Handbook of Polymer Synthesis Second Edition Episode 2 docx

2 Polystyrenes and Other Aromatic Poly(vinyl compound)s Oskar Nuyken Technische Universita ¨ tMu ¨ nchen, Garching, Germany I. STYRENE Discovery of the styrene monomer is credited to Newman [1] who isolated it by steam distillation from liquid amber, which contains cinnamic acid, yielding styrene via decarboxylation. The first polymerization was described by Simon [2]. Commercial styrene polymerization was begun about 1925 [3]. Cracking of ethylbenzene became the major manufacturing route for the monomer. The first commercialization was based on bulk polymerization using the can process [4,5]. Polystyrene production has grown by an average of 2.8% per year over the last 10 years, reaching 8 500 000 metric tons worldwide in 1995 [6]. The most general-purpose polystyrene is produced by solution polymeriza- tion in a continuous process with the aid of peroxide initiation. Suspension polymerization is used for products for which a small spherical form is de sirable. Emulsion polymerization is the method of choice for ABS resins. Polystyrene is a glasslike solid below 100  C. Below this temperature it shows considerable mechani cal strength. Rubber-modified polystyrene is a two-phase system, rubber dispersed in polystyrene being the continuous phase. Advantage is taken of the complex interaction of those systems in many applications in which high stress-crack resistance is needed. Polystyrene is nonpolar, chemically inert, resistant to water, and easy to process. It is the material of choice for many food-packing, optical, electronic, medical, and automotive applications. Tensile strength can be increased by controlled orientation of polystyrene. A. Synthesis [5,7] All current styrene production starts with ethylbenzene followed by dehydrogenat ion over ferrum oxide catalysts yielding crude styrene: ð1Þ Copyright 2005 by Marcel Dekker. All Rights Reserved. Side reactions are reduced by keeping the conversion low or by adding water as a diluent. It is also possible to synthesize styrene by the oxidation of ethylbenzene: ð2Þ ð3Þ ð4Þ An alternative pathway starts from toluene and ethene: ð5Þ ð6Þ It is also possible to synthesize styrene from butadiene: ð7Þ Physical properties and health and safety information for styrene are given in the literature [8]. B. Homopolymerizat ion The C–C double bond of styrene can act either as electron-donating or as electron- withdrawing center. Therefore, it cannot only be polymerized by radicals but also anionically or cationically or by coordination initiato rs. Copyright 2005 by Marcel Dekker. All Rights Reserved. 1. Radical Polymerization Styrene can be polymerized without a chemical initiator simply by heating (spontanous polymerization). The first step is a Diels–Alder reaction. ð8Þ Only the axial isomer can react with a further styrene atom, yielding two radicals, which can start a radical polymerization: ð9Þ Some of the possible dimers, including 1-phenyltetraline, and a trimer have been identified [9–11]: ð10Þ This type of polymerization requires third-order initiation kinetics [12,13]: R i ¼ k d M½ 3 Consequently, the rate of propagation should be of 5/2 order with respect to styrene [13,14]: R p ¼ k p k d 2k t  1=2 M½ 5=2 However, the reaction does not follow a simple kinetic order with respect to mono- mer over the entire range of conversion and temperature. The order is negative below 50  C. At 75  C the polymerization is zero order in monomer for the first 65% conversion. At 127  C the polymerization is first order for the first 85%. Only above 200  C does the polymerization follow theory [15]. a. Initiators. The list of initiators available for radical polymerizati on of styrene is very long [16–18], including azo compounds, peroxides, redox systems and many more. An interesting development is the application of initiators like ð11Þ which decompose to form four radicals. It is even more interesting to have a different half- life for both peroxide groups. This presents novel opportunities for changing the molecular weight and its distribution [19,20]. Copyright 2005 by Marcel Dekker. All Rights Reserved. b. Inhibitors. During shipping and storage styrene needs an inhibitor. The most efficient inhibitors—quinones, hindered phenols, and amines [21]—require traces of oxygen to function. t-Butyl-catechol at 15 to 50 ppm is the most common inhibitor for commercial styrene [22]. It is also possible to use nitrophenol, hydroxylamine, and nitrogen oxide compounds [23]. The inhibitors have to be removed before polymerization, in order to avoid an induction period. Traces of metal such as iron or copper [24] and sulfur compounds [25] are the cause of retardation effects in styrene polymerization. c. Chain Transfer. In styrene polymerization the chain transfer agent can be the solvent, monomer, initiator, polymer, or an added chemical agent. As C tr ¼ k p /k tr increases, the chain transfer agent becomes more effective. Some examples are given in Table 1. The most important property affected by chain transfer is the molecular weight of the polymer. The transfer to monomer has a value of 10 À5 which can be neglected. However, since the transfer constant to the Diels–Alder dimer axial-1-phenyltetralin is about 113 at 80  C [26], this may cause experimental error. Any transfer to polymer would lead to branched structures in the final product. Although this reaction has been inves- tigated to some extent, there is no conclusive evidence that it is an important reaction [27]. The most important aspect of chain transfer is the control of molecular weight by the adequate use of added transfer agent. Mercaptanes are by far the most widely used chemicals for this purpose. d. Termination Reactions. The free-radical polymerization of styrene is termi- nated almost exclusively by the combination of two growing chains [29,30]: ð12Þ Termination is diffusion controlled at all temperatures below 150  C [31,32]. Increasing viscosity leads to a reduction in the termination rate [33]. However, the resulting Trommsdorff effect is comparably small for polystyrene [22]. Table 1 Chain transfer constants in styrene polymerization [28]. Compound C tr T/  C Benzene 2  10 À5 100 Toluene 5  10 À5 100 Ethylbenzene 1.4  10 À4 100 Isopropylbenzene 2  10 À4 100 Styrene 5  10 À5 100 Dichloromethane 1.5  10 À5 60 Chloroform 5  10 À6 60 Tetrachloromethane 1.8  10 À2 100 1-Dodecanthiol 1.3  10 1 130 1-Hexanthiol 1.5  10 1 100 Ethylthioglyconate 5.8  10 1 60 Copyright 2005 by Marcel Dekker. All Rights Reserved. e. Processing [34]. Free-radical polystyrene can be synthesized either by bulk, solution, suspension, or emulsion techniques. Techniques for preparing polystyrene on a laboratory scale are described in detail in Refs. [35–37]. The bulk process needs pure styrene; it is very simple and yields polymers with high clarity. Due to its poor control, this process is not used commercially. In solution polymerization styrene is diluted with solvents, which makes temperature control easier. However, solvents normally reduce the molecular weight and polymerization rate. Both processes can be carried out either in batch or continuously. The advantages are more uniform products and low volatile levels. The main disadvantage is the transportation of highly viscous finished product. Suspension polymerizati on is still an important mode of polystyrene production, although it has lost ground to continuous solution polymerization. The polymerization system contains monomer suspended in water, stabilizing agents, and initiators to speed polymerization. The easy heat control and removal of the finished polymer count as advantages. Contamination with stabili zing agents is considered a disadvantage. Emulsion polymerization requires water as a carrier with emulsifying agents. It yields extremely small particles. Advantages are rapid reactions and excellent heat control. Disadvantages are the contamination of polymer with the emulsifier, water, its deficit in clarity, and the limitation to batch processing. However, this type of processing is important for ABS polymers. 2. Controlled Radical Polymerization The controlled radical polymerization combines the advantages of living ionic systems, as there are narrow molecular weight distributions, linear increase of the DP with the reaction time and the possibility of the formation of block copolymers, with the main advantage of the radical polymerization, the low sensitivity against impurities. The general idea of controlled radical polymerization is to avoid the bimolecular, irreversible termination reactions, typically obtained in a free radical polymerization (combination, disproportionation etc.) by decreasing the number of growing radical chains. Thus, although the reaction itself beco mes comparably slow, the molecular mass can be very well controlled and very narrow molecular weight distributions can be obtained. Early attempts to realize the controlled radical polymerization of styrene involved the concept of reversible termination of growing polymer chains by iniferters (initiation, transfer, termination) [38]. These iniferters based on dithiocarbamates were the first species with photochemically labile C–S bonds. ð13Þ Another way of reversible termination was introduced by the same group [39,40]. They showed, that at the decomposition of phenylazotriphenylmethane both a phenyl Copyright 2005 by Marcel Dekker. All Rights Reserved. and a trityl radical are generated. The phenyl radical initiates polymerization, while the trityl radical does not, due to its mesomeric stabilization. ð14Þ Instead, the trityl radical acts as a radical trap and efficiently terminates polymeri- zation by primary radical coupling. As a result of steric crowding between the pendant groups on the polymer chain and the phenyl groups of the trityl moiety, as much as a result of the stability of the triphenylmethylradical, the C–C bond can redissociate at elevated temperature and add more monomer. ð15Þ Following this approach, Rizzardo et al. and Georges et al. introduced the use of stable nitroxide free radicals, such as 2,2,6,6-tetramethylpiperidinyloxy (TEMPO), as reversible terminating agents to cap the growing polymer chain [41,42]. ð16Þ It has been demonstrated, that at elevated temperatures narrow molecular weight distribution polystyrene (PDI ¼ 1.1–1.3) could be prepared using bulk polymerization Copyright 2005 by Marcel Dekker. All Rights Reserved. conditions. In the polymerization of styrene, temperatures around 120  C are required in order to obtain a sufficient rate of monomer insertion, because of the stability of the C–O bond. A very similar approach is the use of triazolinyl counter radicals as an alternative to the nitroxides [43,44]. The electron spin density is not localized, as in the case of TEMPO, but delocalized in a extended p system. ð17Þ With a polymerization temperature of 120  C, a three-fold higher polymerization rate in comparison to TEMPO could be obtained in styrene polyme rization. Furthermore, in contrast to TEMPO mediated polymerization, polymers up to a molecular weight of 100 000 g/mol can be obtained in good yields. The mechanism of the polymerization process is not identical to the TEMPO mediated polymerization, but the control is introduced by a self regulation process [45]. In 1995, a further approach to controlled radical polymerization, the Atom Transfer Radical Polymerization (ATRP) was indepen- dently reported by Matyjaszewski [46] and Sawamoto [47]. These systems are based on the dynamic equilibrium of a reversible redox reaction between halogen endgroups of the polymers and transition metal catalysts. The catalysts are mainly Cu(I), Fe(II) or Ru(II) complexes with different ligands. The copper based systems usually contain nitrogen ligands like bipyridines, multidentate amines and Schiff bases. A general review over copper mediated ATRP is given in Ref. [48]. The ruthenium based catalysts show a wide variety of structures with arenes, phosphines and halogens as ligands [49]. Iron based catalysts are also applied, most of them are containing bipyridines, trialkylamines, phosphines or phosphites [50]. ð18Þ The ATRP allows the synthesis of very narrow dispersed polystyrenes (PDI 1.1). Recently, a new mechanism for controlled radical polymerization of styrene, the RAFT (reversible addition fragmentation and transfer) process, has been presented [51,53]. This type of bimolecular exchange process employs reversible addition of Copyright 2005 by Marcel Dekker. All Rights Reserved. the radicals to a nonpolymerizable double bond. The RAFT process is best represented by the use of several dithioesters as transfer reagents and the mechanism can be divided in three main steps. The addition of a growing polymer chain to the transfer reagent with subsequently homolytic fragmentation of the S–R bond (transfe r) is the first step: ð19Þ The reinitiation of a new active polymer chain with the resulting radical R . follows. ð20Þ Subsequent addition-fragmentation s teps set up an e quilibrium between the two pro- pagating radicals and the dormant polymeric dithiocarbonylthio compound by way of an intermediate stabilized radical. Th roughout the polymerization (and a t the end) the v ast majority of the po lymer chains are e nd capped by a thiocarbonylthio group (dormant chains). ð21Þ In general, every kind of monomer needs a different dithioester for best results, whereby the most suitable compound for styrene contains phenyl for Z and 2-phenyl- propyl for R. Polystyrene with a PDI down to 1.07 can be obtained by use of this compound [52]. 3. Anionic Polymerization The phenyl group of styrene is able to act as an electron-donating or an electron- withdrawing center. This situation allows the growing end of the polymer to be either a carbeniumion or a carbanion, as shown in more detail in this chapter. Copyright 2005 by Marcel Dekker. All Rights Reserved. a. Initiation. A highly purified monomer is reacted with a strong base. Although several initiators are known, organolithium compounds are the most studied and probably the best understood initiators [54,55]. ð22Þ This initiation is much faster than the propagation step. All styryl anions are therefore formed almost instantaneously. Since no termination occurs, the degree of polymerization can be calculated easily on the basis of the following equation: DP n ¼ ½M ½I Furthermore, one can observe that DP n % DP w % DP z This has been discussed in detail in several reviews [56–58]. b. Propagation. The ideal polymerization of this type (the cationic polymerization follows the same kinetics) obeys the following equation: R p ¼ k p Á½C Æ Á½M where C Æ represents the molar concentration of active ionic chain ends. The rate constant is strongly affected by the solvent [59]: for example k p is 2 L mol Àl s À1 in benzene compared to 3800 L mol Àl s À1 in 1,2-dimethoxyethane at 25  C. In addition to solvent, the counterion affects the rate of polymerization. The effect of the counterions is often explained on the basis of their sizes (e.g., increasing solvation with decreasing size yields a greater concentration of free ions and higher polyme rization rates) [60]. For the growing end of poly(styryllithium), an association of two growing chains has been discussed [61,62]. This complex dissociates if polar solvents such as THF and diethylether are added, resulting in an increase in the polymerization rate. Instead of using a monofunctional initiator, it is possible to use a bifunctional anionic initiator. One of the best described systems involves the reaction between sodium and naphthalene, forming a radical anion that transfers this character to the monomer. The two radical anions combine quickly to form a dianion [57]. ð23Þ Copyright 2005 by Marcel Dekker. All Rights Reserved. ð24Þ ð25Þ ð26Þ Another bifunctional initiator is formed by the reaction of 1,3-bis(1-phenylvinyl)- benzene with organolithium, yielding a dianion with good solubility in hydrocarbons [63–65]: ð27Þ Copyright 2005 by Marcel Dekker. All Rights Reserved. [...]... and Faust and Kennedy [21 6] Higashimura and coworkers [21 7] showed the first living polymerization of a-methylstyrene (a-MeSt), with the HCl-adduct of 2- chloroethylvinylether/SnBr4 initiating system at À78  C in CH2Cl2 Fodor and Faust [21 8] reported the living polymerization of a-MeSt using the cumylchloride, (CH3)3C–CH2–C(CH3 )2 CH2–C(Ph )2 OCH3 (TMPDPEOMe) or the HCl adduct of a-MeSt dimer (DiaMeSt)... mainly of conventional 1 ,2 units Also, copolymerization studies of Mizote et al [24 2 ,24 3] confirm the formation of 1 ,2 enchainments by cationic polymerization of b-methylstyrene C ar-Methylstyrene ð54Þ As styrene o-, m-, and p-methylstyrene are polymerizable by all of the conventional mechanisms A copolymer containing 60% m- and 40% p-MeSt is commercially produced by thermal polymerization 1 Radical Polymerization... to poisoning of the active centers While the m isomer could not be polymerized at all, both the o and p isomers gave varying yields of polymers with AlC12Et (27 %/ 92% ), AlClEt2 (4%/ 82% ), TiCl2(OAc )2 (trace/7%), and TiCl2(OBu )2 (O%/ 12% ) in toluene at À78  C for 6 h The poly(p-MeOSts) had a much higher molecular weight ([Z] ¼ 2) than the poly(o-MeOSts) ([Z] ¼ 0.1) Both the o- and p-MeOSt polymers could... in the presence of ZnCl2 [ 124 ] or EtAlC 12 [ 125 ] 2 Poly(styrene-co-maleic anhydride) (PSMA) Copolymerization of styrene with maleic anhydride yields alternating structures, probably due to the formation of charge transfer complexes [ 126 , 127 ] Statistical copolymers Copyright 20 05 by Marcel Dekker All Rights Reserved Table 2 Styrene (M1) comonomer reactivity ratio; a comprehensive list of reactivity ratios... radical polymerization yielded only dimers [23 2] Only copolymers of b-MeSt have been reported using radical polymerization [23 3], yielding Copyright 20 05 by Marcel Dekker All Rights Reserved phenoxy-phenyl maleimide-b-MeSt copolymers under participation of CT-complex b-MeSt is also a good transfer agent of propene polymerization but becomes partially incorporated into the polymer [23 4] Anionic polymerization... polymer as a function of temperature and from that the thermodynamic values The paper of Wyman and Song [22 6] gives Mw/Mn values in the range 1.5 to 1.8 and molecular weights in the order of 105 g/mol Concerning the design of positive electron-beam resists [22 8 ,22 9], anionic polymerization was used to introduce 2- phenylallylgroups at the end of poly(a-MeSt) chains 4 Living Radical Polymerization p-Br... to pure PS 6 Copolymerization with Divinylbenzene Copolymerization of styrene with small amounts of bifunctional monomers such as divinylbenzene is used for the synthesis of networks The polymerization technique of choice is bead polymerization Polymer porosity can be controlled by the addition of polystyrene, which can be extracted after polymerization has been completed Sulfonation of such networks... a polymeric initiator useful for further polymerizations [157,158]: ð36Þ Copyright 20 05 by Marcel Dekker All Rights Reserved A possibility to synthesize block copolymers by conventional radical polymerization is given by the application of polymeric initiators [1 62] Partial decomposition of the polymeric initiators in the presence of styrene yields block copolymers containing polystyrene and part of. .. chloride/Et2AlCl in methylene chloride [20 9], or 9-anthranylmethyl hexafluorophosphate [21 0] Kennedy and co-workers published a series of papers [21 1 21 3] in which they investigated the polymerization ability of the cationic initiation systems H2O/SnCl4, H2O/BC13, and pentamethylbenzyl chloride/SnCl4 with and without the use of the proton trap 2, 6-di-tert-butylpyridine (DtBP) In the systems H2O/BCl3 in... al [24 8] studied the polymerization of o-MeSt by H2SO4, AlBr3, or BF3–OEt2 in chlorinated solvents at low temperatures and compared it to thermal polymerization in bulk The polymers showed identical IR spectra and it was thus concluded that also with this monomer exclusively, conventional 1 ,2 polymerization had occurred as it did with p-MeSt [21 2] Heublein and Dawczynski [24 9] used the SnCl4/H2O system . synthesized Table 2 Styrene (M 1 ) comonomer reactivity ratio; a comprehensive list of reactivity ratios is given in Ref. [ 120 ]. Monomer 2 (M 2 ) r 1 ( ¼ k 11 /k 12 ) r 2 ( ¼ k 22 /k 21 ) T/  C Ref. Methyl. Emulsion polymerized copolymers of styrene and acrylic esters are important basic materials for coating resins. Copolymerization of styrene with acrylic acid salts (Zn 2 ,Co 2 ,Ni 2 , and Cu 2 ). or suspension polymerizat ion. Alternating structures are derived in the presence of ZnCl 2 [ 124 ] or EtAlC1 2 [ 125 ]. 2. Poly(styrene-co-maleic anhydride) (PSMA) Copolymerization of styrene with