Ind. Eng. Chem. Prod. Res. Dev.
334
1985,24,334-340
Stereoregulation of Methyl Methacrylate Polymerizationt Peter E. M. Allen' Department of Physical and Inorganic Chemistry, University of Adelaide, Adelaide, South Australia 500 1
Davld R. 0. Wllllams Department of Chemical Engineering, Unhrersity of Adelaide, Adelaide, South Australia 500 1
Stereospecific polymerization of methyl methacrylate (MMA) in solution is a rare example of a homogeneous, highly isotactic-specific,vinyl polymerlzatlon. The mechanism of stereoregulatkm and the optimization of efficient, highly isotactic polymerjzetionby Qlgnard reagents Is discussed in detaH. Insertkn of coordinated monomer, by a &enter transition state, into an Mg-C bond at a sterically protected, chirai, growth site is the basis of the proposed mechanism for isotactic polymerization. A ligand-migration mechanism could explain the few cases where organometallic reagents yield PMMA more syndiotactic than radical initiation at the same temperature. This does not happen with organomagnesium reagents. The nature of stereoblock polymers is discussed with particular reference to the difficutty in characterizing their steric triad distributions.
Introduction Isotactic poly(methy1methacrylate) (it-PMMA) was f i t synthesised over a quarter of a century ago (Fox et al., 1958; Miller et al., 1958; Nishioka et al., 1960; Goode et al., 1960, and references cited therein). It was soon established that it presented no threat to the conventional atactic product (at-PMMA), no immediate applications emerged, and commercial interest in this novel crystallizable form of PMMA waned. There are nevertheless a number of deficiencies in the performance of at-PMMA. The physical properties of it-PMMA in the amorphous state are very different; most notably the glass-transition, Tglis 60° lower (Wittman and Kovacs, 1969) and the dimensions of the coiled chain are more extended in solution (Jenkins and Porter, 1982) and, almost certainly, in the amorphous solid state. The crystallizability certainly presents a problem if it-PMMA is to be used as an amorphous material; it is normally lethargic toward spontaneous crystallization, but we find it extremely susceptible to induced crystallization in the presence of organic vapors-methanol in particular. The answer could well lie in blending the two materials. Our tests have shown that the susceptibility of at-PMMA to environmentally induced cracking and crazing is certainly reduced by blending with it-PMMA and the fracturetoughness is enhanced (Truong et al., 1985). Some attention should be given to the syndiotactic triad content (s)of the at-PMMA used in blends. The s-content increases with decreasing temperature of polymerization. Commercial at-PMMA prepared by free-radical initiation at elevated temperatures has an s-content of ca. 0.6. Free-radical initiation at 195 K yields s-contents as high as 0.78 (Bovey, 1960). Syndiotactic st-PMMA forms crystalline stereocomplexes with it-PMMA (Bosscher et d.,1982). We have found that complexing occm in blends In this paper the periodic group notation is in accord with recent actions by IUPAC and ACS nomenclature committees. A and B notation is eliminated because of wide confusion. Groups IA and IIA become groups 1 and 2. The d-transition elements comprise groups 3 through 12, and the p-block elements comprise groups 13 through 18. (Note that the former Roman number designation is preserved in the last digit of the new numbering: e.g., I11 3 and 13.)
-
of it-PMMA with at-PMMA having a high s-triad content. Although stereocomplex formation should be avoided if a truly amorphous material is required, there are circumstances when its presence is an advantage. The stereocomplexed it-st-PMMAs we have examined by electronoptical methods are very porous and the narrow distribution of pore sizes suggests applications as ultrafine filters or semipermeable membranes. Stereospecific Vinyl Polymerization in Homogeneous Solution Academic interest in stereospecific MMA polymerization never waned because it was the paradigm case of isotactic polymerization in homogeneous solution. A run of isotactic triads forms when each new asymmetric backbone carbon has the same configuration as the previous one. This will occur at a chiral growth center when it retains its configuration and monomers present themselves from the same direction and in the same aspect. Syndiotactic growth, where the configuration of the in-chain asymmetric carbons alternates, requires either the growth site to invert chirality between each successive addition of monomer or successive monomers to present themselves from alternate sides or in inverted aspects. There is, however, an intrinsic tendency toward syndiotactic growth at subambient temperatures which is, to a first approximation, unmoderated in polymerizations which are closest to the free-chain-growth model: the radical, free ion, and loose ion-pair mechanisms. The effect arises because the nonbonded repulsions (steric clashes) between side groups of a polyvinyl chain are greater in the isotactic configuration than the syndiotactic. It is most manifest at low temperatures (Fox and Schnecko, 1962; Otsu et al., 1966). Low temperatures are also specified for the formation of syndiotactic polypropene using soluble vanadium complexes (Boor, 1979; Tait, 1979). However, this is believed not to be a free chain growth mechanism but one in which the stereochemistry is directed by nonbonded interactions between the incoming monomer and the ligands of the vanadium growth center, including the attached polymer chain (Zambelli and Allegra, 1980). There does not appear to be a currently extant isotactic polymerization of a nonpolar vinyl monomer in homogeneous solution. It no longer seems likely that the isotactic polymerization of propene in the presence of benzyl Ti and
O196-4321/85/l224-0334$Ol.5O/O0 1985 Amerlcan Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 2, 1985
335
Zr compounds (Giannini et al., 1970; Ballard, 1973) is a
to the organomagnesium systems (Yukiand Hatada, 1979).
case of homogeneous initiation. There was always some resistance to these claims (Billingham, 1974; Soga et al., 1977). Then water was shown to enhance the activity-an activity which remained on the filter when the initiator was filtered (Yermakov and Zakharov, 1975). This could be reconciled with the demonstrations of the initial investigators that their solutions were free of suspended colloid if initiation occurred on the vessel walls at sites created by the reaction of the organometallicreagent with chemisorbed water (Allen, 1980). It is relevant to note that the most efficient initiators of this class of organometallic reagents are obtained when they are chemisorbed on silica or alumina (Ballard, 1973). Recent workers (Martineau et al., 1983) have come to the same conclusion after reexamining initiation of ethylene and styrene polymerization by these compounds. Fifty years ago an investigation of the kinetics and mechanism of a new reaction would have routinely included a test for wall effects There was merit in this practice. It would seem that established examples of homogeneous isotactic vinyl polymerization are confined to polar monomers. In addition to the acrylates, it-poly(viny1pyridine) (Natta and Mazzanti, 1961; Soum and Fontanille, 1981) is particularly relevant. It is unlikely that it is coincidental that these are all polar molecules which yield polymers with polar side groups. Certainly mechanisms postulated make use of both facts. Coordination of monomer to the growth site seems generally accepted in mechanisms for isotactic polymerizations in homogeneous solution. Coordination of a polar polymer side group is an additional requirement in most mechanisms. There are two major classes of initiator used for the synthesis of it-PMMA: organolithium (Glusker et al., 1961; Wiles and Bywater, 1962; Fowells et al., 1967) and organomagnesium compounds. We are primarily concerned with the latter.
Joint monitoring inevitably reveals the source of the discrepancy, e.g., temperature at which the initiator is filtered (Allen and Mair, 1984),but this is not possible when trying to reproduce old experiments reported in the literature. However, if strict protocols are applied, particularly to avoid the effects of local concentration and temperature excesses (Allen and Mair, 1984), the controlling factor is the concentration of THF in the polymerization solution. It has two effects which are mutually opposing. If the first instance, it seems to be a general rule that coordinating solvents militate against the persistence of isotactic propagation in polymerizations at an organometallic growth site. The mechanism is almost certainly inversion of the symmetry of a site (I 1') when solvent exchanges with a ligand coordinated at that site: a polar side group on the covalently bonded, growing chain (-M,,R), monomer (M), or solvent (S)
The Preparation of Isotactic Poly(methy1 methacrylate) Using Organomagnesium Initiators The working parameters for controlling the steric triad distributions of PMMA initiated by alkyl or arylmagnesium halides (RMgX) are solvent composition (tetrahydr0furan:toluene in our experiments), temperature, and stoichiometry (R/X ratio). In practice the stoichiometry and solvent composition are not independently controllable and are jointly determined by preparation procedures. Monomer concentration also affects the steric triad composition (Allen et al., 1981) but the effect is small with the alkylmagnesium halide initiators. With both alkyl and aryl initiators there is a limited range of adjustment because side products arise unless the monomer is in hundred-fold excess over initiator. The order of addition of reagents affects both steric triad composition and yield (Allen et al., 1981). The effect is only a problem in small-scale experiments where mixing is not efficient. It is due to local excesses of one or other of the components and local departures from a thermal steady state. If both initiator and monomer are diluted and efficient mixing is ensured, the effect is insignificant in experiments on the preparative scale. Very slow delivery of initiator (organolithium compounds) by laying the solution on top of the monomer solution also affected steric triad compositions and molecular weight distributions (Hatada et al., 1980b). The number of experimental variables is the basic reason for the notorious irreproducibility of yields and triad compositions encountered. It is not uncommon for two workers in the same laboratory to prepare different products using the same recipe, and this is not confined
-
s +
'hip
\/ M S I
-&
f
S
(1)
S
I'
At high concentrations of a strongly coordinating solvent such as THF, inversion is competitive with the growth reaction and stereorandom atactic polymer is formed. As THF concentration is reduced until inversion is a rare event during the lifetime of a chain, isotactic sequences will be interrupted by an occasional heteroatactic (h)triad and when inversion becomes insignificant wholly isotactic molecules will be formed. The other, contrary, aspect of THF, in polymerizations initiated by Grignard reagents, is that it is essential for the presence of chiral initiation sites in solution. Dialkylmagnesium initiators do not, in our experience, yield itPMMA. The highest i-content of 34% obtained with see-Bu,Mg in toluene at 250 K. THF must be present in the solvent mixture to ensure that sufficient halide remains in solution to form chiral initiation centers. The situation is similar with phenyl- and mesitylenylmagnesium (MsMg) initiators. Bis(diviny1enimino)magnesium yields it-PMMA in both toluene and THF but it is not a soluble initiator (Joh and Kotake, 1970). The consequences of the conflict between dissolving the chiral initiation centers and restraining the inversion of the chiral growth centers is that there is an optimum THF content for isotactic polymerization. This is achieved by pumping off the THF in which the initiator was prepared, digesting the residue in toluene, decanting and filtering through a no. 4 sinter (Allen and Mair, 1984). The composition and initiation efficiency of the filtrate and the steric triad content and the molar mass distribution of the PMMA depend on how much THF is left. Figure 1 illustrates the changes which occur when a solution of t BuMgBr is de-etherated with increasing efficiency and dissolved in toluene. The changes in the composition of the solute, Br/t-BuMg and THF/t-BuMg, are indicated. The simplest solute species having tetrahedrally coordinated Mg is t-BuMgBr,2THF. This stoichiometry could be achieved by briefly heating the dried residue to 353 K under hard pumping. The solubility of THF-solvated MgBr, in toluene is only 6 mM so disproportionation of t-BuMgBr to t-BuzMgand MgBrzcan only be slight as the solutions were clear. It w d d seem likely that the initiation species under these conditions was t-BuMgBr,PTHF which forms a chiral species if one THF is displaced by a coordinated monomer. Initiation under these conditions produced highly isotactic polymer (i = 1.0 f 0.05). Pro-
336 Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 2, 1985 THFiBu 15
5
10
t.aa t 0.05
IC? :
+
2 -
B r Bu
1 -
THF
2
To I
1 IO~THFITOI
Figure 1. The effect of evacuation and heat treatment, as indicated on bottom axis, of t-BuMgBr evaporated to a slurry under vacuum from T H F solution. The composition of the filtered solution after the residue was dissolved in toluene is indicated by the molar ratios: Br/Bu (left axis), THF/toluene (bottom axis), and THF/Bu (top axis). The molar mass distributions of PMMA produced by three batches of initiator solution are indicated by the gel-permeation chromatograms calibrated in terms of log (polystyrene-equivalent molecular weight) shown in the insets. Isotactic initiators, those producing PMMA within the range i = 1.00 0.05, occur within the range of conditions and treatments indicated on the top axis.
*
longed heating at 353 K reduced the THF content of the solute in toluene to its limit: THF/t-BuMg = 1,which was attained after 20 min. That the bromide content should simultaneously rise once again in excess of that of t-BuMg groups might seem curious until it is recalled that alkylmagnesium halides associate through halide bridges and such species as the trimeric sesquibromide +
THF'
determined compmition is close, but not coincident, to that corresponding to a single initiator species: tBuMgBr,2THF. Unfortunately, this condition is difficult to achieve exactly in practice. At higher states of deetheration, where there are two solutes, t-BuMgBr,BTHF and 11, the distribution is bimodal. The less de-etherated solutions, where there are two or more solutes present, produce di- and trimodal distributions and some which are continuously broad. The peaks of the distributions occur at polystyrene-equivalent molecular weights of ca. lo6, lo5, and lo4. The amount of material in each peak is dependent on temperature as well as THF/toluene ratio. The tendency toward isotactic propagation decreased with decreasing temperature. However, with a highly isotactic specific initiator such as t-BuMgBr, it-PMMA could be obtained over the range 200-275 K but at the lower temperatures the de-etheration procedure had to be more scrupulous. The recommended initiator for the preparation of itPMMA was phenylmagnesium bromide (Sandler and Karo, 1974). In our experience it is inferior to t-BuMgBr, at least when the reagents are made in THF. We were unable to achieve PMMA more isotactic than i = 0.81, h = 0.16, s = 0.03, at 270 K where the yield is low (Wallis, 1981). Mesitylenylmagnesium bromide was better; the most isotactic PMMA, attained at 273 K: i = 0.91, h = 0.09, is interesting in that syndiotactic units were undetectable. The heterotactic units in these polymers will occur between isotactic blocks of inverted configuration. Isotactic block polymers were obtained with heterotactic contents up to 0.13. At 250 K s-units appeared (Allen and Fisher, 1985). When the Sandler and Karo (1974) recipe was followed exactly (PhMgBr prepared in diethyl ether) reasonably isotactic polymer was obtained: i = 0.95, h = 0.03, s = 0.02 (Hagias, 1984). PhMgBr was interesting because ex tensiue de-etheration beyond the optimum de-etheration led to a decrease in the isotactic content of the PMMA produced. The solute in toluene was much more halide-deficient (Ph:Mg:Br:THF = 1.43:1:0.57:1.14) than the optimally de-etherated tBuMgBr in toluene. Addition of traces of THF to restore the solution to the THF content of the optimally deetherated solutions caused a further decrease in the icontent. Partial precipitation and filtration of the bromide from optimally de-etherated solution caused a reduction of i-content to that yielded when extensively de-etherated solutions were used (Allen and Fisher, 1985). This confirms the specification of the optimum THF concentration: sufficient to solvate halide containing species, which are essential to provide chiral initiation and propagation centers but insufficient for the presence of uncoordinated THF which can invert the chirality of these centers. When t-BuMgC1, n-BuMgBr, and n-BuMgC1 are prepared in the more volatile diethyl ether they can be de-etherated more thoroughly than we achieved with THF so that the isotactic directing power is lost completely. Slurries in toluene at 195 K have yielded PMMA of s-content 50.78 (Hatada et al., 1983), almost identical in fact with that of polymers produced by radical polymerization in ethyl acetate at these temperatures (Bovey, 1960), though there is no suggestion that these are radical mechanisms. So far nothing has been said about the effects of the organic group of the Grignard reagent. It was thought at one time that only branched cyclic or aryl groups yielded highly isotactic products and this is in some way related to steric effects. The latter may in fact still be relevant, but not in the simple manner we once believed. To some extent the difference between initiators is the difficulty
M9/BiM9/B\M/Bu (11) \sf
' s f h
F
have THF/t-BuMg = 1while Br/t-BuMg > 1. There is no loss of isotactic-directing power but the efficiency of initiation has decreased with these ultimately de-etherated initiators. Further change can only be brought about by increasing the temperature, which leads to decomposition and loss of activity. Efficient initiation of highly isotactic polymerization extends into the less efficiently de-etherated range until THF/t-BuMg = 10. We normally used THF/t-BuMg = 5-6, Br/t-BuMg = 1.25 f 0.10 for routine preparations. The high bromide contents when THF was in excess of that needed to solvate monomeric t-BuMgBr can be accounted for by the presence of several solute species including solvated MgBr, and the dimeric sesquihalide tBuMg2Br,,2THF. The early investigators of isotactic polymerization of MMA noted that the molar mass distributions were exceedingly broad (Glusker et al., 1961). Workers in our laboratories subsequently found that the distributions were in fact polymodal (Allen and Bateup, 1978) which led to the concept of enieidic polymerization (Plesch et al., 1965) being adopted; growth centers established early in the reaction operate entirely independently, other than competing for monomer, each producing polymer of steric content and molar mass characterized by the stereochemistry and reactivity of the particular center (Allen, 1980). The gel-permeation chromatograms of characteristic polymers produced using initiators of the indicated compositions are shown diagrammatically in Figure 1. The distribution closest to monomodal is obtained when the
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 2, 1985
in attaining the optimum conditions for generation and persistence of chiral centers. Our claim (Allen and Mair, 1984) that t-BuMgBr is the most robust and reliable initiator of it-PMMA is basically an assertion that the optimum conditions prevail over a wider range of experimental conditions than with other initiators. By careful control, highly isotactic PMMA (i L 0.89) has been prepared with n-BuMgBr (previously regarded as a nonstereospecific initiator), sec-BuMgBr and iso-BuMgBr. Hagias (1982) has prepared it-PMMA comparable with the best attainable with t-BuMgBr (i = 1.00 f 0.02) by using isoBuMgBr. In the case of PhMgBr and c-hexylMgBr (i I 0.7) it is conceivable that optimum conditions may exist over a narrow range of experimental conditions which have eluded us. A crucial factor in initiation is the position and relaxation time of the Schlenk equation 2RMgX
R2Mg
+ MgX2
(2)
since RMgX is a potential precursor of chiral initiating species and RzMg is not. A full equilibrium and dynamic characterization has been completed only for iso-BuMgBr in THF (Allen et al., 1982) but there is little doubt that the equilibrium moves in favor of RMgX with increasing temperature (Parris and Ashby, 1971) in all cases. We find that the NMR resonances of RMgX decrease in intensity relative to those of R,Mg at low temperatures in all initiating solutions we have used. In THF and toluene solutions the 'H resonances of the two species remain distinct in the case of t-BuMgBr, s-BuMgBr and n-BuMgBr over the temperature range 250-330 K, which means that the rate of exchange is slow on the time scale of the NMR experiment. At the concentration of our experiments this implies that the second-order rate coefficients of the forward and back reactions are less than 1 dm3 mol-' s-l. Under these conditions reaction of both species with monomer is likely to occur before the equilibrium is greatly perturbed. iso-BuMgBr passes into fast exchange (second-order rate coefficients 1 10 dm3 mol-l s-l) in THF between 270 and 275 K. PhMgBr, MsMgBr, and cHexMgBr all pass from fast to slow exchange on the 13C NMR scale in both THF and toluene (Allen and Fisher, 1985). If the coalescence temperature of the RMgX and R2Mg resonances is taken as an indication of the rate of exchange, the exchange of the PhMg and c-HexMg compounds in toluene is faster than MsMg. This correlates with the higher isotactic specificity of MsMgBr. However, the correlation may not be direct. The exchange reactions are believed to proceed through carbon-bridged associates and are retarded by large and C-1 branched R groups. On the other hand, large and C-1 branched R groups tend to enhance isotactic specificity and this may be a direct steric influence on the initiation reaction rather than an indirect effect through the exchange reaction. The other relevant equilibrium in the initiator solutions is solvent exchange. We have occasionally observed the separation of the THF resonances in toluene solutions at ca. 210 K. This is probably due to the equilibrium of coordinated and free THF passing into slow exchange, with the implication that it is fast above 210 K. The Mechanism of Stereoregulation The propagation reaction is believed to be an insertion of a coordinated monomer into a covalent organometallic bond, probably (Bateup and Allen, 1977) M
+
RM,MgX RM,MgX
e f M
I11
-
RM,+,MgX
( 3)
337
The kinetics of the polymerization vary from an internal order of zero with respect to monomer to internal first order, in the case of BuMgBr initiators, in a manner which is wholly consistent with the mechanism of reaction 3 (Allen et al., 1984). In the case of PhMgBr the kinetics are different, possibly because of the intervention of termination reactions. The mechanism is not an anionic polymerization. An anionic polymerization is accelerated by polar solvents which shift ion pair and ionization equilibria in favor of the more reactive free ions and solvated ion pairs. This occurs when organosodium or ceasium compounds are used to initiate the polymerization of MMA (Allen et al., 1972; Kraft et al., 1978). However, when organomagnesium compounds are used, the rate decreases with increasing proportions of the polar solvent component (Bateup and Allen, 1977). They have been described as pseudo-anionic, but this is not a very useful classification of a reaction which shows no evidence of involving ions at all. MMA polymerizations initiated by organolithium compounds have always been regarded as anionic (Fowells et al., 1967). We have no direct experience of them, but certain similarities in behavior to the systems we have investigated have raised the question whether they are truly anionic (Allen et al., 1981). Polymodal molecular-weight distributions indicate the presence of different independent growth centers which maintain their identity throughout the reaction. Allen and Bateup (1978) argued that trimodal distributions obtained in alkylmagnesiuminitiated systems are evidence for an insertion-in-a-covalent-bond mechanism. It was held that equilibria between different ionic species would be too labile for growth centers to persist in a particular form long enough to yield polymodal distributions. If this argument is cogent for organomagnesium systems it also applies to those organolithium systems where polymodal distributions have been reported (e.g., Hatada et al., 1979). There seems little doubt that the organic compounds of the heavier group 1metals initiate genuine anionic polymerization of MMA. However they do not yield it-PMMA. The mechanism we propose for stereoregulation through the complex I11 in reaction 3 is a direct descendent of models based on Li ion pairs. We adopt from them the concept of coordination of monomer at the metal as well as coordination of a carbonyl group of the polymer chain. Molecular models suggest that Fowells' six-membered ring is less strained than the earlier eight-membered ring (Glusker, 1961). The mechanism is shown diagrammatically in Figure 2. Complex I11 is a carbonyl complex which must rotate about 0 Mg and C-C to bring C=C into alignment with Mg-C, to form the 4-center transition state IV. Insertion of C=C into Mg-C, through cis-opening with decoordination of 0 Mg leads to a new Mg-C, bond with the new C1 having the same chirality as the previous one, now C3 If the next monomer complexes at the site vacated by the monomer inserted, the chirality of the Mg growth center is retained. If, however, it complexes at the site shown vacant in I11 and IV, chirality of Mg is inverted. This is prevented if the ligand on this site is the carbonyl group on the chain C3 as shown in IVa. After each insertion the six-membered ring expands to an eight-membered Glusker ring which must relax to a Fowells ring after the monomer has coordinated. This ensures retention of chirality of the growth center. There is an alternative conformation in which the coordinated monomer could approach the 4-center transition state which would lead to inversion of the C, configuration on insertion. This is an approach from the right-hand side of the Mg-C1 bond as shown in I11 and IV. The rotation of the coordinated
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338 Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 2. 1985 H
5
Figure 2. Proposed mechanism for the passage of complex I11 into a 4-canter transition state of an insertion reaction (IV). IVa shows the growth site protected from inversion by the fourth coordination s i t e of Mg being occupied by %CO. IVb indicates how 'CCOOCH, aiid 'CC'OOCH, prevent the rotation of the coordinated monomer to enter a ¢er transition state on the right-hand side of Mg-'C
iwriomer into this position from that shown in I11 and IV is restricted by the nonbonded interactions with the C,COO(;H, and the coordinated C3--COOCH3,IVb. Such a ceiiter will generate a run of isotactic units until its chirality is inverted by the chain-C0 being displaced from its coordination site by monomer or solvent. In the case of alkylmagnesium bromides, 'I'HF is the only potent cause of inversion; the monomer effect is small (Allen and Mair, 1984). Monorrier has a more significant effect on chiral growth ceiitws generated by PhMgBr, which may account for the lower kotactic-specificityof this initiator (Allen and Fisher, 198'11 Rare displacement of the chain-CO from its coordinafion site leads to the insertion of an occasional h-triad in essentially isotactic molecules. When the rate of'displacement becomes more rapid than the growth reactioti the triad distribution is governed by Bernoulliantrial statistics (Bovey, 1972) in which the configuration of a new chain end is not influenced by the configuration of other units in the chain. Under intermediate conditions stereoblock polymers are formed. Stereoblock PMMA Stereoblock polymers have a non-Bernoullian triad distribution. 'They are most commonly defined in terms of the parameters of a first-order Markov distribution Rovey, 1972) but they can arise from mechanisms operating according to higher-order Markov or Coleman-Fox (Coleman arid Fox, 1963) statistics. We prefer to define them in more general terms: polymers in which the mean letigtli of sterically identical blocks exceeds that of' the fjerrioulliari case. It we are concerned with isotactic and syndiotactic blocks the triad distributions corresponding t o stereoblocks are those defined by the domain bounded by the Ihriioulliaii hyperbola and the S-I axis of Figure 3. Free-iadical polymerization normally produce PMMA with triad distributions close to Bernoullian. So do many ionic mechanisms. The organomagnesium initiators yield rion-Bernoullian PMMA except at low temperatures and liigh THV concentration. Vigilre -3 illustrates the effects of THF concentration and teiriiit-iature oil the triad frequency of PMMA prodtlced by t h iwst strongly isotactic-directinginitiator t-BuhlgBr. 'I'he two r t i e c t s must be distinguished. 'I'WF reduces the c-coritenl by causing inversion of the growth centers leading 1 0 ti diange from isotactic chain growth to one where the rontigurstion is governed by Rernoullian trial statistics.
e,
0
d
I
Figure 3. The effects of temperature and T H F concentration on steric triad composition of PMMA produced with t-BuMgBr a t constant initiator, (Bu/MMA = IO-*) and monomer mol fraction (xMm= 0.1). [THF]/[toluene] < 0.1 (0,0 ) ;0.1-0.25 (a,.); 0.25-1 (A,A); 1-2 ( 0 , e), and >2 (V, V), open symbols: 273 K, filled symbols: 200 K. The compositions are the average of many samples produced under each defined range of conditions. The hyperbolic curve indicates triad compositions permitted to a Bernoullian distribution. The four data points 195, 273, 315, 373 are triad compositions of PMMA produced by radical initiation in ethyl acetate a t the four cited temperatures (K) (Bovey, 1960).
The effect of decreasing temperature is a reduction of the frequency of i-triads, a fundamental phenomenon observed with other chain-growth mechanisms (see above). At 200 K the full range, isotactic to Bernoullian, is encompassed in the change from a toluene to a T H F medium. A t 273 K, Bernoullian statistics are not attained even in THF. Dialkyl- and diarylmagnesium compounds initiate PMMAs confined to the Bernoullian distribution-stereoblock range. Alkyl- and mesitylenylmagnesium halide initiators cover the range from Bernoullian to isotactic (or nearly isotactic), but an individual initiator does not necessarily cover the whole range at any one temperature. In convering this range the line defining the attainable compositions traverses a different region of Figure 3 at each temperature, so that a range of interesting polymers (e.g., s = i, h varied) can be prepared. Triad frequencies provide no basis for identifying distribution more complex than Bernoullian. Tetrad and pentad frequencies, accessible through high-resolution NMR spectrometry, can be used to test for first- and second-order Markov distributions (Bovey, 1972). The polymers we have prepared are more complicated. The mechanism proposed is basically similar to a Coleman-Fox two-state system. In one state, the active center, protected by the coordination of the polymer chain, retains chirality. This involves 2, or possibly 3, in-chain monomer units so it is conceivable that this state operates according to Markov statistics of up to third order. The growth center passes into the second state when the coordinated polymer chain is displaced by solvent or monomer. In this state the aspect and direction of delivery of monomer into the four-center transition state (IV) is uncontrolled. It could be quite random, in which case retention or inversion of configuration on insertion of each new monomer is random and so Bernoullian statistics prevail. The simplest, and most likely, product is a stereohlock PMMA consisting of isotactic blocks and Bernoullian, or near-Bernoullian, blocks. Even this simplest distribution is inaccessible to currently available testing techniques. Calculation of persistence ratio and mean block length (i and s taken together) (Bovey, 1972) gives some crude characterization. However, calculation of the individual mean lengths of
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 2, 1985
339
i-blocks and s-blocks depends on the h-triad appearing only a t the junction between an i- and an s-block. An additional problem is compositional homogeneity. The molar mass distributions are commonly polymodal, so the question arises as to whether the stereoblocks are in fact mixtures of distinct populations having different chain configurations. Fractionation gives a partial answer. The fractions separate primarily on the basis of molar mass. In some cases there is no significant difference in the triad composition of the high and low molar mass fractions; in some cases a stereoblock polymer fractionates into two stereoblock fractions of significantly different triad composition. Extensively de-etherated PhMgBr in toluene yielded a t 250 K a very broad molar mass distribution of triad content i = 0.38, h = 0.23, s = 0.39 Extraction with T H F yielded material of molar mass lo4 and i = 0.55, h = 0.22, s = 0.23. The literature contains other examples of both cases: fractions having the same composition or significant fractionation of triad compositions. However, it does seem that when there is a range of triad frequency sufficiently broad to fractionate it is a range of stereoblock polymers. There is no evidence to suggest that they are not stereoblock polymers but mixtures of isotactic with syndiotactic or atactic PMMA.
insertion reaction of a precoordinated monomer into the Mg-C covalent bond of a chiral growth center. An optimum THF concentration is determined by the necessity of having sufficient halide in solution to form chiral growth centers but insufficient THF to cause significant inversion of chirality. Increase in T H F concentration'induces increasing inversion, formation of stereoblocks and, in the limit, polymers having a Bernoullian distribution of triads. As t,emperature is decreased, the tendency to isotactic placement decreases. This is partly intrinsic (it is observed in radical polymerization) and partly due t,o changes in the Schlenk equilibrium of the initiator. There is no evidence that PMMA more syndiotactic than attainable by radical polymerization at the same temperature is formed in homogeneous solution with alkyl- or arylmagnesium reagents. The stereoblock polymers cannot be fractionated into it- and st-PMMA, though in some cases triad-enriched stereoblock fractions can be separated. The detailed triad distributions cannot be obtained; the mechanism suggest8 that they may consist of isotactic and near-Bernoullian distribution blocks. Registry No. MMA, 80-62-6; PMMA (homopolymer), 901 1-
Syndiotactic and Heterotactic PMMA A chiral growth center operating in the syndiotactic mode must invert chirality regularly between each addition of monomer. This is a highly improbable occurrence at a growth center such as IV if the growth step is an insertion of monomer into the Mg-C bond at the end of the polymer chain. However, if the reaction is a ligand migration, the polymer chain migrating and adding to the coordinated monomer, the site vacated will be the one occupied by the next monomer to coordinate. The chirality of the center is then inverted. Such a mechanism would yield syndiotactic chains. No polymers we have prepared exceed the s-content attainable by radical initiation at the appropriate temperature so we have no need to postulate an inverting chiral center. If we look at the st-PMMAs produced using other orgammetallic systems (Yuki and Hatada, 1979), most are prepared at 195 K where free chain growth runs predominantly syndiotactically and their s-contents do not exceed that of a radical-initiated polymerization a t that temperature. For reference the triad frequencies of PMMA prepared in ethyl acetate at four temperatures (Bovey, 1960) are shown in Figure 3. There are two cases where an inverting chiral center may be involved: bis(pentamethy1enimino)magnesium in T H F or toluene at 195 K: i = 0, h = 0.07, s = 0.93 (Joh and Kotake, 1970) and triphenylmethylcalcium chloride in 1,2-dimethoxyethane (trace THF) a t 210 K: i = 0, h = 0.12, s = 0.88 (Lindsell, 1981). The system aged A1Et3/TiC14in toluene (i = 0, h = 0.06, s = 0.94 a t 195 K) is not a homogeneous one (Abe et al., 1965). We have tested the first of these 3 recipes but were unable to produce s contents in excess of 0.68 (Hagias, 1984). Heterotactic propagation requires a much more elaborate mechanism of stereoregulation than is attainable with models we have proposed. No highly heterotactic PMMA appears to have been prepared, though there is a notable case: octylpotassium in T H F a t 195 K (Hatada et al., 1980a), i = 0.11, h = 0.53, s = 0.36, where the h-content is significantly in excess of that permitted to a Bernoullian distribution.
Literature Cited
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Conclusions The formation of it-PMMA initiated by toluene solutions of Grignard reagents is consistent with a four-center
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Receiued for reuiew September 17, 1984 Accepted January 28, 1985 The work described was
by the
Research
Grants Scheme. It was presented in part at the 14th Australian Polymer Symposium, Ballarat, Victoria, February 1984.
COMMUNICATIONS Selective Synthesis of Acrylonitrile from Acetonitrile and Methanol over Basic Metal Oxide Catalysts Catalytic synthesis of acrylonitrile from acetonitrile and methanol was achieved by using basic metal oxide catalysts based on magnesium oxide. At elevated temperature (>350 "C), acrylonitrile was yielded selectively (>go%) with minor amounts of propionitrile and methacrylonitrile. The ratedetermining step of the reaction is the abstraction of the proton from the methyl group of acetonitrile by the surface basic site on the oxide catalyst, followed by C=C bond formation with methanol adsorbed on the oxide surface.
Introduction Acrylonitrile is one of the most important chemicals in the chemical industry. The current industrial synthetic method is a partial oxidation process, called the SOH10 process, where propylene is oxidized by molecular oxygen in the presence of NH, over multicomponent bismuth molybdenum oxide catalyst (Callahan et al., 1970; Grasselli et al., 1982). In recent years, acrylonitrile synthesis from C1 chemicals such as CO, CH,OH, and CH4 has increased in interest. For example, acrylonitrile can be synthesized oxidatively from CH4and acetonitrile, which is synthesized from CO, Hz and NH, catalytically (Monsanto process) (Khcheyan et al., 1979, 1980: Olive and Olive, 1979). We have developed another general method for synthesizing acrylonitrile selectively from acetonitrile and methanol using binary metal oxide having surface basic properties as a catalyst. In this method, the C-H bond of the methyl group of reactant must be activated by inductive electron withdrawal by the unsaturated substituent such as carbonyl, cyano, or phenyl groups, converting the methyl into vinyl group by the addition of methanol. It is thus widely applicable for synthesizing a,@-unsaturatedcompounds. In this communication, we report the catalytic synthesis of acrylonitrile from acetonitrile by using methanol over various metal ion contained magnesium oxide catalysts and the role of the surface metal ion is discussed.
Experimental Section Catalyst Preparation. All chemicals used were of regent grade quality and were commercially available. Various metal ion contained magnesium oxide catalysts, M-MgO [M = Cr(III),Fe(III),Al(III), Cu(II), Ni(II)],were prepared by impregnating magnesium oxide (Soekawa Rika, 99.92%) with the corresponding nitrate solution. The metal ion conntent (wt%) was based on the concentration of metal ion in the preparation solution. All the catalysts were heated in a nitrogen stream for 2 h at 600 0196-432118511224-0340$01.50/0
0
!
2
3
0
5
Time on stream ( h )
Figure 1. Acetonitrile conversion and selectivity to acrylonitrile in the reaction of acetonitrile and methanol over Cr-MgO (3.1 wt % ) catalyst at 350 "C.
"C before the reaction in order to decompose the metal nitrate and to desorb water and COz. The catalyst was used in the form of pellets. Reaction Apparatus and Conditions. The reaction at atmospheric pressure was carried out in a conventional flow system equipped with a quartz reactor and a tubular furnace. The reactant mixture (acetonitrile/methanol = 1/10) was introduced into the flow line by a syringe pump and evaporated in a preheater tube. Nitrogen was used as the diluent and the flow rate was maintained at 70 mL/min. Quantitative analysis of the products was carried out by using gas chromatography [Adsorb P-1 (3 m, 170 "C, He carrier) for reactant and acrylonitrile, and Molecular Sieve 13X (0.5 m, 25 "C, Ar carrier) for HZ,CHI, and CO]. Results and Discussion Figure 1 shows the plot of acetonitrile conversion and selectivity to acrylonitrile as a function of time in the reaction of acetonitrile with methanol over Cr-MgO (3.1 w t % ) catalyst at 350 "C. An initial decrease in the con0 1985 American Chemical
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