growing chains. Anionic polymerization involves several kinds of anionic species propagating simultaneously. M . Szwarc distinguishes between free ions, contact ions pairs, solvent-separated ion pairs, triple ions, complexed ion pairs, and others. H e determined the activation energies of the various types of growth and the heats of the different transformations. If the propagation step is much faster than the initiating step, two or more polymers differing in chain length and stereospeciflcity are obtained. This was demonstrated in the polymerization of methyl methacrylate with Grignard or organolithium catalyst. V . N . Sokolov and I. Y. Poddubny propose a mechanism for the stereospecific polymerization of dienes. They also assume a dynamic equilibrium during the catalyst complexing and the subsequent polymer chain propagation. In their experiments, butenyllithium- and butenylmagnesium chloride were used as catalysts. Catalysts of the Ziegler-Natta type are applied widely to the anionic polymerization of olefins and dienes. Polar monomers deactivate the system and cannot be copolymerized with olefins. J. L . Jezl and coworkers discovered that the "living" chains from an anionic polymerization can be converted to free radicals by the reaction with organic peroxides and thus permit the formation of block copolymers with polar vinyl monomers. In this novel technique of combined "anionic-free radical" polymerization, they are able to produce block copolymers of most olefins, such as alkylene, propylene, styrene, or butadiene with polar vinyl monomers, such as acrylonitrile or vinyl pyridine. Cationic Polymerization. Cationic polymerization is initiated by the transfer of a cation from the catalyst to the monomer. It allows a wider choice of monomers with double bonds, including carbonyls, cyclic ethers, and lactones. The ion may be within a carbonium or an oxonium ion. Friedel-Crafts halides, like A1C1 3 or A 1 ( C 2 H 5 ) C 1 2 , are strong Lewis acids and initiate the polymerization directly. Weak Lewis acids need a xiv


cocatalyst to aid cation transfer. Such cocatalysts may be a small amount of water, HC1, an ether, or another solvent. It is even possible that i n the case of a strong Lewis acid a small amount of water or another proton donor acts as a cocatalyst: BF3 + H 2 0

BFHOH . . . . H

BF3OH . . . . H + C H 2 = C H - » C H 8 — C H . . . . B F 3 O H "

R


R

BF3OH " represents the negative counterion, also called gegenion. Propagation proceeds through the insertion of a new monomer molecule on the carbonium ion between the two charges. J. P. Kennedy and G . E . Milliman studied the cationic polymerization with a weak Lewis acid, like trialkylaluminum. They required a Lewis base, like tert-butyl chlo ride, as a cocatalyst. They assume the formation of a complex catalyst in the polymerization of isobutylene and other olefins. V . A . Kormer and co-workers studied the sterospecific polymerization of 1,3-butadiene with bis(7r-allylnickel halides) and Lewis acids. They found that the strength of the bond between the transition metal and the growing chain influences the structure of the polybutadiene formed. The initiation mechanism for cationic polymerization of cyclic ethers, vinyl amines, and alkoxy styrenes has been investigated by A . Ledwith. H e used stable cations, like tropylium or triphenylmethyl cations with stable anions, like SbCl 6 ", and distinguished between three initiation reactions: cation additions, hydride abstraction, and electron transfer. One of the typical examples of cationic polymerization, in which the propagating species is the oxonium ion, is the polymerization of tetrahydrofuran. P. and M . P. Dreyfuss studied this polymerization with the triethyloxonium salts of various counterions and established an order of Θ

(C2HB)8O

Θ

· sbci

>

+ q

6

/

CH.,—CH 2

+SbCl

r\Ti C H 2 — C H 2 — Ο

5

2

CH 2 C1

xv


stability for P F 6 " , SbF 6 ", BF 4 ~, and S b C l G " anions. It had been difficult to copolymerize tetrahydrofuran or other oxonium type species with carbonium type species like styrène. Y. Yamashita discovered that certain monomers, like trioxane or 1,3-dioxolane would form carbonium ions with ( C 2 H 5 ) 2 O B F 3 , but oxonium ions with ( C 2 H 5 ) 3 O B F 4 , and would copolymerize with either one of the above species. During the initial polymerization of trioxane with ( C 4 H ) O B F 3 in melt or solution, no solid polymer is formed, and the reaction medium remains clear. Using a high resolution N M R spectroscope, C. S. H . Chen and A . D i Edwardo observed the appearance of soluble linear polyoxymethylene chains. In the cationic copolymerization of trioxane with 1,3-dioxolane, V . Jaacks found also that a soluble copolymer forms first and turns later into a crystalline copolymer of different composition. Crystallization and polymerization proceed simultaneously in the solid phase. Polymerization with Complex Catalysts. H i g h density polyethylene reached a domestic production of 1.25 billion pounds in 1968. It is made either with a stereospecific Ziegler-Natta catalyst or on a supported chromium oxide catalyst. The latter forms a complex with the silicaalumina and is activated by treatment with air and steam at elevated temperature. The mechanism is such that electrons are donated to the catalyst in order to be returned under polymerizational-promoting conditions, consequently lowering the energy of the system:


9

CH2—CH2

CH2—CH2

—CrO—CrO—

CH2—CH2 1

2

CH2—CH2

—Cr—O—Cr—O—

Θ Θ CH2—CH2—CH2—CH2 —Cr—O—Cr—O— It is, however, more complex than this because the silica-alumina support affects also the polymerization. A . Clark describes the correlation between the chromium trioxide and the silica-aluminum support. H e also shows the effect of the catalyst activation temperature on the molecular weight of the polyethylene formed. Cycloolefins can be polymerized by a ring-opening mechanism with complex tungsten-aluminum catalyst, according to K. W . Scott and co workers. The polymer formed is unsaturated and is either elastomeric or rigid, depending on the monomer and degree of polymerization. xvi



Polymers

at Will

and

the

Limitations

A typical example showing that we are able to build macromolecules at w i l l is given by C. P. Pinazzi and co-workers i n the first chapter of the second section, Chapter 27. They report how model polyenes can be built and how they react. In Chapter 28 K. F. O'Driscoll illustrates the limita tions i n polymerization. For every vinyl monomer, a ceiling temperature exists, above which depropagation exceeds polymerization. If two vinyl monomers are copolymerized at a temperature at which one depropagates, the polymer formed w i l l have an unusual composition and sequence distribution. Occasionally, low molecular weight polymers are required as sealants, adhesives, or in rocket solid propellants. F. P. Baldwin and co-workers report on the manufacture of a low molecular weight carboxy terminated product made by depropagating ozonization of a higher molecular weight polyisobutylene. Polymers of High Strength and Toughness. A l l polymer chains, regardless of their detailed structure, have about the same strength of ca. 2,000,000 p.s.i.g. since they are formed by C — C , C — N , C — O , or C—S covalent bonds. However, the tensile strength of commercial thermoplastics is only 3000-17,000 p.s.i.g. and depends primarily on the interaction between the polymeric chains. Interaction between chains is affected either by secondary forces, such as the van der Waals' forces, dipole-dipole attraction, and hydrogen bonding or by primary forces, such as crosslinking by ionic or covalent bonding. Its effectiveness can be increased by lengthening the chains or by arranging the chains in a crystalline structure. In selecting a polymerization process, it is impor tant to know the phase of the monomer and polymer since the phase affects the configuration and crystallinity of the product. V . A . Kargin shows the importance of the phase in Chapter 30 on the formation of liquid crystals and the analogy of the nucleation of polymers to the nucleation i n crystallography. The possibility of fracture on impact can be reduced by dispersing an elastomeric phase uniformly through the rigid material, as it is done in polyblends or better in grafting vinyl monomer upon rubber. H . Bartl and D . Hardt describe the manufacture of a tough rigid P V C by grafting vinyl chloride upon an elastomeric ethylene—vinyl acetate copolymer. The internal cross-linking of styrene copolymers is dealt with in the chapter of R. N . Haward, Β. M . Parker and E . F. T. White, who prepared a copolymer of styrene and hydroxyethyl methacrylate and crosslinked the copolymer with hexamethylene diisocyanate. Light Weight Polymers. In many instances, lighter weight, thermal insulation, and cushioning effects are considered more important than xvii



strength. During the last 15 years, a number of light weight thermoplastic and thermoset foams with closed and open cellular structure have been developed using volatile foaming or chemical blowing agents. A . R. Ingram reviews various processes for making expandable polystyrene which can be molded or extruded into foam. A new expandable resin is a copolymer of 1-butene with sulfur dioxide where excess S 0 2 acts as a foaming agent. Its manufacture by suspension polymerization is reported by J. Châtelain. Non-flammable Plastics. Non-flammable plastics are required as structural material i n commercial buildings and for many automotive, electronic, and electrical applications. From the thermoplastics, only the halogen-containing polymers, polyamides, polycarbonate, poly(phenylene oxide), polysulfone, and polyimides are self-extinguishing. Flammable plastics can be made self-extinguishing by incorporating fire retardants or by adding mineral fillers. A novel version of making A B S (acrylonitrile-butadiene-styrene graft copolymer) non-flammable, without reducing softening temperature, is by incorporating bis(dibromopropyl) fumarate as a fourth monomer. This is described by W . C u m mings and R. E . Stark. Modified Surfaces. It is frequently desirable to change the surface of a polymer. Nonpolar surfaces of plastics are characterized by static electricity buildup, non-wetting, poor adhesion, low printability, and poor dyeing. These disadvantages can be overcome by grafting polar vinyl monomers upon the surface by irradiation. A . S. Hoffman describes radiation grafting of polyelectrolytes upon nonpolar surfaces, and A . Chapiro and co-workers discuss radiation grafting of acrylic acid and vinyl pyridene upon Teflon films. On the other hand, it might be desirable to reduce the hygroscopicity of certain polymers. J. C . Arthur deals with radiation grafting of vinyl monomers upon cotton cellulose. Transparent Polymers. Amorphous thermoplastics, like poly (methyl methacrylate ), polystyrene, S A N , P V C , or the cellulose esters are transparent and used for glazing, photographic film, blown bottles, or clear packaging containers. Only a few crystalline thermoplastics, like poly(4methyl-l-pentane), where the crystalline and the amorphous phases have almost identical refractive indexes, or polycarbonate, which has smaller crystals than the wavelength of light, are also transparent. R. Kosfeld and co-workers analyzed the mobility of methyl groups i n polycarbonate, poly (methyl methacrylate) and poly (α-methyl styrene) by N M R spectroscopy. Generally, to make glass-clear polymers, we must prevent crystalli zation. This can be accomplished by placing bulky side groups on the main chains. Changing from a crystalline to an amorphous configuration xviii



results in less rigidity and lower heat deflection temperature. To main tain high temperature performance, the backbone must be stiffened by placing aromatic rings into the main chains. Two typical examples are a glass clear amorphous aromatic polyamide with three methyl side chains and a phenylated polyphenylene. The polyamide is made by polycondensation of trimethylhexamethylenediamine with terephthalic acid, as described by G . Bier. The phenylated polyphenylene is made by a Diels-Alder reaction of biscyclopentadieneone with diacetylene, as described by J. K. Stille and co-workers. Heat-Resistant Polymers. One of the greatest disadvantages of the commodity plastics i n comparison with natural building materials is their low softening temperature. B y building an inflexible aromatic or heterocyclic ring into the chain, the softening temperature can be raised significantly. Linear p-polyphenylene by itself is a very crystalline, brittle, insoluble, and infusible polymer because its chain is essentially inflexible. Chain flexibility may be introduced by inserting flexible hinges, like alkyl, — O — , — C O — , — N H — , — Ν , — , —S—, or — S O , — groups between the rings. Poly-p-xylylene

has — C H 2 — C H 2 — as a flexible hinge and a melting point of 400 ° C . It is made by a novel vapor deposition polymerization process, as described by its inventor, W . F. Gorham. Poly(phenylene oxide) has — Ο — as a flexible hinge. It is made by

oxidative coupling with cuprous chloride in pyridine. Beginning with a phenol having two methyl side chains, the polymer remains amorphous with a heat deflection temperature of 2 0 4 ° C . (at 66 p.s.i.g.) and can be thermoprocessed. The process and its mechanism are reviewed by G . D . Cooper and A . Katchman. xix



Oxidative coupling is also to be used to prepare azopolymers from

aromatic diamines with cuprous chloride or dimethyl acetamide or its mixture with pyridine, as reported by H . C. Bach and W . B. Black. Bisphenol-A (ρ,ρ-isopropylidene diphenol), made by the reaction of phenol with acetone, is one of the components in the condensation poly merization of polycarbonate and phenoxy, as well of polysulfones and polysulfonates. CH3

CH;,

CH:,

CH;,

The polysulfones are made by condensation polymerization of the potas sium salt of bisphenol-A with dichlorodiphenyl sulfone, as discussed by S. R. Schulze and A . L . Baron. The polysulfonates are made from bisphenol-A and disulfonyl chlorides. They are more brittle than polysulfone and have been suggested by R. J. Schlott and co-workers to be used in coplymers with linear polyesters to improve the hydrolytic stability of the latter. The polyimides are the best developed class of high temperature polymers with exceptional oxidation resistance. They are generally based on the condensation polymerization of aromatic dianhydrides with di amines forming a soluble polyamic acid, followed by intermolecular cyclodehydration to the insoluble polydiimide. Their maximum use temperature is 358 ° C . Recently H . Reimschuessel prepared a polyimide xx


from β-carboxymethylated caprolactam by opening the lactam followed by intermolecular cyclodehydration:

-CH

ring

>N—CH2—CH2—CH2


^CH.—cr

Semi-ladder polymers, possessing aromatic and hetero rings, are prepared by condensation polymerization and cyclodehydration from aromatic dicarboxylic or dicarbonyl compounds with aromatic tetraamines of diaminophenols or aminothiophenols. They are used in composites with glass cloth or high strength whiskers, or as adhesive for metal to metal bond. They are characterized by good strength properties from very low temperatures (as — 2 6 0 ° C . ) to very high temperatures ( 3 2 0 ° - 6 5 0 ° C . ) . C. Berry and co-workers discuss novel high temperature materials devel oped for the A i r Force: B B B (poly(bisbenzimidazo benzo phenanthrolinedione)) and B B L , both made by condensation polymerization of naphthalenetetracarboxylic acid with either tetraaminobiphenyl or tetraaminobenzene. These polymers are finding increased use in space-craft technology. Conclusion

The first 26 chapters represent a cross-section of the numerous proc esses available for the polymerization of common monomers. Through the understanding of process kinetics and through novel technology, we are able to build better polymers from these monomers. The remaining 22 chapters should leave the reader with the impression that we can build new or modified polymers at w i l l and design them for specific end uses. NORBERT A . J. PLATZER

Springfield, Mass. January 1969

xxi


Addition and Condensation Polymerization Processes

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