456
Ind. Eng. Chem. Prod. Res. Dev. 1986, 2 5 , 456-463
desired extent of cross-linkingcan be achieved by adjusting the level of these two materials. Infrared spectra of the particles support the assumed hydroxy/melamine crosslinking reaction. Since the radical polymerization is generally faster than the hydroxy/melamine condensation reaction, it is likely that only a fraction of the melamine cross-linking reaction occurs during the initial polymerization. Most of the melamine is probably trapped, unreacted or partially reacted, in the particles; the balance of the condensation reaction likely occurs upon further refluxing. Decreasing the amount of oligomer IV in the dispersion composition produces particles of larger size (Tables I and 11);however, increasing the amount of hydroxy and melamine materials causes no apparent change in the particle size. The addition of acid catalyst seems to result in formation of larger particles (dispersions B’ and D’, Table 11). The particles (A’ and C, Table 111), when included in the base coat of the base-coat-clear-coat high-solids paint composition, with a 3-min flash, provide gloss and distinctness of image only comparable to that of the composition without any additive (Table 111);however, aluminum control, as determined by visual inspection, in the case of compositions A’ and C (3-min flash) is far superior to the case of the one without any additive. When the flash time is reduced to 1min, use of particles A’ and C provides coatings with significantly improved aluminum control, gloss, and distinctness of image (Table 111). Particles A’ and C seem to provide superior resistance to attack of the base coat by solvents in the clear coat. The accelerated weathering of base-coat-clear-coat metallic paints con-
taining dispersed particles is slightly inferior to that of one without particles; the difference in loss of gloss, however, is rather small (Figure 2). Conclusions Capped poly(l2-hydroxystearic acid) reacts with 1,4butanediol diglycidyl ether to produce a dihydroxy oligomer. The oligomer can be successfully reacted with isophorone diisocyanate and hydroxyethyl methacrylate to obtain a new divinyl oligomer. The divinyl oligomer can be copolymerized with methyl methacrylate and hydroxyethyl methacrylate, in the presence of hexamethoxymethyl melamines, to produce stable polymeric dispersions. The dispersions, when included in the base coat of a silver metallic base-coat-clear-coat high-solids paint system, produced coatings with enhanced gloss and distinctness of image. Literature Cited Andrew, M. S.; Backhouse, A. J. U.S.Patent 4242384, 1980. Chattha, M. S. J. Coat. Techno/. 1980, 52(671), 43. Chattha, M. S.;Cassatta, J. C. J. Coat. Techno/. 1983. 55(700), 39. Chattha, M. S.: Cassatta, J. C. J. Coat. Techno/. 1985, 57(730), 41. Chattha, M. S.:van Oene, H. I n d . Eng. Chem. Prod. Res. Dev. 1982, 2 1 , 431. Maklouf, J. M.; Porter, S., Jr. U S . Patent 4 147 688, 1979. Porter, S.,Jr.; McBane, B. N. US. Patent 4025474, 1977. Porter, S.,Jr.; McBane, B. N. U S . Patent 4075 141, 1978. Schoff, C. f r o g . Org. Coat. 1976, 4, 189. Sullivan, T. R.; Christenson, R. M.; Das, S. K.; Dowbenko, R. U.S. Patent 4 055 607, 1977. Theodore, A. N.; Chattha, M. S. J. Coat. Techno/. 1982, 54(693), 77. Theodore, A. N.; Chattha, M. S.J. Coat. Techno/. 1985, 57(721), 67, Waite, F. A. J. Oil Colour Chem. Assoc. 1971, 54, 342.
Received f o r review Seutember 3. 1985 ’ Accepted February 4; 1986
Mechanism of Rare-Earth Catalysis in Coordination Polymerization Henry L. Hsleh and Gene H. C. Yeh’ Philllps Petroleum Company, Bartlesville. Oklahoma 74004
The microstructure and stereospecificity of a variety of 1,3diene polymers prepared with lanthanide coordination catalysts were examined. In contrast to the conventional d-orbhl transition-metal catalysts, the lanthanide catalysts would polymerize 1,3diene monomers prevailingly in a 1+addition manner. The cis and trans 1,Cstereoregular polymerizations depend on the steric and electronic structures of the dienes. Under the same lanthanide catalysts the 1,4-dimethylbutadiene (e.g., trans ,trans -2,4-hexadiene) leads exclusively to trans-I ,4 polymers, the 2,3-dimethylbutadiene gives mainly cis-I ,4 polymers, and the 1,3dimethylbutadene (e.g., trans-2-methyl-l,3-pentadiene) produces mixed translcis (60140) 1,Cpolymers. The ligands in the lanthanide catalyst and the polymerization conditions have relatively little effect on the stereoregulation. I t is proposed that the polymerization mechanism involves the bidentate coordination of the diene monomer through both double bonds in the cisoid conformation, followed by an anti-syn isomerization of the growing allylic unit prior to monomer insertion. The rate of anti-syn isomerization is affected by the structure of the monomer.
Introduction The stereospecific polymerization of l,&dienes, particularly butadiene and isoprene, with transition-metal catalysts has attracted a great deal of attention in the past three decades. The mechanistic aspects of the stereospecific polymerization of dienes is one of the most interesting problems in view of both polymer chemistry and organometallic chemistry. Despite the large amount of work that has been devoted to the diene coordination polymerization with various transition-metal catalysts, the mechanism governing the stereoregulation of diene hom0196-4321/86/1225-0456$01.50/0
opolymerization and copolymerization still remains obscure. Various proposals on the diene polymerization mechanism have been reported in the past without unified agreement. The mechanistic considerations and postulates frequently appeared in the literature, including bidentate (cisoid) or unidentate mode of monomer coordination (Natta et al., 1964; Adman, 1966),R- or u-allylic structure of the growing polymer chain end (Tsutsui et al., 1969; Hughes and Powell, 1972), anti-syn (cis-trans) R-allylic isomerization (Tolman, 1970; Kormer et al., 1969; Dolgo0 1986 American
Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25. No. 3, 1986 457
Table I. Comparison o f 4 f Lanthanide Ions and 3d Transition-Metal Ions transition-metal lanthanide ions ions metal orbitals 4f normal oxidation states +3 ionic radius 1.06-0.85 8, (large size) common coordination 6, 7, 8, 9 (10, 11, 12, etc.) manifold coordination possibilities coordination geometry trigonal prism square antiprism dodecahedron
3d +2, +3, +4, +5, etc. 0.75-0.60 8, 4, 6
square planar tetrahedron octahedron
Table 11. Effect of Halogen Atom and Transition the Cis Content of Polybutadienes cis-1,4, 70 transition metal F c1 Br titanium" 35 75 87 93 98 91 cobalt" nickel" 98 85 80 uraniumb 98 98 lanthanideC 96 96 97
Metal on
I 93 50 10 98 97
"Throckmorton, M. C. Kautsch. Gummi Kunstst. 1969,22, 293. *Tris(*-allyl) uranium halides. Bruzzone, M. et al., Rubber Chem. Technol. 1974,47, 1175. cNdC13-C2HSOH-(C2H5)3Al.
plosk et al., 1973), back-biting coordination (Furukawa, 1975), and bimetallic or monometallic complexes mechanism (Natta and Mazzanti, 1960; Boor, 1967), etc. Some of these mechanistic proposals are confined only to specific transition-metal catalysts and do not agree with all experimental results. The stereospecific polymerization of 1,3-dienes using lanthanide (rare-earth-metal) catalysts extended the conventional d-orbital transition-metal catalysts to f-orbital elements with unique and interesting stereochemical properties (Table I). The lanthanides, which have large size, less electronegativity than d-orbital elements, manifold coordination capabilities, and f-valence orbitals, are a relatively new family of coordination polymerization catalysts in contrast to the d-orbital transition metals (Hsieh and Yeh, 1985; Yeh and Hsieh, 1984). In the stereospecific polymerization of dienes, the lanthanide coordination catalysts are known to be highly stereospecific for producing high-cis polybutadiene and high-cis polyisoprene as well as high-cis copolymerizations of the two monomers (Hsieh and Yeh, 1985; Shen et al., 1980). The distinguished feature of lanthanide catalysts is that the microstructure of diene polymers is relatively unaffected by the nature of the ligands and halides attached to the lanthanide metals, whereas the ligands and halides are significantly important to the d-orbital transition metals (e.g., Ti, Ni, and Co) with regard to the stereoregulation of the polymer (Table 11). It has been found in our laboratory that lanthanide catalysts can actively and efficiently polymerize ethylene, butadiene, isoprene, trans-1,3-pentadiene, trans-1,3-hexadiene, trans-1,3-heptadiene, trans-1,3-octadiene, 2ethylbutadiene, 2,3-dimethylbutadiene, trans,trans-2,4trans-3hexadiene, trans-2-methyl-l,3-~entadiene, methyl-1,3-pentadiene, and 1,3-cyclohexadiene. The substituted diene monomers can generally afford more stereoregular polymers than those of butadiene monomer. The study of these monomers may, therefore, provide much information on the mechanism of the stereoregularity of lanthanide catalyst systems as well as produce
novel polymers with unique and useful properties.
Experimental Section Butadiene and isoprene were purified and dried by distillation through a column packed with activated alumina. Other 1,3-dienes such as 1,3-pentadiene, 2,3-dimethyl-1,3-butadiene, and hexadienes were purified by distillation over calcium hydride under dry nitrogen and stored in a freezer before use. Cyclohexane and toluene were dried by draining them through a column filled with activated alumina against a countercurrent flow of dry nitrogen. The reagent grade anhydrous lanthanide compounds, obtained from ICN-K&K Laboratories, Inc., or Great Western Inorganics, were stored in a desiccator before use and were handled in a glovebox. All air- and moisture-sensitive chemicals were handled either in a glovebox or on the bench top under an inert atmosphere of dry nitrogen or argon. The liquid, reagent grade organic chemicals (alcohols and aromatic chlorohydrocarbons,etc.) were dried over activated alumina or molecular sieves prior to use. Aluminum alkyls diluted in heptane as 10-25 w t % solutions were obtained from Texas Alkyls. The lanthanide catalysts such as MdCl,-nL, where L is an electron donor ligand compound, such as ethyl alcohol, butyl alcohol, or a long-chain alcohol, and n = 1-4 mostly, were prepared from the reaction of anhydrous NdC1, with a calculated quantity of L ligand at the elevated temperature under dry nitrogen in pressure bottles fitted with perforated crown caps over toluene-extracted self-sealing rubber gaskets. The resulting mixtures, which were diluted and dispersed in dry cyclohexane as slurry or suspension, were charged by syringe to the polymerization bottle, followed by the required amount of aluminum alkyl cocatalysts. Polymerizations were normally carried out in capped beverage bottles by adding the solvent and purging with dry nitrogen, monomer, lanthanide catalyst, and aluminum alkyl cocatalyst. The polymerization bottles were tumbled in a constant-temperature bath for the desired time. Polymer solution was terminated by adding two parts of 2,6-di-tert-butyl-4-methylphenol (Ionol) as antioxidant and precipitated in methyl alcohol or isopropyl alcohol. The coagulated polymers were dried overnight at 60 OC in a vacuum oven. For the polymerization kinetic study, the rates of polymerization were determined from the total solids content of small samples withdrawn via a hypodermic syringe at appropriate time intervals. Inherent viscosity was determined in toluene at 25 "C. The microstructure of the homopolymers was determined either by IR spectra or by 'H and I3C NMR. The molecular weight (MW) and molecular weight distribution (MWD) were obtained by GPC analyses. Glass transition temperatures were derived from the dynamic-mechanical spectrum by using a Rheovibron viscoelastometer. Results and Discussion Two types of lanthanide catalysts were used on the basis of their physical appearance: (1)apparently soluble catalysts, which appear to the naked eye to be soluble in a hydrocarbon solvent; and (2) hydrocarbon-insoluble catalysts, which normally would form swollen and well-dispersed suspensions in hydrocarbons. Both hydrocarbonsoluble and -insoluble lanthanide catalysts developed in our laboratory are highly active and efficient catalysts for stereospecific diene polymerization. Apparently soluble catalysts can more fully control the MW of the diene polymers than do hydrocarbon-insoluble catalysts. In order to understand how lanthanide catalysts stereoregulate a polymerization, a variety of 1,3-dienes were
458
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 3, 1986
Figure 1. chain).
0-
and r-allyl-metal bond (M = metal; P
--
polymer
examined. Lanthanide catalysts are active for the stereospecific polymerization of the following types of 1,3dienes: (1) butadiene; (2) 2-alkyl-1,3-butadiene (e.g., isoprene and 2-ethylbutadiene, etc.); (3) 4-alkyl-1,3-butadiene (e.g., trans-1,3-pentadiene, trans-1,3-hexadiene, trans-1,3-heptadiene, trans-1,3-octadiene, etc.); (4) 1,2dimethyl-1,3-butadiene (e.g., trans-3-methyl-l,3-pentadiene); ( 5 ) 1,3-dimethyl-1,3-butadiene(e.g., trans-2methyl-1,3-pentadiene); (6) 1,4-dimethyl-1,3-butadiene (e.g., trans,trans-2,4-hexadiene); (7) 2,3-dimethylbutadiene; and (8) 1,3-cyclohexadiene. On the basis of polymerization studies of these diene monomers with lanthanide catalysts, the following mechanistic considerations are proposed: A. Mode of Monomer Coordination. The general mechanism of coordination polymerization is that the monomer is coordinated (complexed) to the transition metal prior to repeated insertion into the growing chain. In view of lanthanide’s large size, expansive coordination possibilities, and vacant 5d inner shell (i.e., highly available coordination sites) as well as the results of 1,3-dienes polymerization predominantly in a 1,4-addition manner, it is possible that the diene monomer coordinates bidentately with both double bonds in the cisoid conformation to the lanthanide active center and then is incorporated in a 1,4 fashion into the growing chain. The diene monomers with sterically unfavored cisoid conformation such as cis-1,3pentadiene and cis,trans-2,4-hexadiene were not polymerized with lanthanide catalysts. These results suggest that the bidentate coordination of the diene monomer is required to show the polymerization activity with lanthanide catalysts. B. Structure of the Growing Polymer Chain. The essential common characteristic of diene polymerization with transition metals is that the active center of the growing polymer chain is a metal-carbon bond. In general, the last monomer in the growing polymer chain is bonded to the transition metal by an allylic-type bond (Hughes and Powell, 1972). The incoming monomer, which is bound as a a-complex to the transition metal, is then inserted into the transition-metal-allyl bond. That is, the growing polymer chain with either a (r- or a .rr-allylicbond to the transition metal migrates to the coordinated monomer and simultaneously generates a new metal-carbon bond and free coordination sites. For the purpose of better understanding the mechanism of 1,3-diene polymerization with lanthanide catalysts, it is necessary to clarify the structure and properties of the allyl complexes of lanthanide metals. Spectra data indicate that the bonding of organolanthanide complexes containing an allyl ligand consists prevailingly of a a-allyl-metal bond (Tsutsui and Ely, 1975; Tsutsui et al., 1976). However, the steric and electronic factors due to the substituents of the allyl ligand may determine the u- and a-allyl rearrangements (Figure 1). It is believed that the a-allyl-lanthanide bonding is relatively stable and not constrained in view of possible f-orbital participation in bonding coupled with the large size and manifold coordination sites of the lanthanide ion.
anti
wn
Figure 2. Anti and syn .rr-allyl polymer chain (Nd = neodymium; P = polymer chain).
n
an,, * lIl”l
o-lllyll
$y“
1
‘11$
Figure 3. Anti,syn isomerization and C,--C:, bond rotation o t internal o-allyls.
100
1
8ot
i
75
10 20 30 45 AI/FJd M O L A R R A T I O
50
Figure 4. Effects of temperature and Al/Nd ratio on the CIS content of polybutadiene
Two isomeric forms are possible for the r-allyl complex of the growing polymer chain, Le., anti (cis) and syn (trans), which are in equilibrium (Figure 2). The anti-syn allyl isomerization is reported to proceed by rotation about the C2-C3 bond through a-u rearrangements via a metal-C, bonded a-allyl intermediate (Graham and Stephenson, 1977) (Figure 3). The rate of anti-syn isomerization is strongly dependent upon the structural nature of the diene monomer. For example, the polymerization of a 1,4-disubstituted diene, such as trans,trans-2,4-hexadiene, with lanthanide catalysts gives exclusively trans- 1,4 polymers (Yeh and Hsieh, 1984). This is because the presence of the terminal methyl groups at the C1 and C, positions of the growing chain end would cause the rapid isomerization of the initially formed anti allyl unit to the sterically more stable syn species, which leads to trans-1,4 polymers. In addition, the anti-syn isomerization is also affected by the polymerization temperature, the catalyst concentration, and the ratio of organoaluminum to lanthanide. In the polymerization of butadiene with lanthanide systems, the organoaluminum cocatalysts play an important role in stabilizing the allylic active centers and the catalytic activity. The increase in the organoaluminum concentration would give the allyl complex a sufficient lifetime; it will increase the rate of isomerization from the less stable anti form to the thermodynamically more stable syn species. An increase in the butadiene polymerization temperature also shifts the anti- syn equilibrium toward the thermodynamically stable syn species. The above explanations are consistent with the experimental data (Hsieh and Yeh, 1985) (Figure 4). The trans content of butadiene polymers increased at the expense of cis content
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25,
as the polymerization temperature or the Al/Nd ratio increased. C. Termination of Propagation Chain End. The polymerization of dienes with lanthanide coordination catalysts proceeds by a chain mechanism with “living” characteristics. I t has been found that under certain conditions, such as lower Al/Nd ratio, lower polymerization temperature, or the use of R3Al as cocatalyst instead of R3A1H, the propagation chain ends remain “living”, a condition in which transfer of the growing polymer chain and the termination of the active chain are almost absent. The “living” polymer chain in the lanthanide systems can lead to successful block copolymerizationsby incremental addition of monomer (Hsieh and Yeh, 1985). Attempts have also been made to investigate the structure of the growing chain end in the lanthanide catalyst system by terminating or coupling this “living” chain with suitable reagents such as carbon dioxide and radioactive alcohols. It was found by Yang et al. (1982) and by our group that the addition of CO, at the end of polymerization gave a polymer with carboxylate groups, as analyzed by IR spectra. This result suggests that the lanthanide-initiated polymer chain end behaves as a coordinated-anionic polymerization and reacts with COz to form a carboxylate ion. I t was also found that the MW and intrinsic viscosity of the product would increase upon careful quenching with a trace amount of CO,. This indicated that the active polymer chains may react with CO, to form a chain extension resembling the ”living” polymer chain of butyllithium anionic polymerization. Tritiated methanol (CH30T) and 14CH30Hquenching technique further confirmed that the polymerization with lanthanide catalysts proceeded according to the coordination anionic mechanism (Quyang et al., 1983; Shen et al., 1982). The general mechanism of the lanthanide catalyst systems can, therefore, be regarded as coordination anionic polymerization. “Coordination” means the monomer is coordinated to the lanthanide metal prior to insertion by the growing chain. “Anionic” indicates that the chain end behaves as an anion and can initiate or react with a coordinated monomer (an electrophile). Kinetic studies of the rate of several 1,3-diene homopolymerizations and copolymerizations with lanthanide catalysts also suggested that the growing chain shows a characteristic feature of coordination anionic mechanism. The activity order of homopolymerization and copolymerization of 1,3-dienes is as follows: butadiene 2 isoprene > trans-1,3-pentadiene >> dimethyl-substituted butadienes. The electron-donating ability of alkyl groups which makes the double bond of diene monomer more electronrich (i.e., less electrophilic) as well as the steric factor of alkyl groups would slow down the propagation (repeated monomer insertion) to the anionic growing chain. So the polymerization activity of butadiene is significantly higher than those of alkyl-substituted dienes. D. Bimetallic or Monometallic Active Species. The lanthanide (Ln) catalyst systems reported in the literature and in our laboratory always required the presence of an organoaluminum cocatalyst with relatively high ratios of Al-to-Ln concentration (generally, Al/Ln = 10-60). The monometallic organolanthanide compounds such as C5H&nC1, (C5H5= cyclopentadienyl) on their own showed no polymerization activity. It needs to combine with RBAL to form an active catalyst for butadiene polymerization (Yu et al., 1983). Recently, the identification or isolation of a Ln- and Al-containing bimetallic complex active for ethylene and butadiene polymerizations was reported
No. 3, 1986 459
Figure 5. Butadiene polymerizationmechanism (Mt = lanthanide).
(Ballard et al., 1978). These results indicated that the active catalytic species may consist of bimetallic complexes and possibly involves an alkylated lanthanide metal ion. The alkylated lanthanide complex thus formed would initiate the incoming coordination monomer and generate a growing chain with an allyl unit at the chain end. Unfortunately, the simple allyl complexes of lanthanide were extremely unstable and cannot be isolated. Numerous attempts to prepare pure, neutral (allyl),Ln complexes were unsuccessful. The study of (allyl),Ln itself as a polymerization catalyst may further confirm the monometallic and bimetallic active centers of the lanthanide catalysts. Polymerization of Dienes. 1. Polymerization of 1,3-Butadiene. The butadiene polymers obtained with lanthanide catalysts have a high cis-1,4 structure and the cis content can be varied from less than 75% to as high as 99 % . The cis content usually can be increased by reducing the molar ratio of Al/Nd and the polymerization temperature. However, the cis content is not affected by the organic ligands and the halides in the lanthanide system. The 1,4-structures always remain high (total cis and trans contents are greater than 99%) even with changes in polymerization parameters such as temperature, catalyst concentration, and Al/Nd ratio. These results are in accord with the proposed mechanism of the bidentate coordination of diene monomer, followed by the formation of an anti allyl unit in the growing polymer chain end. The initially formed anti allyl unit would lead exclusively to a cis-1,4 polymer if the rate of the propagation step with incoming monomer is faster than the rate of anti (cis) to syn (trans) isomerization (Figure 5). An increase of trans-1,4 units in the butadiene polymerization was observed which was due to the increase in anti-syn isomerization as the polymerization temperature or the Al/Nd ratio was increased (Figure 4). The relatively low 1,2-vinyl content (usually less than 1%) of butadiene polymers indicates that the internal a-allyl-metal bond is significantly unfavored compared to the external a-allyl-metal bond (Figure 1). Faller et al. (1971) also reported that the external a-allyl species would be formed much more often and faster than the internal a-allyl bond. The less stable internal a-allyl species is presumably due to the difference in electronic and steric factors between the secondary carbanion of internal a-allyl and the primary carbanion of external u-allyl species. The 2’ carbanion at the C3 position is less stable and less reactive than the lo carbanion at the C1 position. This accounts for the lower percentage (less than 1%)of 1,2polymerization. 2. Polymerization of Isoprene. The cis content of the polyisoprenes obtained with lanthanide catalysts is normally greater than 94% and up to 99%. The cis contents decrease with the increase in polymerization temperature and organoaluminums. I3C NMR spectra showed that the
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 3, 1986
480
-
/,CH2
Me -C
......... Nd
\',.
C H -Cn2-
HC:.
2 \
2
CHz
/.:
......... Nd
Me-C.:
P
Table 111. Microstructure of Poly(trans-1,3-pentadiene) Preoared with Lanthanide Catalysts insoluble soluble lanthanide lanthanide catalyst catalyst
cis-1,4, % trans-1,4, % trans-1,2, 70 cis-1,2, YO
. . . . . . . .Nd
\. c-cn--P
I MP
4
1
polym temp, 5°C 88 0 12 0
polym temp, 50°C 59 5 33 3
polym temp, 5°C ca. 78 0 ca. 22 0
Me
Me
I P
-
,.3
polym temp, 50°C 80 0 20 0
I
1
CH
CH
Figure 6. Possible r-allyl complexes of polyisoprene growing chain. CH,
CH,
. .
cn,
CH -CHz--^P 3
CH
I
4
5
x
\ CH> -externat o-allyl
anl,.r.allyl
Internal
0
P
allyl
CH?
Figure 7. u-Allyl growing chain of polyisoprene.
cis-1,4 structure consists essentially of head-to-tail linkages with no irregular head-to-head and tail-to-tail linkages. 13C NMR spectra also confirmed the absence of both trans-1,4 and 1,2-addition structures in the polymer. The 3,4-addition with isopropenyl pending groups is usually around 3-5% and appears to be randomly distributed along the polymer chain. In the isoprene polymerization with lanthanide catalysts, the bidentate monomer coordination would lead to four possible allyl complexes in the growing chain end (Figure 6). Since polymers with 3,4-vinyl pending groups (Le., 1,Zaddition) were not observed, this result suggests that polymerization mechanism probably did not occur through the 3 and 4 allyl complexes of Figure 6. 13C NMR monomer sequence dyad analyses of the copolymerization of isoprene with trans-l,&hexadiene by lanthanide catalysts also indicated the absence of structures 3 and 4. In addition, isoprene has a high tendency to give metal s-allyl species of 1 and 2 with a methyl group a t the Cz position rather than the 3 and 4 species with a methyl group at the C3 position, due to the higher stability of the 1 and 2 species. The initially formed anti allyl unit of 1 (cis-polymer) does not have a tendency to isomerize into the 2 species (trans-polymer) due to the steric factor of the methyl group a t the C2 position. So, trans-polymers were not increased even with the increase in the polymerization temperature and the Al/Nd ratio. However, the 3,4-addition (i.e., 1,2-polymerization)increased as the temperature or Al/Nd ratio was increased. In the anti allyl 1 species, the s-allyl can rearrange into either C1 external a-allyl (cis-1,4) or C3 internal 0-allyl (3,4-addition) depending on the nucleophilicity and the steric factor of the C1 and C3 atoms (Figure 7). The reactivity with incoming coordinated monomer a t C3 is lower because C3 is more substituted than C,; therefore, the formation of a 3,4-addition is slower than that of a 1,4-polymerization. This result suggests that the external 0-allyl is significantly more stable than the internal a-allyl and leads predominantly to cis-1,4 polymers. 3. Polymerization of trans - 1,3-Pentadiene and 1Alkylbutadienes. Only the trans isomer of 1,3-pentadiene underwent polymerization with lanthanide coordination catalysts. The cis monomer, which was unfavored
.
. .
CH -CH-P
'
IMe 7 Y
!!
Figure 8. r-Allyl growing chain of poly(trans-1,3-pentadiene).
cisoid conformation via bidentate coordination, was not polymerized. The microstructures of polymers prepared with both hydrocarbon-soluble and hydrocarbon-insoluble lanthanide catalysts at the different temperatures are listed in Table 111. Prolonged polymerization time is necessary to have good conversion at 5 O C or lower temperatures. Polymer characterized by 13C NMR, 'H NMR, X-ray diffraction, and melting point determination showed that the polymers obtained with either insoluble catalyst at 50 "C and lower or soluble catalyst at lower temperature consist of predominantly cis-1,4 isotactic structure (Yeh and Hsieh, 1984; Xie et al., 1979). The trans-1,2 content increased with the increase of polymerization temperature. The amounts of cis-1,2 and trans-l,.i linkages are not detectable or significantly low. The possible modes of s-allyl growing chains of trans1,3-pentadiene in the lanthanide systems are shown in Figure 8. The structures of 7 and 8 species probably do not exist because 13CNMR showed no 3,4-addition polymer. Recently Kormer et al. reported that in the 1,3pentadiene polymerization with Ni-based catalysts the 7 and 8 structures were not detected in the IH NMR spectra (Kormer and Lobach, 1977) only 5 and 6 species are largely predominant. The initially formed anti s-allyl 5 species would lead exclusively to cis-l,.l-polymers. The tacticity of cis-1,4polymer was reported to depend on the chiral orientation (geometric factors) of the incoming monomer with respect to the anti allyl group in the growing chain (Bolognesi et al., 1982). The incoming monomer, which generates a new anti allyl group of the same chirality as the preceding one, will result in cis-l,4-isotactic polymers. The formation of 1,2-polymerization is influenced by steric factors due to the presence of substituents at the allyl group in the growing chain. Both C1and C3 atoms in anti 5 and syn 6 configurations bear a substituent (Me and
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 3, 1986
Table IV. Microstructure of Poly(trans-1,3-hexadiene) Prepared with Lanthanide Catalysts catalyst system (solubility in hvdrocarbon) insoluble soluble Dolvm temD. 50 50 - . "C Cis-i,a, % 56.5 35 trans-1,4, % 14 30 trans-1,2, % 22.5 19 cis-1,2, % 7 16
tranrmnr
cwiani
111.
