CARBOCATIONIC POLYMERIZATION Introduction Cationic polymerization is a chain-growth reaction where the active center is positively charged. In the case of carbocationic polymerization, the active center is a carbenium ion. The term cationic polymerization also covers polymerizations where the active center contains a heteroatom (eg, oxonium, sulfonium, ammonium, or phosphonium ion); these are not included in this chapter. The focus will be on carbocationic polymerization. In order to get a perspective about the importance of carbocationic polymerization in polymer science and engineering, its positioning in the overall picture needs to be considered. The estimated total volume of polymers produced commercially by cationic polymerization is around 3–3.5 million tons per annum (1). This constitutes about 3% of the total synthetic polymer market. Polymers produced commercially by carbocationic polymerization make up about 2% of the total polymer market. On the basis of these numbers, carbocationic polymerization appears to be insignificant. The main products are poly- and oligoisobutylenes, poly(butenes), hydrocarbon resins, and poly(vinyl ethers)—these will be discussed in more detail under Industrial Carbocationic Processes. Estimated worldwide production capacities are listed in Table 1. It can be seen that isobutylene-based products are the most important; they range from viscous oils to high molecular weight elastomers, such as butyl and halobutyl rubbers (see BUTYL RUBBER). The latter exemplify most strikingly the practical importance of carbocationic polymerization. Butyl and halobutyl rubbers are used as inner tubes or inner liners in vehicle tires, because of their exceptional impermeability toward gases and moisture (3,4). Had butyl and halobutyl 382 Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.
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Table 1. Worldwide Production Capacities of Commercially Produced Carbocationic Polymersa Polymer Isobutylene-based elastomers (butyl, halobutyl) Polybutenes Polyisobutylenes Resins C9 + indene–coumarone C5 + dicyclopentadiene Vinyl aromatics Polyterpenes Poly(vinyl ethers) a Ref.
Thousand Metric Tons/year 760 650 100 350 274 128 25–30 20–25
2.
rubbers not been discovered, we would have to pump up our car and bicycle tires daily as the air would escape through the rubbers used to build the body of these tires. Butyl and halobutyl rubbers have very high damping properties and therefore are used in machinery mounts to reduce vibration; unique examples include dampeners for bridges and foundations for earthquake-resistant buildings. They also have outstanding oxidative, environmental, and chemical stability, rendering them suitable for applications as industrial rubber goods, seals, adhesives, condenser caps, and pharmaceutical stoppers. An interesting application is chewing gums (considered essential by our children as any parent can testify). Emerging applications of polyisobutylene-based rubbers include biomedical implants that could save lives (5). These examples highlight the importance of carbocationic polymerization. This article discusses the most important aspects of carbocationic polymerization, with references to more detailed information for specialists.
Historical Background Carbocations are extremely reactive. Because of their high reactivity, identification or direct observation of carbocations is extremely difficult—although they are the intermediates of many organic reactions. The existence of carbocations became generally accepted in the 1920s and 1930s through the work of Whitmore, Ingold, and Meerwein (6). Analytical proof was first given by Olah in 1963 (7,8). Reacting SbF5 and alkyl halides in “superacid” media, he was able to prepare carbocations stable enough for 1 H NMR characterization. He proposed to use the term carbenium ion for classical trivalent sp2 carbocations (such as CH3 + ) and carbonium ion for the pentavalent carbocation (CH5 + ) (7,8). This nonclassical entity has a two-electron, three-center-bound pentacoordinated carbon; ammonium (NH4 + ), oxonium (R3 O+ ), or sulfonium (R3 S+ ) ions can be considered as similar “nonclassical” structures. The first example of carbocationic polymerization, the resinification of turpentine oil by sulphuric acid (9), dates back a couple of centuries. Other examples
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from the 1800s include the polymerization of pinene with SnCl4 and BF3 (10), turpentine with BF3 (11), and styrene and isobutylene with sulphuric acid (11,12). Hunter and Yohe laid out the first mechanism of carbocationic polymerization in 1933 (13). According to their proposal, the metal halide–monomer interaction leads to the formation of a “zwitterion” (the German word zwitter stands for “between”): MtXn + CH2 C(CH3 )2 →MtXn− CH2 (CH3 )2 C+ Polymerization then proceeds by reaction of monomer with the carbenium ion. In this system, the metal halide was considered the “catalyst.” For thermodynamic reasons, this mechanism was criticized (14,15) and later abandoned as Polanyi’s school put forth the idea of coinitiation on the basis of the study of the isobutylene–TiCl4 system (16). Polanyi suggested that the true initiator in this system would be derived from the reaction between a cationogen such as water, and TiCl4 , the coinitiator (eq. 1): TiCl4 + H2 O + CH2 C(CH3 )2 →CH2 (CH3 )2 C+ TiCl4 OH −
(1)
This reaction can be written in a more generalized form (eq. 2): MtXn + I B + CH2 C(CH3 )2 →I CH2 (CH3 )2 C+ MtXnB −
(2)
where I represents the true initiating species, such as a proton or carbenium ion, and B represents a basic group, eg, a hydroxide or a halide. The metal halide coinitiators are still called “catalysts” in the industry, as the processes used today were originally developed many decades ago. It would be helpful to all if the scientifically accepted terms were more commonly used. Classical carbocationic polymerization is very rapid and therefore difficult to control, and is riddled with side reactions and incomplete monomer conversion. In contrast, living (or controlled) carbocationic polymerization opened new avenues for macromolecular engineering, ie, the design and synthesis of well-defined structures with efficient processes (17–20).
Monomers for Carbocationic Polymerizations The most common monomers in carbocationic polymerization are alkenes, often termed “vinyl” monomers. Carbocationic polymerization of vinyl monomers proceeds by the electrophilic attack of the initiating and/or propagating carbenium ion on the double bond of the monomer. In order to sustain the chain reaction of propagation in competition with termination or transfer reactions, the carbenium ion has to be sufficiently stable and the monomer has to be sufficiently nucleophilic to ensure long enough lifetime of the active center to propagate. Stability of Carbenium Ions. In terms of thermodynamic stability, carbenium ions can be ranked on the basis of gas-phase hydride affinity, heats of ionization, ionization equilibrium in solution, and SN 1-solvolysis rate (21). The “stability scales” obtained by these different methods correlate reasonably well
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Table 2. Ranking of Carbenium Ions by Stability Carbenium ion
R H 0 , kJ/mol (R H → R+ + H − )a
log k (R Cl, EtOH, 25◦ C)b
1306 1143 1034 — 963 963 — 940 921 — — —
— — −10.8 −9.4 −7.1 −6.8 −5.6 −4.6 −3.4 −1.7 0.8 1.8
CH3 + CH3 CH2 + CH3 CH+ CH3 Ar CH2 + (CH3 )3 C+ Ar CH+ CH3 (CH3 )3 CH2 C(CH3 )2 + (CH3 )2 C CH CH2 + Ar C(CH3 )2 + CH3 O Ar CH+ CH3 (Ar)3 C+ [CH3 O Ar]2 CH+ a Hydride
affinity of carbocations (21). Original data from Ref. 22. rate constant of the corresponding alkyl chlorides in ethanol (21). Original data from Ref. 23. b Solvolysis
(21). Table 2 lists selected carbenium ions in order of increasing stability, along with their hydride affinity and the solvolysis rate constants of the corresponding alkyl halides. It can be seen that as the stability of carbenium ions increases as a result of inductive or resonance effects, their reactivity (electrophilicity) decreases. Nucleophilicity of Monomers. Figure 1 shows cationically polymerizable monomers in order of increasing nucleophilicity. The numbers listed are nucleophilicity parameters (N) developed by Mayr (24,25). N values were derived by measuring the rate of reaction between diarylcarbenium ions and various monomers using UV–vis spectroscopy and are discussed more later in this article. Optimum combination of carbocation stability and monomer nucleophilicity is necessary for effective polymerization. Indeed, isobutylene positioned in the middle of both the carbocation stability and monomer nucleophilicity scales can be polymerized effectively to yield high molecular weight polymers, while the other monomers listed above mostly yield low molecular weight products by carbocationic polymerization. Examples of Active Monomers. A vinyl monomer can undergo carbocationic polymerization only if the double bond is the most basic site of the monomer. For example, acrylates and vinyl acetate cannot be polymerized cationically
N OEt N: −2.63
−1.15
0.78
1.07
1.12
2.39
3.9
Fig. 1. Ranking of monomers by nucleophilicity.
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+
H3C
+
H3C
+
H3C
C H2
+
Fig. 2. Release of steric hindrance by isomerization in the carbocationic polymerization of β-pinene.
because the carbonyl oxygen of the ester group is more basic than the vinyl group. If the stability of the chain-carrier carbenium ion forming from a particular monomer is too high or too low, it cannot undergo carbocationic polymerization; N-vinyl amines readily react with cationic initiators, but the formation of stable ammonium salts prevents their polymerization. Alkyl vinyl ethers can be polymerized readily, although the oxygen is certainly more basic than the vinyl group. However, in this case protonation of the vinyl group leads to the formation of a carbocation stabilized via charge delocalization. Another example of these n,πdonor monomers is N-vinylcarbazole. In these monomers the combination of an unsaturation with an electron-donating heteroatom leads to very easy polymerization (26); the monomers are highly nucleophilic and form very stable carbenium ions (see Table 2 and Fig. 1) As a consequence, their polymerization can be initiated by initiators that would be inefficient with less-reactive monomers. However, the high stability of their carbenium ions restricts their ability to copolymerize with monomers of lower nucleophilicity. Ethylene, propene, or n-butenes only give oligomers because of the low stability/high reactivity of the corresponding carbenium ions. Hydride transfer from oligomers prevents the formation of high molecular weight product. For example, the primary cation formed from ethylene will abstract a hydride ion from the secondary carbon of the oligomer. In addition to carbocation stability and monomer nucleophilicity, steric factors are also important. For example 1,1-diphenylethylene, a very reactive monomer can only be dimerized, because of the formation of a sterically buried carbenium ion (27). Other monomers that cannot be polymerized to high polymers because of steric congestion in the transition state are 2,4,4-trimethyl-1-pentene, 1,2-diphenylethylene (stilbene), 3-methylindene, and 2-methylenenorbornane. In some monomers, such as β-pinene (Fig. 2) or methylene–norbornene, isomerization of the initially formed carbenium ion helps overcome the steric restriction (27,28): Monomers containing more than one double bond (conjugated or nonconjugated dienes and trienes) with similar reactivity lead to ill-defined mixtures of cyclized, branched, or cross-linked polymers (27). Monomers active in carbocationic polymerization have been periodically reviewed over the years (26–30). Isobutylene remains the monomer with optimum combination of nucleophilicity, carbocation stability, and steric factors, yielding high molecular weight elastomers.
Initiating Systems Carbocationic polymerization can be initiated by a wide variety of chemical and physical methods. Examples are initiation by Br¨onsted acids, Lewis acids, Lewis
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acids in combination with a proton or carbenium ion source, stable carbenium ions, UV or γ -irradiation, photochemical reactions, and the application of a high electric field. The inverse relationship between stability and reactivity of carbenium ions plays an important role in the initiation step. In order to achieve high initiator efficiency, it is essential to match the stability of the propagating carbenium ion with that of the initiating species. For instance, primary and secondary alkyl halides were found to have low initiation efficiency in the polymerization of isobutylene coinitiated by (C2 H5 )2 AlCl at −50◦ C, because of the high reactivity and hence short lifetime of the corresponding ions compared with the growing tertiary carbocation (31). Better initiation efficiency was achieved using tert-butylchloride, because of the positive inductive and hyperconjugative stabilizing effect of the three methyl groups. However, further stabilization of the carbenium ion by aryl groups resulted in decreasing initiation efficiency. Hundred percent initiation efficiency was achieved when 2-chloro-2,4,4-trimethyl pentane was used in conjunction with TiCl4 (32). This can be explained with the effect of back-strain. The solvolysis rate of 2-chloro-2,4,4-trimethyl pentane was found to be 21 times higher than that of tert-butylchloride, attributed to back-strain, ie, the release of steric strain during rehybridization from sp3 to sp2 (33). On the basis of the relative rates of reactions between selected alkyldiphenyl carbenium ions and tertiary carbocations with allylsilanes, the 2,4,4,6,6-pentamethyl-2-heptyl cation was a perfect match for a polyisobutylene cation with 33 repeat units (34). The stability and nucleophilicity of the counteranion (sometimes called “gegenion,” based on the German equivalent word) is equally important in the initiation step as well as in the subsequent propagation, chain-transfer, and termination steps. Detailed reviews of various initiating systems are available (15,35–37). The most important initiating systems will be discussed briefly. Bronsted ¨ Acids. The ability of Br¨onsted acids to initiate carbocationic polymerization is strongly dependent on their acidity (pK a value), the basicity and nucleophilicity of the counteranion derived from the Br¨onsted acid, and the stability of the carbenium ion formed from the monomer (37). Highly nucleophilic counteranions will result in the formation of covalent bonds between anion and cation, preventing polymerization. Highly basic counteranions will promote chain transfer by proton elimination. Strong Br¨onsted acids, such as trifluoromethane sulfonic acid (triflic acid) or perchloric acid, are capable of polymerizing most of the cationically active monomers. The kinetic order of the initiation reaction with respect to the acid is typically higher than one, indicating the involvement of additional acid molecules in the initiation step (38). Weaker Br¨onsted acids such as hydrogen halides can only polymerize monomers, yielding very stable carbenium ions, such as N-vinylcarbazole (39,40). Although the pK a values of trichloro- or trifluoroacetic acid are higher than those of hydrogen halides, they can readily polymerize monomers such as styrene derivatives or cyclopentadiene (37). The likely reason for this is the participation of a second acid molecule in the ionization step and/or in the stabilization of the counteranion (eq. 3): HAc + >C
C
C
C< Ac
HAc
[H >C CC CC C 1, the reactivity ratio decreases with decreasing temperatures. Table 6 lists some selected data. This trend is opposite to that observed in radical polymerization, in which reactivity ratios approach unity with increasing temperature; this was attributed to similar entropies of activation (177). Apparently, entropy effects cannot be neglected in carbocationic copolymerization (178). It has been observed that in carbocationic copolymerization the molecular weight of the resulting copolymer is always lower than that of the individual homopolymers prepared under identical conditions (179). This was explained by the high ktr /kp ratio of the favored cross-propagation step (reaction of the most reactive chain end with the monomer giving the most stable carbenium ion), compared to the respective homopolymerization ktr /kp ratios (173). Copolymerizations involving dienes such as the copolymerization of isobutylene with isoprene are important from the industrial point of view (3,4). Isoprene acts as a strong chain-transfer and terminating agent in the carbocationic polymerization of isobutylene (161), and at high concentration it leads to a crosslinked, insoluble product (180). Because of the limited composition range available for analysis, determination of the reactivity ratios in this system is rather difficult. Reactivity ratios published for the isobutylene/isoprene system are listed in Table 7. Interestingly, controlled copolymerization was achieved in the isobutylene/2,4-dimethyl pentadiene-1,3 system, explained by the internal strain exerted by the methyl groups of the 2,4-dimethyl pentadiene-1,3 incorporated into the copolymer, and the presence of electron-pair donors forming in situ from the initiators (190).
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Table 7. Copolymerization Reactivity Ratios of the Isobutylene (1)/Isoprene (2) System Initiating system AlCl3 EtAlCl2 AlCl3 EtAlCl2 +Cl2 AlCl3 EtAlCl2 EtAlCl2 EtAlCl2 EtAlCl2 EtAlCl2 AlCl3 AlCl3
Solvent
T,◦ C
r1
r2
CH3 Cl −103 2.5 ± 0.5 0.4 ± 0.1 CH3 Cl −100 2.17 0.5 CH3 Cl −90 2.3 CH3 Cl −35 2.5 EtCl −95 2.26 0.38 EtCl −90 2.27 0.44 CH3 Cl 88%/hexane 12% −80 2.15 1.03 CH3 Cl 50%/hexane 50% −80 1.90 1.05 CH3 Cl 12%/hexane 88% −80 1.17 1.08 Hexane 100% −80 0.80 1.28 C5 /CH2 Cl2 −70 1.56 ± 0.19 0.95 ± 0.17 CH3 Cl −80 1.6
Reference 181 182 183 184 185 185 186 186 186 186 187,188 189
Copolymerization studies provide a very useful tool to assess the effect of polymerization medium, coinitiator, counteranion, and additives on polymerization kinetics. However, the interpretation of results is difficult because of the interrelationships between these parameters. For examples and details see Reference 164.
Controlled Carbocationic Polymerization Traditionally, carbocationic polymerizations have been considered uncontrollable, owing to the high reactivity of the chain carriers. Similar to radical polymerization (qv), they were treated as random processes, governed by the statistical probability of side reactions such as chain transfer and termination, relative to propagation. Real control in polymerization processes was first achieved in anionic systems (see ANIONIC POLYMERIZATION). The term “living polymerization” was introduced by Szwarc (191), although controlled polymerizations without chain-breaking processes had been observed experimentally much earlier (18,192). Similar to the history of living anionic polymerization, living conditions in carbocationic systems had been achieved experimentally as early as 1974 (193,194), before systematic research had begun to be carried out into living carbocationic conditions. The breakthrough was the successful production of high molecular weight and nearly uniform polymers with controlled carbocationic polymerization, by Kennedy’s group (19). Carbocationic polymerizations reportedly exhibiting living characteristics have been called “pseudocationic,” “quasiliving,” then “truly living.” All of these provide control over the molecular weight of the polymer, defined as the ratio of converted monomer to initiator: Mn (g/mol) = M(g)/I(mol)
(19)
where M is the amount of monomer converted into polymer, and I is the molar amount of initiator. According to Szwarc’s classical definition, in living polymerizations M n increases linearly with conversion, and in the case of instantaneous initiation, the polymers produced in a batch reactor will have a polydispersity
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described by the Poisson distribution: Mw /Mn = DPw /DPn = 1 + 1/DPn
(20)
where M n and DPn , and M w and DPw are the number- and weight-average MW and DP, respectively. Under controlled conditions, nearly uniform polymers can be produced at sufficiently high MWs. Since the active centers do not enter into side reactions, their concentration remains constant and the monomer consumption rate will obey a linear first-order dependency. The early experimental examples of living carbocationic polymerizations were riddled with broad molecular weight distribution, and thus were not acknowledged to be controlled polymerizations. In fact, the achievement of carbocationic living polymerization was considered to be impossible, because of the inherently high reactivity of carbocations, which tend to enter into irreversible termination and chain-transfer reactions via β-proton expulsion (142). It was then recognized that the key to controlled carbocationic polymerization was the promotion of an equilibrium between dormant and active propagating chain ends and thereby providing sufficient time for synthetic manipulations. Importance of reversible termination in order to obtain control over the polymerization was first outlined by the original “quasiliving” concept (137) (eq. 21) Pn
Pn*
inactive
active
Pn* + M
Pn+1*
(21)
Remarkably, the same key concept applies in living radical polymerizations. It has been suggested that the term “temporary deactivation” is more appropriate than “reversible termination,” to emphasize control (20). In controlled carbocationic polymerizations the initiating systems are usually Lewis acids in conjunction with a carbocation source such as an alkyl, allyl, or aralkyl halide, ester, ether, or epoxide (18). The Lewis acid coinitiators reactivate or reinitiate the inactive polymer chain to produce the active growing carbocations. The existence of the dynamic equilibrium between dormant covalent and active ionic species in carbocationic polymerizations is now generally accepted. Ligand exchange and dynamic NMR studies supported the evidence of the involvement of ionic species even in polymerizations that were thought to proceed by a “pseudocationic” or “insertion” mechanism (18). The temporary deactivation also allows for MWD control. This concept was introduced by Puskas, and co-workers (195) and is valid for all polymerization systems with temporary deactivation. According to this treatment, MWD is governed by the run length l, which is defined as the average number of monomer units incorporated into the polymer chain during a productive ionization period: DPw /DPn = 1 + l/DPn
(22)
This general equation reduces to the Poisson distribution when l = 1. The magnitude of l will define the narrowness of the MWD. At sufficiently high MW, l = 25 still yields DPw /DPn < 1.1. The MWD criteria for living polymerizations
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can then be defined as follows: DPw − DPn = l = constant
(23)
Since the Poisson distribution is a subcase of the general definition of MWD for l = 1, Poisson or very narrow MWD does not seem to be necessary in the definition of living polymerization. Living (controlled) carbocationic polymerization can be viewed as a process where the most probable distribution of monomer incorporation during an active ionization period is superimposed on a Poisson distribution of the repeating events of ionization periods. Depending on the lifetime of the active cationic species, the run length l can be l 1, resulting in broader MWD. For instance, at l = 50, and at DPn = 50, the MWD will be M w /M n = 2. With increasing conversion, the MWD will get progressively narrower. The temporary deactivation process seems to protect the active species from side reactions long enough for synthetic manipulation. It is understood that chainbreaking processes might be identified in polymerizations, which are considered living today (20). The real power of controlled polymerizations in general, and controlled carbocationic polymerizations in particular, is their ability to control the polymerization process in order to design and build new polymeric structures. Since certain monomers can only be polymerized via a carbocationic mechanism, the significance of controlled carbocationic polymerization is evident. Polyisobutylene is a very important example—it has a fully saturated backbone with excellent stability and is thus a prime candidate for building model structures (Fig. 10) for systematic structure–property studies. Structure–property relationship studies are considered to be of critical importance. Hopefully, controlled carbocationic polymerization will make an important contribution in this area (196). From an industrial point of view, controlled carbocationic polymerization has serious drawbacks. Except for block copolymer production, narrow MWD is a disadvantage, since broad distribution or polydispersity improves the physical properties and/or processability of commodity polymers (3). In controlled carbocationic polymerization, one initiator molecule produces only one polymer chain. Since the coinitiator, usually a Lewis acid such as TiCl4 and BCl3 , is used in at least a tenfold excess over the initiator, the catalyst efficiency, a term used in industry to express the activity of the initiating system in terms of grams of polymer per grams of Lewis acid, is very poor in comparison with commercial processes (a few hundred vs several thousands). However, the ability of controlled carbocationic polymerization to produce new polymeric structures in a controlled manner may override these and other concerns. Anionic living polymerizations have long been commercialized, producing branched polybutadienes for high impact polystyrene production, styrene–isoprene–butadiene terpolymers and various functionalized
Star-branched
Brush
Dendritic (arborescent)
Fig. 10. Model polymer structures for structure–property relationship studies.
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rubbers for tire production, and styrenic block copolymers (thermoplastic elastomers or recyclable rubbers). Under laboratory conditions, anionic polymerization is considered a difficult experimental technique (197). The relative newcomer, controlled carbocationic polymerization, is similarly viewed as a difficultto-master method in the laboratory. In addition, it often is perceived to be inferior to living anionic polymerization. Ironically, large-scale commercial ionic polymerizations require less stringent conditions than laboratory procedures, because of the much smaller surface area-to-volume ratios. Commercial examples of controlled carbocationic polymerizations include poly(vinyl ethers) used in liquid-crystal technology (198), polystyrene–polyisobutylene–polystyrene block copolymers and functionalized polyisobutylenes, which have been test-marketed recently by Kuraray and Kaneka Co. of Japan (19). Several reviews provide detailed description of controlled carbocationic polymerization (18–20,199–201). It should be mentioned here that controlled polymerization of p-hydroxy styrene was also achieved in aquoeus media (202). This may prove to be a revolutionary breakthrough in controlled carbocationic polymerization.
Industrial Carbocationic Processes There are relatively few surveys available on industrial carbocationic polymerization (1,2,203,204). In this section, the most important commercial polymers produced by carbocationic polymerization will be discussed. Polyisobutylene. The polymerization of isobutylene by strong acids at room temperature was first reported in 1873 (12). The reaction yielded a “sticky liquid” (mainly dimers and trimers). In 1929, Otto at I.G. Farbenindustrie discovered that high molecular weight polymer could be prepared by cooling the reaction mixture with dry ice (205). Presently low, medium, and high molecular weight polyisobutylenes are produced commercially, with significantly more volume for the former two (see BUTYL RUBBER). The unique properties of polyisobutylene are low permeability, good thermal and oxidative stability, ozone resistance, chemical resistance, high hysteresis (mechanical dampening), and tack. The BASF process to produce high molecular weight polyisobutylene was developed on the basis of Otto’s pioneering work. In the original process a mixture of ethylene and isobutylene was sprayed on a moving belt along with a solution of BF3 in ethylene (206); the reaction heat was removed by the evaporation of ethylene. The original ESSO process was developed by Standard Oil of New Jersey (now known as ExxonMobil) and used methyl chloride and AlCl3 to produce high molecular weight polyisobutylenes (1). These processes most likely have been modified, although the fundamentals may be the same. The molecular weight of polyisobutylenes produced by these processes ranges from few hundred thousand to few hundred million. High molecular weight PIB is used as binder in sealing tapes. It is also used to modify asphalt to improve weathering and low temperature properties as well as toughness and abrasion resistance of the mix. Polyethylene and polypropylene modified with polyisobutylene display improved barrier, flex, and stress-cracking properties. Polyisobutylene compounded into natural rubber (NR) or styrene–butadiene rubber (SBR) improves aging, cut growth, flex, and stress-cracking properties. Low (few hundred) and medium (few thousand) molecular weight polyisobutylenes are produced at higher temperatures to reduce molecular weight. BASF uses a batch
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process, polymerizing isobutylene in isobutane with the CH3 OH/BF3 initiating system. Medium molecular weight is achieved by using higher purity isobutylene and lower temperatures (−25 to −40◦ C, compared to −10◦ C for low molecular weight polymer). Infineum (a joint venture between ExxonMobil and Shell) uses a continuous process in hexane, initiating the reaction with a suspension of AlCl3 in hexane and regulating molecular weight with the process temperature (1,2). Low and medium molecular weight polyisobutylenes are used as viscosity modifiers, fuel and lubricating oil additives, tack improvers in adhesive formulations, and primary binders in caulking and sealing compounds. Blending them into waxes used for paper coating reduces the penetration of the wax into the paper, and improves moisture resistance. In lubrication the main advantage of polyisobutylene is that it decomposes without residues. In the mid 1990s Shell started to commercialize an amine-functionalized low molecular weight PIB as fuel/oil additive. Emulsification of sludge particles by functionalized PIB is an important aspect of this application. Isobutylene-Based Elastomers. Butyl elastomer is a random copolymer of isobutylene and a small amount (0.7–2.2 mol%) of isoprene. The dominant form of the incorporated isoprene is 1,4-trans. This copolymer was the first example of low unsaturation elastomers. The starting point of the development of butyl rubber was a collaborative research effort carried out by Otto (I.G. Farbenindustrie) and Thomas (Standard Oil Development Co.) in 1933–1935. Thomas suggested the use of the AlCl3 /ethyl chloride polymerization system, instead of BF3 /ethylene. The goal was the production of elastomers that could be cured to yield cross-linked polyisobutylene-based rubbers. First butadiene was copolymerized with isobutylene to provide the necessary double bonds for curing sites. Later butadiene was replaced by isoprene because of its higher reactivity as a comonomer, and ethyl chloride was replaced by methyl chloride as diluent because of its higher stability (207,208). Currently the AlCl3 -coinitiated copolymerization of isobutylene with isoprene in methyl chloride is the basic technology for the production of butyl elastomers. The first butyl plants using this technology, located in Baytown and Baton Rouge, Louisiana, and Sarnia, Canada, were constructed as part of the Synthetic Rubber Procurement program during World War II. It is interesting to note that this was the second most important program next to the Manhattan Project. Although numerous incremental process improvements were reported since World War II, production of butyl rubber is still based on the original technology (3,4). Butyl rubber is produced in a continuous slurry process at low temperatures (−90 to −100◦ C) using methyl chloride as diluent and a solution of AlCl3 dissolved in methyl chloride as the initiating system. Small amounts of water or HCl as cationogen sources are used to promote initiation. The reactor resembles a heat exchanger, with the slurry circulating inside the pipes that are cooled by boiling ethylene in the jacket. The product is separated from the diluent and unreacted monomers by coagulating it in hot water. More detailed information can be found in References 3,4, and 203. A solution process called the “Russian process” was developed in the former Soviet Union (209). Polymerization is carried out in isopentane at around −90◦ C, initiated with water/alkylaluminum chloride(s). The most important physical properties of butyl rubber are essentially the same as those of poly-isobutylene; low permeability, good chemical and thermal stability due to the low unsaturation content, and high damping. Butyl elastomers
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are cross-linked (cured) with sulphur to yield cross-linked rubbers. The most important application of butyl rubbers is tire inner tubes. Other applications include tank liners, vibration dampeners, protective clothing, tire curing bladders, railway pads, wire and cable coating, belting, hoses, and chewing gum base. Halogenation of the isoprene enchainments in butyl elastomers yields halobutyl elastomers. Reaction of the isoprene units with elementary bromine or chlorine leads to the formation of allylic halide units along the chain—a substitution as opposed to addition across the double bonds. This leads to much faster curing, which permitted the development of tubeless tires, as adhesion of halobutyl to the rest of the tire could be achieved. Halobutyls are primarily used by the tire industry to form the inner liner of the “tubeless” tire. Other applications are similar to that of butyl. Chlorobutyl is also used in pharmaceutical stoppers. Over the years additional polyisobutylene-based elastomers were developed and commercialized. Butyl partially cross-linked with divinyl benzene with a high gel content has high resistance to cold flow (creep), improved green strength (green strength refers to the ability of a yet uncured tire to keep its shape, which relates to the 300% modulus of the raw elastomer) and elasticity, and good recovery after deformation. These properties are especially important to prevent the sag and cold flow of caulking compounds, sealants, and adhesives (210). Unlike regular butyl, it can be cured by peroxides via the unreacted pendent vinyl groups of the divinylbenzene units. Star-branched butyls are produced by the addition of polystyrene–polybutadiene block copolymer to the reactor (211). A fraction of the growing chains grafts onto the block copolymer; the star-branched portion of the final product is about 10–15 wt%, with the rest of the chains being linear. This combination leads to improved green strength, coupled with enhanced processability. It should be noted here that commercial high molecular weight isobutylenebased elastomers are characterized by the so-called Mooney viscosity, not by molecular weight or molecular-weight distribution. This standardized low shear viscosity measurement is used to grade all synthetic and natural elastomers and rubbers (212). Interestingly, this highly prevalent method in industry is rarely ever mentioned in textbooks or encyclopedias. Polybutenes. Polybutenes are ill-defined copolymers of isobutylene with butenes (203). They are low molecular weight (M n = 300–2000 g/mol), clear oily or viscous liquids, with 85–95% isobutylene content. They are produced by the carbocationic polymerization of C4 refinery stream, using AlCl3 or in some cases BF3 as coinitiator, and impurities or purposefully added HCl or H2 O as initiators. The C4 stream is a product of crude oil distillation, containing a mixture of C4 hydrocarbons such as isobutylene, cis- and trans-2-butenes, 1-butene, and butane. Molecular weight is controlled by temperature (−10 to +30◦ C) and initiator concentration. Intensive chain-transfer reactions result in exo and endo terminal double bonds. The exo double bond is more reactive and is preferred for further functionalization with, for example, maleic anhydride or phosphorous pentasulfide. The most important use of polybutenes is in the production of lubricating oil additives. Functionalized polybutanes improve wear resistance because of their ability to emulsify small metal particles. Hydrocarbon Resins. Hydrocarbon resins are ill-defined, low molecular weight products of the carbocationic polymerization of aliphatic or aromatic mono and diolefins, ranging from viscous liquids to high melting point thermoplastics.
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These resins are generally grouped into three main categories depending on the source of the raw material (1,203,204). The indene–coumarone resins are coal tar based. Aliphatic, aromatic, and dicyclopentadiene resins are made from distillate fractions of cracked petroleum. Polyterpene resins are produced using monomers isolated from turpentine oil (α- or β-pinene) or etheral oils of lemon, orange, mandarin, or caraway (d,l-limonene or dipentene and d-limonene). Commercial production of the coal tar based indene–coumarone (benzofuran) resins started in the early 1900s, using sulphuric acid as initiator in the temperature range of −10 to +50◦ C. The subsequent use of AlCl3 and BF3 -based initiating systems resulted in a lighter-colored product. Polymerization is carried out in bulk or naphtha. The primary use of indene–coumarone resins is in adhesives, coatings, inks, and additives for the rubber industry. Starting from the mid-1900s, petroleum-based aromatic resins gradually replaced indene–coumarone resins. Petroleum-based resins consist of aliphatic, aromatic, and dicyclopentadienebased polymers. Aliphatic resins are produced from C5 feed stream, a mixture of isoprene, cyclopentadiene, and piperylene (1,2-pentadiene) along with various monoolefins. Cyclodienes lead to gel formation and they are removed before polymerization. Aromatic resins are produced from C8 –C10 petroleum distillate fraction (styrene, α–methylstyrene, vinyl toluenes, indene, methylindene, dicyclopentadiene, cyclopentadiene/methyl–cyclopentadiene dimer, and methyl– cyclopentadiene dimer). Petroleum-based resins are produced by carbocationic or thermally induced free-radical polymerization. In the case of dicyclopentadiene resins, free-radical polymerization is the dominant mode of synthesis. The most frequently used Lewis acid carbocationic initiators are AlCl3 and BF3 . AlCl3 leads to higher molecular weight products, but the loss of unsaturation via cyclization is more pronounced. Frequently the Lewis acid is complexed with ethers, phenols, or aldehydes to reduce reactivity, increase solubility, and alter product properties (molecular weight, molecular weight distribution, and chain structure). Petroleum resins have excellent tack properties, so they are used to improve tack in hot-melt and pressure-sensitive adhesive formulations, floor coverings, sealants, caulks, mastics, and rubber compounds. They are also used in printing inks as binder and protective coatings, and in paints and varnishes to improve gloss and water resistance. Water-white resins are a recent commercial development. They are made by the homo- or copolymerization of pure vinyl aromatic monomers such as styrene or alpha-methylstyrene. Polymerization is carried out in aromatic solvents using BF3 or complexes of BF3 at 15–40◦ C. A controlled amount of water is added in order to improve initiation efficiency. The molecular weight of the product is low as a result of the extensive chain transfer processes at this temperature. Water-white resins are used where color stability is important. The production and application of polyterpene resins are very similar to those of petroleum-based resins (1,203,204). Poly(vinyl ethers). Homopolymers of vinyl ethers are produced via carbocationic polymerization. Polymerization is carried out either in bulk or solution using BF3 or its complexes, AlCl3 , or SnCl4 . Commercially important poly(vinyl ethers) are made of methyl, ethyl, isobutyl, and octadecyl vinyl ethers [T g = −34, −42, −19, and 50◦ C (1)]. Depending on their glass-transition temperature and molecular weight, commercial poly(vinyl ethers) are viscous oils, tacky resins, or
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elastomeric solids. Poly(vinyl ethers) are added to pressure-sensitive adhesives in order to increase tack. They are also used as viscosity improvers in lubricants or as plasticizers in elastomers and coatings.
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JUDIT E. PUSKAS The University of Western Ontario GABOR KASZAS Rubber Division, Bayer Inc.
CARBOHYDRATE POLYMERS.
See Volume 9.
