STEP-GROWTH POLYMERIZATION

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STEP-GROWTH POLYMERIZATION TIMOTHY E. LONG Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0212 S. RICHARD TURNER Polymers Technology Group, Research Laboratories, Eastman Chemical Company, Kingsport, TN 37662

Introduction Requirements for Successful Step-Growth Polymerization Polyesters Polycarbonates Polyamides Polyurethanes Polyimides

Introduction Step-growth polymers are produced by the reactions of functional groups of monomers in a stepwise progression from dimers, trimers, etc. to eventually form high polymer. When this polymerization process is accompanied by the elimination of small molecules during the reaction step, the process is called polycondensation. An excellent review of the history of step-growth polymers is contained in Morawetz's "Polymers, The Origins and Growth of a Science" (1). The first example of step-growth polymer dates back over 100 years with the reaction by Baeyer of resorclnol and formaldehyde (2). Years later Baekeland patented and commercialized these chemistries when they were done under pressure and with suitable fillers present (3). The struggle of Staudinger to gain acceptance of macromolecules is well documented and perhaps is a reason why most of the early major discoveries and development of the science involved in step-growth polymers were made in industry. Carothers, armed with the concept of macromolecules, as championed by Staudinger, envisioned that large molecules could be synthesized by the direct addition of diols and diacids to form polyesters and diamines and diacids to form polyamides (4,5). Out of this pioneering work by Carothers (and Flory who joined Carothers at DuPont) came the

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T.E. Long and S.R. Turner

basic principles of condensation polymerizations (6, 7) and the initial connmercial linear synthetic step-growth polymer, Nylon 6,6. It is interesting that polyesters were the original focus of Carothers' research, but were abandoned since the melting points of the resulting polyesters were too low to be useful. Whinfield (8) from ICI discovered that changing the aliphatic diacids, which were the basis of Carothers' studies, to an aromatic diacid (terephthalic acid) led to a high melting polymer (256 °C) when it was condensed with ethylene glycol. This discovery led to the development of poly (ethylene terephthalate) (PET) which, even today, Is one of the fastest growing, if not the fastest growing, step-growth polymer. Another of the basic step-growth polymer families, polyurethanes and polyureas, arose from basic work at Farbenfabriken Bayer by Bayer (9). These chemistries were investigated as alternatives to the polyamides of DuPont. Polycarbonates and polyimides are two other important step-growth polymers of considerable commercial importance that will be discussed in this chapter. Today the commercial volume of basic step-growth polymers is approximately 85 billion pounds (1996) and is growing at a healthy rate (10). Progress continues to be made along many fronts with step-growth polymers. Unlike polyolefins and other vinyl polymers, where catalyst and process changes can lead to different types of monomer enchainments with concomitant mechanical and other property changes, significant variation in the properties of step-growth polymers requires the Incorporation of new monomers into the polymer backbone. For the majority of the engineering resin applications, where most step-growth polymers find applications, the development and commercialization of new step-growth monomers and polymers are challenging tasks. This chapter will review the basics of the chemistries that govern the build-up of molecular weight In step-growth polymerizations. This is followed by summary discussions on the major classes of step-growth polymers i.e., polyesters, polycarbonates, polyamides, polyurethanes, and polyimides. Other step-growth polymers such as poly(arylene ethers), polyketones, and polysulfones are also important commercially, but are not discussed in this chapter.

Requirements for Successful Step-Growth Polymerization As discussed in the Introduction, Carothers (and Flory) derived several mathematical equations that facilitate our understanding of the experimental and manufacturing requirements for successful preparations of high molecular weight macromolecules (6,7). Equation 1 defines the number average degree of polymerization (Xn) In terms of the extent of reaction (p) in a difunctional step growth polymerization process, and it is quickly appreciated that very high levels of monomer conversion (p > 0.99) lead to higher number average degrees of polymerization and molecular weight.

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Consequently, the ability to achieve high (greater than 99%) degrees of functional group conversion is essential for the preparation of high molecular weight polymers. In most instances, the thermal and mechanical properties of many thermoplastics increase linearly and eventually plateau with an increase in molecular weight. Molecular weight is known to influence many thermal and mechanical properties of commercial products including crystallization rate, strain-induced crystallization, impact properties, toughness and other mechanical tensile and flexural properties. Xn = 1/1-p where

Eq.1

Xn = number average degree of polymerization p = the degree of functional group conversion

The importance of achieving high degrees of functional group conversion and high molecular weight products points to several other related requirements for successful step-growth polymerization. Industrial and economic considerations require that the polymerizations proceed quickly in order to achieve high degrees of conversion at economically feasible rates. In addition, perfect reaction stoichiometries or potential processes that in-situ generate perfect stoichiometries are required. Consequently, careful monomer charging and ultra-pure monomer sources are required in step-growth polymerization processes. The presence of side reactions also will deleteriously effect the reaction stoichiometry resulting in the inability to prepare high molecular weight products. Although one could intuitively propose the use of longer polymerization times to achieve higher molecular weight products, the importance of relatively slow side reactions during the polymerization process becomes more important when using either longer polymerization times or higher polymerization temperatures. For example, the manufacture of PET is accompanied by the formation of diethylene glycol (DEG), vinyl esters, and acetaldehyde during the polymerization process. DEG is incorporated into the chain during the polymerization and its presence in the backbone has been shown to negatively effect the chemical resistance, crystallization rate, thermal stability, and ability for strain-induced crystallization in many PET product applications. As discussed earlier in the introduction, step-growth polymerizations proceed via the reaction of any two species in the reaction mixture, and step-wise reactions result in the formation of high molecular weight polymers. In many instances, step-growth polymerization results in the formation of a condensate, and earlier nomenclatures referred to step-growth polymerization processes as condensation polymerization processes. Although the current scope of suitable chemistries has eliminated the strict requirement for the formation of a condensation by-product, many commercial stepgrowth polymers including polyesters and polyamldes do involve an equilibrium between the monomeric reactants and polymerization by-products. Consequently, the presence of this equilibrium requires that the synthetic process completely eliminate the polymerization by-product in order to achieve high molecular weight. A salient feature of step-growth polymerization processes is the gradual Increase in molecular weight throughout the polymerization process. This observation is a direct result of the step-wise addition of reactive species (monomers, dimers, trimers, tetramers, etc.) to form high molecular weight. In contrast, high molecular products are

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obtained very quickly in chain polymerization processes and are typically accompanied by highly exothermic reactions. Consequently, the gradual increase in melt viscosity and the absence of highly exothermic reactions facilitates the use of bulk polymerization processes in step-growth polymerization. For example, polyesters and polyamides are manufactured commercially in the absence of solvents and the final products are directly usable without further isolation and purification. Although molecular weights gradually increase throughout most of a step-growth polymerization process, the final stages of the polymerization process involve the rapid increase in molecular weight. It is well known that the melt viscosity for thermoplastics increases with the 3.4 power of the weight average molecular weight. It is not surprising, therefore, that significant attention has been devoted in recent years to the development of novel agitation and reactor designs to facilitate the transport of the viscous melt and the transport of polymerization by-products in the latter stages of step growth polymerization processes (11). The requirements for a successful step growth polymerization process are summarized as follows: (1) High conversion reactions (greater than 99%) (2) Absence of deleterious side reactions resulting in the loss of functionality (3) Controlled functional group stoichiometry (4) High monomer purities (5) Efficient removal of any polymerization condensates (6) Relatively fast polymerization rates Although high molecular weight is often required for the maximization of thermal and mechanical properties, the synthesis of difunctional oligomers (less than 10,000 g/mole) Is accomplished in step-growth polymerization using monofunctional reagents or a stoichiometric excess of a difunctional monomer. The difunctional oligomers are suitable starting materials for the preparation of crosslinked coatings, adhesives, or segmented block copolymers. Due to the reactive nature of many polymers prepared using step-growth polymerization processes, reactive endgroups and internal reactive functionalities offer the potential for subsequent deriviatization, chain extension, or depolymerization. For example, the depolymerization of polyesters is easily accomplished with the addition of a suitable difunctional acid or glycol (12). In addition, polyester and polycarbonate blends are easily compatibilized in a twin screw extruder to prepare optically clear coatings (13). It should also be noted that commercial processes often require that the polymerization process minimize the use of solvents and the in-situ fonnation of salts. In most instances, the final polymeric products are directly charged to extrusion and molding operations without further expensive purification steps. It is easy to understand, based on the above discussion, the serious restrictions that severely limit the number of suitable organic reactions that have been used for the successful preparation of high molecular weight products via step-growth polymerization. Although many synthetic organic reactions appear to be suitable for the preparation of macromolecules via a step-growth polymerization process, most organic reactions do not meet all the necessary requirements and have not been utilized in commercial products. A challenge remains to broaden the scope of suitable

Step-Growth Polymerization

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polymerization chemistries and processes leading to new families of high performance polymeric products.

Polyesters Commercial importance and aoolications. As stated in the Introduction, polyesters were apparently the first targets of Carothers (4), but were not attractive for commercial applications because of low melting points. The insertion of aromatic groups into the backbone using terephthalic acid was the discovery that overcame this deficiency (8). From this beginning poly(ethylene terephthalate) (PET) has by far become the most important polyester with over 60 billion pounds produced in 1997. The first major application area for PET was In textile fibers and today about 70% of the PET produced is used in fibers. The fastest growing area is for application in food and beverage containers consuming around 20% of the PET produced with a growth rate of 10-12 %/year. The remaining 10 % are consumed as extruded film (photographic, magnetic tape, etc.) and in engineering plastics, primarily as glass fiber reinforced composites (14). The high growth rate in the container area is based on ease of processing, barrier, aesthetic properties of the containers, recycle ease (12), and cost. The reheat blow molding process, used to produce containers, utilizes the unique strain hardening characteristics of PET and permits the high speed manufacture of high volumes of containers. Copolyesters containing small amounts of 1,4-cyclohexanedimethanol (CHDM) or isophthalic acid (IPA) are often used instead of the homopolymer of PET in the bottle applications because these copolyesters process easier. Other, specialty polyesters are increasingly becoming important as commercial step-growth polymers. The commercialization of 2,6-dimethylnaphthalene dicarboxylate (DMN) has made the commercialization of poly(ethylene naphthalate) (PEN) possible. This polyester has a Tg of 125 °C compared to PET with a Tg of 80 °C. In addition its barrier to oxygen in containers is approximately 5 times that of PET (15). PEN homopolymer, copolymers, and blends are currently being investigated in refillable containers, beer containers, and small volume containers requiring enhanced barrier properties. Future obstacles to the larger penetration of PEN and copolymers in the marketplace will include the necessity for less expensive monomer grades in commodity beverage container markets and the wider application of 2,6-naphthalene monomer in higher value, niche applications. In addition, higher processing temperatures required for PEN for higher melt viscosities often hamper manufacturing and molding operations. Opportunities in packaging of oxygen sensitive foods have led to considerable interest in Ws-hydroxyethyl resorcinol (HER), which has been shown to provide significantly improved barrier (16). Other specialty monomers that have been recently commercialized include 1,4-cyclohexanedicarboxylic acid (CHDA), which has found applications in tough coatings (17) and 1,3-propane diol (PDO). PDO has been available for many years, but recent developments in catalytic (18) and biochemical (19) processes have lowered the cost of PDO to permit the commercialization of poly(trimethylene terephthalate) (PTT) as a carpet fiber (20). In addition to applications in containers and fibers, semi-crystalline polyesters, which are reinforced with glass fibers (GFR), find many uses in the automotive and

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7. E Long and S. R. Turner

electronics industry. Poly(butylene terephthalate) (PBT), which was originally commercialized in the mid 1970s as a replacement for phenolic resins in automotive applications, is used extensively in these kinds of applications. Some semi-crystalline PBT is used unfilled, but the majority is used in GFR composites. The unique combination of toughness, fast crystallization rates, resistance to creep, and ease of compounding in additives such as flame retardants has led PBT to be the polyester of choice for glass filled applications in the electronics, automotive, and fiber cable industries, despite having a lower melting point than PET. The global demand for PBT in 1997 exceeded 7 billion lbs. and the long-term average annual growth rate is predicted to be approximately 6.2% (21). GFR PTT has been shown to have some unique properties when compared to GFR PBT and PET and proposed to have considerable potential for applications in the composite area (22). The polyester of TPA and CHDM, poly(1,4-cyclohexyldimethylene terephthalate, (PCT), which has a melting point of 290 °C, has been found to be particularly useful, as a GFR composite, in electronic and automotive applications where higher temperatures are needed (23). Amorphous polyesters are also important in polyester applications. PET can be extruded into amorphous sheet (aPET) and this clear, transparent sheet can be thermoformed into useful articles. Copolyesters that contain mixtures of EG and CHDM with TPA or IPA (glycol modified PET or PETG) have proven to be excellent clear sheeting materials for signs, etc. and excellent materials with wide processing windows for thermoformed articles (24). Polyesters derived In part from unique rigid aromatic monomers including hydroquinone and biphenyl derivatives offer the potential for new families of high temperature, high performance polyester resins. Although liquid crystalline polyesters (LCPs) were discovered in the late 1960s (25), this family of engineering thermoplastics continues to receive intense academic and industrial attention. In addition to inherent flammability resistance, LCPs offer exceptional moldability due to the shear-induced alignment of the rigid polyester backbones. Therefore LCPs have become very important in the manufacture of small parts for the electronics and other industries. Ticona (formerly Hoechst-Celanese) has pioneered the commercialization of allaromatic liquid crystalline polyesters based on 2-hydroxy-6-naphthaoic acid (HNA) and p-hydroxybenzoic acid (PHB) (26). Many other industrial competitors including Eastman Chemical and DuPont have recently entered this technical arena based on recent bullish projections for LCP market growth. Synthesis. Most commercial polyesters are prepared by the direct esterification of diacids with diols or the transesterification of methyl esters and diols. Many other polyesterification reactions are known to fit the rigorous reaction requirements to give high molecular weight polymers (27). Acid chlorides, in particular, are used to prepare polyesters that are not stable in the melt phase. Other chemistries using silyl derivatives can also be employed (28). Equation 2 depicts the metal catalyzed polymerization of terephthalic acid and ethylene glycol. Typical manufacturing operations, for semicrystalline polyesters, involve the preparation of a low molecular weight oligomer in the melt phase followed by polymerization in the solid state (29). The solid state polymerization process offers two advantages. First, the melt viscosities for the high molecular weight products are avoided. Secondly, the solid state process

Step-Growth Polymerization

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occurs at lower temperatures than the melt phase leading to reduced levels of polymerization by-products such as acetaldehyde. In fact, the solid state process also provides a mechanism for the devolatization of by-products that are formed in the higher temperature, melt phase polymerization process. A variety of metal catalysts have been employed to obtain PET and other polyesters at commercially acceptable rates with minimization of side reactions (30).

0

/—\

HO—C—/

O

Catalyst

VC-OH

+

HOCHjCHgOH

^

**

Eq.2 Catalyst HO-CHpHp-C—<^

j-C-^

^—C-O-CHgCHpH +

H2O

^^

>~C--0--CH2CH204;7- +HO-CH2CH2OH PET

Ester formation based on the reaction between an aromatic carboxylic acid and an aromatic phenol is not suitable for polyester manufacture, and all-aromatic polyesters require the in-situ acetylation of the aromatic phenols. Condensation of the acetate and aromatic carboxylic acid readily occurs at 250 °C with the liberation of acetic acid, and the polymerization mechanism is often referred to as acidolysis. Although the polymerization mechanism appears straightfonvard, Hall and coworkers (31) have recently elucidated the very complex nature of this polymerization process. Equation 3 summarizes the polymerization chemistry to prepare all-aromatic polyesters via the acidolysis route. Industrial attention has focused on the development of suitable manufacturing processes that can handle the corrosive reaction environment and the high polymerization temperatures required in viscous melt phase. Significant attention has also been devoted to the preparation of liquid crystalline polyesters derived from aliphatic glycols and biphenyl dicarboxylic acids.

986

T. E. Long and S. R. Turner ^_/=\

^

HO-C-^^)—C-OH

/ = \ +

HO-^

^ O H

— ^ ^

Eq. 3 HO'

W /=\ °

^

^

/=\

- { I ^ H I I - ^ H Q - O - .

fi

^

CH3-L,,,

Polycarbonates Commercial importance and applications. Polycarbonates had their industrial birth almost 2 decades later than the other major step-growth polymers, polyamides, polyesters, and polyurethanes with their commercialization based on the work of Fox and Schnell. (32,33) Since the early 1960's polycarbonates have become an extremely important and fast growing clear amorphous thermoplastic for injection molding and extruded sheet products. The workhorse polycarbonate resin is based on bisphenol A (BPA PC) and has a unique set of properties, which include a very high glass transition temperature of 145 °C and excellent toughness. These properties along with the low color and excellent clarity of products produced from BPA PC have propelled consumption to approximately 2.6 billion lbs. in 1996 with a growth rate of 8-10% a year (34). Conventional BPA PC has found wide acceptance as the plastic of choice for applications where a combination of optical properties (color, transparency), impact resistance, and resistance to thermal flow are important (35). Specific applications include the replacement for glass in areas where impact resistance is important, e.g. in areas where the resistance to vandalism is needed. BPA PC finds many applications in transportation such as instrument panel covers in automobiles and other vehicles. Currently major research and development efforts are undenvay to develop scratch resistant coatings for polycarbonate to permit the replacement of glass in automotive window applications (36). Polycarbonate resins are used widely as substrates for data recording. Compact disc (CD) technology is based on BPA PC as the substrate. Specially designed polycarbonates, based on bisphenols other than BPA, have been studied for the requirements for advanced optical data storage systems. Higher heat deflection temperatures as well as better flow and lower birefringence are all desired features for these applications (37). Considerable research has been done using different bisphenols to change these important propertles(38,39) but generally flow and impact suffer as the Tg is raised.

Step-Growth Polymerization

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The enchainment of 4,4"-{3,3,5-trimethylcyclohexylidene)diphenol into the polycarbonate backbone has been found to raise the Tg from 150 °C to 239 °C without sacrificing the flow characteristics and impact properties of BPA PC (40). Synthesis. There are several excellent reviews describing laboratory and industrial synthetic process for polycarbonates (41). Polycarbonates are prepared in solution or interfacially using bisphenol and phosgene in the presence of a base to react with the hydrochloric acid that is liberated. Currently the commercial process of choice is the interfacial method using the sodium salt of bisphenol A with phosgene with methylene chloride serving as the organic solvent (Eq. 4). Due to the toxicity of phosgene and methylene chloride and the problems with disposal of the large amounts of sodium chloride that are generated during production of polycarbonate, there has been a very large interest in developing melt phase processes (42,43,44). Catalyzed transesterification based on the reaction of bisphenol A with diphenyl carbonate is accomplished at temperatures that go as high as 320 °C under vacuum in order to obtain the high molecular weights needed for good mechanical properties. Various lithium salts and other additives are used as catalysts. Considerable research continues in this area to minimize the degradation reactions, color formation, etc. that accompany these high temperature melt phase processes. O II CI—C-CI =A

^^—^

Eq.4

^'^3/=r\

MOH

/ = \

CH3

^H ^-^ ^

Methylene Chloride

^^—^

C H ^^-^ ^

ff

Polvamides Commercial importance and aoolications. As stated earlier Nylon 6,6 was the first of the modern day thermoplastics with properties based on an intentionally designed molecular structure. Nylon nomenclature is based on the number of carbon atoms in the diacid and the diamine, e.g. Nylon 4,6 is the polyamide of tetramethylene diamine and adipic acid. Polyamides remain very important commercially today with a host of various backbones based on different diamines and diacids being available in the market place. Close to 6 billion lbs. of synthetic polyamides (this number includes Nylon 6, which is produced by a ring opening polymerization) were produced in 1998 (45). Nylons are used in a wide variety of applications. Originally nylons were used in fiber applications and this still constitutes a large market for nylons. Their high tensile strength and good dyeability make them superior in fiber performance. By varying the diamine and the diacid in aliphatic nylons a wide variety of polyamides with much different property profiles can be prepared. Systematic variation in melting points, glass transition temperatures, water uptakes, and mechanical properties has led to the ability

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T.E. Long and S.R. Turner

to design these polyamides with very specific sets of properties for applications and has resulted in aliphatic nylons finding a wide range of applications (46). Partially aromatic nylons are generally based on mixtures of terephthalic acid and isophthalic acid with various linear diamines (47). Very high melting points and excellent resistance to solvents and other chemicals are important characteristics of these kinds of nylons. This has led to their development as useful high temperature thermoplastics. Thus partially aromatic nylons find many applications where high temperature resistance and solvent resistance are required, e.g. automotive under the hood applications and electronic applications where resistance to high temperature processing is important. In many cases these applications are as a glass fiber reinforced (GFR) composite. Crystalline nylons often form superior GFR composites because of the ability to tailor glass surface chemistries to give strong interactions with the polar amide functionalities along the backbone. In addition these rigid structures have been found to be readily toughened with functionalized low Tg olefins, which are designed to interact or react with terminal amine groups in the polyamide (48). The high level of hydrogen bonding in nylons often leads to excellent barrier properties. Nylons therefore find use as food packaging resins. A partially aromatic nylon (I) based on adipic acid and m-xylylene diamine is of considerable current interest because of its high barrier to oxygen and to carbon dioxide. This material has great potential as a high barrier layer when sandwiched between two poly(ethylene terephthalate) layers in containers used for beer and other foods that easily spoil when

p NH

If

-0 -NH 4 c H ^ -Jn

exposed to oxygen (49).

Nylons that crystallize very slowly and thus can be used as transparent films are based on copolyamide structures containing branched diamines or mixtures of diamines and diacids. Such transparent structures also find use in the food packaging areas. All aromatic nylons, or aramids, possess the highest tensile strengths of any man-made synthetic step-growth polymer and thus now enjoy a very high level of usage in the high performance fiber area. Due to the extremely high melting points of aramids (decomposition occurs in most cases before melting), all synthesis and processing of aramids is done via solution based processes. The first materials of this class were the all meta linked polymers, i.e. m-phenylenediamine and isophthalic acid (Nomex, DuPont). Fibers based on Nomex are important because of their fire and chemical

989

Step'Growth Polymerization

resistance. The para linked analogues (p-phenylenediamine and terephthalic acid) have extremely high tensile moduli and have become fibers of considerable importance in high performance composites (Kevlar, DuPont and Twaron, Akzo) (50). (There are many good references on aramids, this one details the history of the development of the synthesis and properties of aramids.) Svnthesis. There are many excellent reviews that have been written on the synthesis of nylons (51). Most aliphatic and partially aromatic nylons are synthesized commercially by melt phase polycondensation processes. In cases where crystalline polyamldes are prepared and melt phase viscosities limit desired molecular weight formation, solid state polymerization processes have been employed (29). For nylons, the formation of the "nylon salt" is very useful in purifying the monomers and obtaining the exact stoichiometry to be able to get a high degree of polymerization (Eq. 5). Unlike polyesters, the equilibrium in a polyamide condensation lies far to the right and thus the polymerizations can be charged stoichiometrically and initially run under pressure to react all the diamine and maintain the stoichiometry. O

O

HO-C>fcHj]-,C-OH

+

H,N-fCHj^NH,

0-G-j-CHjf,C-0 "Nylon Salt" HsN^CH^-jiNH;

i

HEAT

-H O 2

- G - h c H 2 - i c - N H - [ - C H , - ^ NHJn

Nylon 6,6

Eq. 5

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T.E. Long and S.R. Turner

Solution phase polymerization processes can be used to prepare step-growth polyamides and, for aramids, is the only way that these materials can be prepared since most examples decompose before they melt. There are several published routes to make polyamides in solution. The most utilized is based on starting with the diacid chloride and the diamine. It is necessary to use a base to take up the liberated acid in these reactions in order to keep from protonating the diamine. Interfacial techniques, with the acid chloride in the non polar solvent and the diamine in water with a base such as sodium hydroxide present, is a facile way to quickly obtain high molecular weight polyamides. Aramids are prepared directly from the acid chlorides and diamines in strong polar aprotic solvents such as N-methylpyrrolidone (NMP) or dimethylformamide (DMF). It is necessary to use salts such as lithium chloride or calcium chloride to assist in solublizing the aramid so that it does not precipitate and stop the polycondensation at low molecular weight (52). A solution method that involves the direct polymerization of aromatic diacids and aromatic diamines, as well as of aromatic amino acids, has been studied extensively (53). When, for example, an aromatic diacid is reacted with an aromatic diamine in NMP which contains an aryl phosphite such as triphenyl phosphite and a base such as pyridine in the presence of a salt such as lithium chloride, high molecular weight aramids can be formed (54). These kinds of polymerizations have been found to be very sensitive to the conditions and the monomer/polymer structure and have not found commercial success. A much different solution process involves the use of diaryl halides and diamines to prepare aramids via the Heck carbonylation reaction (55). This route to aramids was first disclosed (56) using dibromo aromatics. Relatively low molecular weight polymers were formed. Diiodo aromatics were found to react at a much higher rate and to give polymers of much higher ultimate molecular weight, presumable due to fewer side reactions (57) (Eq. 6). This process eliminates the use of corrosive and moisture sensitive acid chlorides, but requires the use of expensive palladium catalysts. Polyamides continue to show steady growth in applications in the marketplace. Recently introduced ethylene-carbon monoxide alternating copolymers (58) have been targeted at many basic polyamide applications because of a combination of high melting point and good solvent resistance along with good barrier properties. Based on the inexpensive starting materials, these copolymers should have significant cost advantages over nylons if "world-scale" volume production is reached. Despite this and other threats, thermoplastic and amorphous polyamides will likely continue to be attractive commercial thermoplastic engineering resins in the future. The great flexibility in designing specific structures with specific properties from readily available building blocks by well-proven condensation techniques will assist in the continued growth of these materials.

991

Step-Growth Polymerization

'^"\^^°^\ /

NH;

I

NH,

PdCljLg DMAC, Base, CO

Eq. 6

""0~°~G^"""

Polvurethanes Commercial importance and applications. O. Bayer in Germany originally investigated polyurethanes and polyureas as alternatives to DuPont's polyamides in the 1930's (59). The polyureas turned out to be very difficult to process and characterize, in contrast to polyurethanes, which could readily be molded and spun into fibers. In 1998 approximately 9 billion lbs. of polyurethanes (PURs) were produced for three major classes of applications in rigid and flexible foams, elastomers, and coatings (60). The great flexibility in choosing the starting polyisocyanate and the polyol leads to the capability to design polyurethanes with a wide-range of properties. Most of the flexible foam is based on toluene diisocyanate (TDI) with various polyols. These foams are used primarily for cushioning applications, e.g. car seats, furniture cushions, bedding, etc. The technology for blowing these foams, flame retarding, stabilizing, etc. is very involved and key to the enormous commercial success. Rigid PUR foams generally are based on polymeric methylenediphenyl isocyanates (PMDI) and are used as insulation in transportation vehicles, appliances, etc. These foams are characterized by their dimensional stability, structural strength, and insulation performance (61). The polyols most widely used are generally based on polyether or polyester backbones. Polyurethane elastomers are based on hard-soft segment type polymeric structures and can exist as cast elastomers or as thermoplastic elastomers. These elastomers generally possess good chemical and abrasion resistance and maintain their properties over wide temperature ranges. The hard segments that phase separate in the elastomer are primarily based on methylenediphenyl isocyanate (MDI).

992

T.E. Long and S.R. Turner

The reversible nature of aromatic isocyanate and alcohol reactions has been exploited by the commercial development of engineering thermoplastic polymers that are designed to break down at melt temperatures to form an easily processable low viscosity material and then refonn as the polymer solidifies (62). The low melt viscosity achieved by this reversible linkage allows the injection molding of smaller parts and decreases the cycle time of making parts. This basic concept has been extended to "thermally reversible" polyesters (63). Svnthesis. The synthesis of polyurethanes is an example of a step-growth polymerization that is not an actual condensation since no small molecules are eliminated during the reaction. This reaction is shown in Eq. 7. Urethane formation can be done in solution, in bulk, or interfacially (64). The reactions are very fast and can proceed far below room temperature at high rates. Catalysts have been developed to allow the reaction rates to be varied from seconds to hours. The inclusion of these catalysts is very important in many applications that involve the reactive processing of the monomers. Another way the reactivity of these systems is controlled is by blocking the isocyanate group with a group that comes off on heating to regenerate the reactive isocyanate group. This chemistry has found considerable application in coating technologies.

HO—R—OH

+

0=G=N—R'—N=C=0-

—R—0~G-N

R'—N-C-0-

Eq.7

Since isocyanates can react with many nucleophiles the control of purity of the monomers is very important to obtain the desired structures. The presence of water leads to amine formation and the elimination of carbon dioxide. This reaction can lead to urea links in the backbone and the released carbon dioxide can serve as a foaming agent. An excellent review has been published of the myriad of chemistries that are involved in polyurethane technologies (65). Polvimides Commercial importance and applications. Efforts to prepare aromatic polyimides via the imidization of a soluble poly(amic acid) precursor were initiated in 1956 in the DuPont laboratories and within only one month, A. L. Endrey prepared the first poly(amic acid) film (66). Research efforts at DuPont were fueled by the presence of parallel research efforts in novel diamines and high temperature polyamides. Polyimides have since received significant attention over the past three decades in the aerospace and electronics industries due to the exceptional thermal and chemical stability of the rigid

Step-Growth Polymerization

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heterocyclic backbone. Polyimides are recognized as one of the most thermally stable organic polymeric materials available today and research activities dealing with polyimide homo- and co-polymers continue for many high performance applications. Although DuPont's Kapton polyimides were initially developed in the late 1950s and commercialized in late 1965, this family of high temperature polymers continues to receive significant research attention and many review articles have been published that summarize their interesting structure-property relationships (67,68,69). Recent efforts have focused on the control of the dielectric constant and water sensitivity for polyimides in microelectronics applications and the Improvement of solution and melt processability while maintaining thermal stability in composite applications (70). Other polyimide applications include gas separation membranes, photosensitive materials, electronic packaging, and Langmuir-Blodgett films. These very diverse applications are due to low relative permittivity and high breakdown voltages in combination with their chemical resistance, tough and flexible mechanical performance, and thermal stability exceeding 450 °C (71). Other Kapton applications have included aerospace wire and cable insulation, substrates for flexible printed circuits, electrically conducted films, and various applications requiring flame resistance. Segmented block copolymers containing polyimides and poly(dimethyl siloxanes) have received intense attention during the past two decades for applications ranging from adhesives and composites to circuit boards and protective coatings (72,73). In addition, NASA has devoted significant attention to the utility of polyimides as new aerospace materials due to their Intrinsic stability to both high-energy radiation and aggressive atomic oxygen. In general, polyimides derived from dianhydrides and diamines produce insoluble products after imidization and are generally processed by casting the poly(amic acid) intermediate onto a suitable substrate and subsequently heating to induce quantitative imidization. Recent advances include the preparation of thermoplastic polyimides and subsequent melt processing. General Electric has devoted significant attention to the commercial manufacture of Ultem polyether imide and it is often recognized as a premiere soluble and processible thermoplastic material. Chemistrv and Svnthetic Methodologies. Condensation polyimides are classically prepared via the addition of an aromatic dianhydride to a diamine solution in the presence of a polar aprotic solvent such as NMP, DMAc and DMF at 15-75 °C to form a poly(amic acid) (74). The poly(amic acid) is either chemically or thermally converted to the corresponding polyimide via cyclodehydration. The general chemistry for this twostage, step-growth polymerization process for the preparation of Kapton polyimide is depicted in Equation 8. It is important to note that the formation of the polygamic acid) is an equilibrium reaction and attention must be given to ensure that the fon^/ard reaction is favored in order to obtain high molecular weight poly(amic acids). If the final polyimide is insoluble and infusible, the polymer is generally processed in the form of the poly(amic acid). Caution must be exercised when working with classical poly(amic acid) solutions due to their hydrolytic instability, and shelf life is limited unless properly stored at low temperatures. This is due to the presence of an equilibrium concentration of anhydride and their susceptibility to hydrolytic degradation in solution. On the other hand, poly(amic diesters) can be stored for indefinite periods of time without degradation due to the inability to form an intermediate carboxylate anion and have

T.E. Long and S.R. Turner

994

been utilized reproducibly in microelectronics applications (75). Earlier studies have shown that the formation of tri- and tetramethylesters of dianhydrldes increased the likelihood of N-alkylation side reactions and a corresponding decrease in the molecular weight and mechanical properties of the final product. A similar side reaction has been observed in attempts to prepare polyamide esters using dimethyl esters of terephthalic acid (76). Melt processible thermoplastic polyimides are prepared by the addition of flexible units, bulky side groups, or as mentioned earlier, flexible difunctional oligomers. Examples of these modifications include GE's Ultem (ether units), Amoco's Torlon (amide units), Hoechst-Celanese's P150 (sulfone units), and GE's Siltem (siloxane segments). Ultem polyimide is manufactured by General Electric and is an injection moldable thermoplastic poly (ether imide). This commercial product exhibits high mechanical properties including modulus and strength, excellent ductility, and high thermal stability. Although polyimides have been prepared using a myriad of other synthetic

Q

.0

P +

NH;

r\

\

//

NH,

^Xh

Eq.8

+

HgO

-Jx

methodologies including aryl coupling with palladium catalysts, poly(amic silylesters), silylethers with activated halides, and trans-imidization, this chapter has focused on only two of the commercial polyimides In the marketplace today. Many specialty polyimides for composite applications have also evolved including LARC-TPI and GPI (Mitsui Toatsu), and Hoechst-Celanese's fluorinated polyimides. Due to the tremendous scope of polyimide research and applications, several excellent comprehensive texts have been devoted to this family of high temperature polymers.

Step-Growth Polymerization

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