Znd. Eng. Chem. Res. 1990,29,1443-1454
1443
Synthesis, Characterization, and Kinetics of Functionalized Polybutadiene Using a Homogeneous Rhodium Hydroformylation Catalyst Patrick L. Mills* Department of Chemical Engineering, Washington University, 1 Brookings Drive, Box 1198, St. Louis, Missouri 63130-4899
Samuel J. Tremont and Edward E. Remsen Central Research Laboratories, Monsanto Company, 800 North Lindbergh Boulevard, Q3B, St. Louis, Missouri 63167
The liquid-phase hydroformylation of a commercially available low molecular weight polybutadiene whose microstructure consists of 12 wt % 1,2-polybutadiene and 88 wt % cis/truns-1,4-polybutadiene using Wilkinson's homogeneous rhodium catalyst with excess triphenylphosphine is examined. Through the use of both 13C and 'H NMR, it is shown that this catalyst system results in a polymer product whose olefin units are selectively converted to the corresponding internal and terminal branched aldehydes with negligible formation of hydrogenation products. Gel permeation chromatography reveals that the polybutadiene molecular weight distribution, which is initially unimodal, evolves into a bimodal distribution during the formation of hydroformylated products functionalized with 10-80 wt % aldehyde. By taking the ratio of molecular weight distributions that are based upon aldehyde specific and total polymer mass detectors, the addition of aldehyde to the polybutadiene is shown to be uniform over the entire range of polymer molecular weight. A reaction kinetic study leads to a power law type rate form whose parameters are comparable to those reported in the literature for the hydroformylation of olefins using rhodium or cobalt homogeneous catalysts. The conversion of olefins to aldehydes using homogeneous hydroformylation catalysts is an important reaction that has applications ranging from laboratory-scale synthesis to commercial-scaleproduction of selected organic chemicals, pharmaceuticals, and other related products (Cornils, 1980; Davidson, 1984; Gates et al., 1979; Masters, 1980; Parshall, 1980). A particularly well-known example is the conversion of straight or branched chain terminal olefins to the corresponding terminal or iso-branched aldehyde products using homogeneous cobalt- or rhodiumbased catalysts. The overall reaction can be described by the following stoichiometry: RCH=CHZ
olefin
+ CO + HZ
+
RCHZCHZCHO
terminal
aldehyde
+ RCH(CHB)CH3 iso-branched aldehyde
(1)
Typically, the terminal aldehyde in eq 1is the preferred product since it can be used in subsequent conversion reactions to produce a variety of derivatives, such as alcohols, acids, esters, ethers, and amines. However, a limited number of applications exist where the iso-branched aldehyde is the desired product. Additional details on both the fundamental and more practical aspects of hydroformylation chemistry and processes are provided in the starting references cited above. Most of the previous literature on hydroformylation chemistry and processes involves reactions between small molecules where the number of carbon atoms in the reactant rarely exceeds 10-16, with 3-6 carbon atoms being more typical. While olefins or species having olefinic structures are the most commonly used carbon source in commercial processes, numerous examples exist where hydroformylation is used to synthesize specialty chemicals from non-olefinic substrates (Cornils, 1980). Thus, hydroformylation chemistry and processes represents an important area in the selective conversion of many different
* Author
to whom correspondence should be addressed. 0888-5885/90/2629-1443$02.50/0
types of small organic molecules to specific reaction products having an aldehyde functionality. Although hydroformylation technology for small organic molecules has been the subject of a significant amount of fundamental research and commercial application, an analogous knowledge base does not exist for higher molecular weight molecules and polymers. One of the first practical applications of hydroformylation chemistry to long-chain substrates was proposed by Robinson (1966) in which homogeneous cobalt catalyst precursors were used to convert C18-Czoconjugated diolefins present in certain petroleum refining streams to the corresponding aldehydes. Application of hydroformylationto polymers was reported by Ramp and co-workers (1966) in pioneering work, who examined the hydroformylation of 1,4-polybutadiene (1,4-PBD),styrene-butadiene rubber, and other polyolefin copolymers using the same catalyst. Subsequent conversion of the hydroformylated products to functionalized polymers containing oximes, alcohols, acetals, and other derivatives was also investigated. A few years later, Cull and Mertzweiller (1968) described the use of cobalt carbonylphosphine ligand catalyst precursors for hydroformylation of selected polyolefins and polyolefin copolymers with subsequent hydrogenation to the hydroxymethylated products. Applications in this latter effort were primarily directed toward preparation of novel coatings where one of the components consisted of either the hydroformylated or hydroxymethylated product (Cull, 1967; Cull and Mertzweiller, 1967; Mertzweiller et al., 1968; Yuhas et al., 1967). The preparation of composite materials containing hydroxymethylated polybutadiene functionalized with a C,-C, polycarboxylic acid anhydride has also been described (Mertzweiller et al., 1969). A common feature of the above applications is that homogeneous cobalt complexes were employed as catalysts for both hydroformylation and hydrogenation of the POlyolefin to either the polyaldehyde or hydroxymethylated product. To obtain reasonable reaction rates and reaction times in batch reactors, temperatures and pressures be0 1990 American Chemical Society
1444 Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990
tween 422 and 505 K (300-450 O F ) and 10342 and 13 790 kPa (1500-2000 psi) were necessary. These elevated conditions introduced the undesired effect of promoting spontaneous gelation and cross-linking through an aldol condensation type mechanism once the aldehyde concentration in the polymer product exceeded a certain critical value (Ramp et al., 1966). While the addition of gelation inhibitors or reagents which depolymerize polyaldehydes is a reasonable method for reducing gelation and reversing cross-linking in laboratory-scale synthesis, this has disadvantages from a commercial processing perspective since it introduces additional separation steps, and an alternate approach is preferred. Homogeneous rhodium catalyst complexes provide an alternative to cobalt catalysts since they exhibit greater intrinsic activity for olefin hydroformylation a t lower temperatures, pressures, and substrate concentrations (Cornils, 1980). When applied to polyolefin polymers, they have potential for eliminating gelation and cross-linking of the hydroformylated product (Sanui et al., 1974). In the presence of excess phosphines, rhodium phosphine ligand type catalysts, such as HRh(CO)(PPh,),, also have the additional advantage of reducing the isomerization of the olefin bond. Double-bond migration and isomerization increases the ratio of n-aldehyde to isoaldehyde products and is a well-known characteristic of cobalt catalyst complexes (Brown and Wilkinson, 1970). The first application of a rhodium phosphine system to the hydroformylation of polyolefin polymers was reported by Sanui et al. (19741, who functionalized a selected polypentenamer with up to 20 mol 7% aldehyde and used the resulting aldehyde polymer to prepare nitrile derivatives. Azuma et al. (1980) followed this same approach for the hydroformylation of cis-l,4-polybutadiene and polypentenamer and also synthesized various hydrogenation products. The effect of the degree of functionalization on selected basic polymer properties, such as the glass transition temperature and melting points, was also studied. Rempel and co-workers have studied the selective functionalization of various polybutadienes and assorted polyolefin copolymers using rhodium phosphine complexes to yield both the hydroformylated and hydrogenated polymer products (Mohammadi and Rempel, 1987,1988, 1989). Unlike some of the previous studies cited above involving polyolefins, characterization of the polymer products was performed in more detail using IR, NMR, and GPC as part of an effort to obtain a better understanding of the reaction chemistry and polymer microstructure. Kinetic studies of the homogeneous-hydrogenation of 1,2-polybutadiene (1,2-PBD) having M, = 10000 using RhCl(PPh& catalyst precursor in toluene solvent showed that the reaction rate was first order with respect to the total rhodium metal concentration and the olefin concentration, was inversely proportional to the excess triphenylphosphine concentration, and exhibited Michaelis-Menten type rate behavior with respect to hydrogen (Mohammadi and Rempel, 1989). These same characteristics were also observed for an acrylonitrilebutadiene copolymer in an earlier study (Mohammadi and Rempel, 1987). The results were interpreted by using a catalytic cycle involving the formation of various rhodium catalyst complexes. Previous results (Hedrick and Gabbert, 1981, 1982; Gabbert et al., 1982) showed that a new class of polymer materials based upon a copolymer of nylon 6 and low molecular weight polyols could be prepared by using a reaction injection molding (RIM) technique. The key chemical steps are shown in Figure 1 for reference. The
X HD-----OH
+ X
+
FC-N-C-R-C-N-C I R ! !
I
POLYOL 0
'
7-y-R-C
-----)
BIS-ACYLLACTAM A
0
NH(CH ,+-C--O------O-C(CH
R
R
POLYETHER
I,.\
AX
POLYESTERAMILIE PREPOLYMER
L
POLYAMIDE
i ,
J
CAPROLACTAM
POLVETHER
T 7 Figure 1. Chemistry of the Nylon block polymerization in a RIM application showing the incorporation of the hydroxymethylated polybutadiene.
physical properties of the RIM product were sensitive to the poly01 properties, such as the fraction of hydroxy groups, degree of unsaturation, molecular weight distribution, and molecular weight averages. The preferred polyols were the ones derived from commercially available mixed 1,2- and 1,4-polybutadienes in which a specified fraction of the olefinic structures were converted to the hydroxymethylated product without introducing saturation. This requires that both the hydroformylation of the mixed polybutadienes and the subsequent hydrogenation of the polyaldehyde products be conducted under carefully selected oxo conditions so that a detailed understanding of the kinetics and polymer microstructure is essential. One objective of this work is to examine the hydroformylation reaction kinetics of a commercially available mixed polybutadiene system using a Wilkinson's type rhodium triphenylphosphine ligand catalyst in the presence of excess triphenylphosphine. Emphasis is placed here upon the development of a suitable hydroformylation reaction rate expression that accounts for the effect of key kinetic variables, such as olefin and catalyst concentrations, and reaction temperature. Another objective is to examine the microstructure of the hydroformylated reaction products by identifying the type of functional groups and the resulting molecular weight distribution of the functionalized polymer. A final objective is to compare the results where possible to literature results on polymer modification and olefin hydroformylation to determine the similarities and differences between the two systems.
Experimental Section Apparatus. All reaction experiments were performed in a standard 0.3-L Hastelloy C high-pressure reactor obtained from Autoclave Engineers. A diagram of the reactor and other key components is shown in Figure 2. The entire system was located within a special-purpose cell having explosion proof walls at the Monsanto High Pressure Experimental Facility. All valves were mounted inside the cell and operated through the use of reach-rods, while other control functions were manipulated by using remote electronic controls or air operators. The reactor was operated as a well-stirred batch liquid with a constant gas pressure being maintained in the head space. The gas, which typically consisted of a 1:l CO:H2 blend obtained from standard cylinders, was supplied to a reactor from a reservoir (SR in Figure 2) through a series of block valves and a Tescom dome-loaded downstream pressure regulator. The volume of the reservoir and associated lines leading up to the regulator was determined to be 190.94 cm3by the standard technique of gas pressure equilibration from a reference volume. An accurate value of this volume was used in subsequent data reduction calculations to
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1445
PRESS
,
.or0 2
5
Fr.
RR
ADD
RR RD PT AA MV
Reach Rod Valva
S or SV
Soleoold Valva
Ruplurr D l t c Pretrurr Transducer
Alr Actualed Valvr Manual Valvr
300ML AUTOCLAVE
Figure 2. Stirred batch reactor system used for the polybutadiene hydroformylation reaction kinetic experiments.
evaluate the instantaneous value of the global reaction rate and the cumulative gas uptake. The gas pressures in the reactor head space and small reservoir were measured by pressure transducers (Validyne) having absolute accuracies of f34.4 kPa at 3442 kPa (500 psi) and f344.2 kPa at 34418 kPa (5000 psi). The reactor was heated by a standard tubular furnace supplied with the autoclave with temperature control being provided by a PID (Love) controller. The temperatures of the outer reactor wall and batch liquid were measured by Type T thermocouples, with the latter being maintained within f 2 K of the desired set point over the range 353-383 K. Cooling of the liquid batch upon completion of an experiment was performed by directing tap water to the internal autoclave coils. During the course of a typical kinetic experiment, the temperatures and pressures of the small reservoir and reactor were sampled a t a fixed time interval and stored by using a custom-designed microprocessor-based data acquisition system. Upon termination of an experiment, the stored data were uploaded through an appropriate protocol converter to a VAX 8600 host computer for subsequent data reduction and plotting. A typical set of raw experimental data is given in Figure 3. Materials. The polybutadiene used in this study was a mixed polymer whose microstructure consisted of 10-15 wt % 1,2-polybutadiene, 50-60 wt % trans-1,4-polybutadiene, 25-35 w t 70cis-l,4-polybutadiene, and residual (10.5 wt %) toluene as stated by the manufacturer. It was obtained from Revertex, Ltd., and is commercially produced by the anionic polymerization of butadiene monomer using organolithium catalysts in toluene reaction
solvent. For simplicity, it is referred to here as LX-16. Analysis by gel permeation chromatography (described below) yielded number- and weight-average molecular weights of 5200 and 7200, respectively, for a polydispersity index of 1.38. Quantitative 13C NMR analysis yielded a microstructure of ca. 12 wt 70 1,2-polybutadiene and 88 w t % (balance) cis- and trans-l,4-polybutadiene,which agreed with the manufacturer’s specifications. The hydroformylation catalyst precursor, hydridocarbonyltris(triphenylphosphine)rhodium(I), HRhCO(PPh3)3,was obtained from Strem Chemicals. Excess triphenylphosphine ligand, which was added to the reaction mixture to achieve a predetermined PPh3/Rh molar ratio for reduction of the olefin isomerization and to eliminate hydrogenation activity, was obtained from Aldrich. Purities of the catalyst precursor and ligand were typically greater than 99.5 wt %. Toluene was used as the reaction solvent and used as received from commercial sources with a purity in excess of 99 mol 70. Methanol was found to be an effective solvent for separation of the crude hydroformylated product from the reaction mixture via precipitation and was used as received from commercial sources. Carbon monoxide and hydrogen were obtained as a blended gas in a standard cylinder where the nominal composition of each component was 50 mol %. The actual composition was determined by using a Carle gas analyzer and found to be 49.14% CO, 49.6% Hz, and 1.26% (balance) Nz, 02, CHI, and HzO. cis-3-Octene and trans-3-octene were used as model compounds to mimic the hydroformylation of a typical olefin monomer in the mixed polybutadiene. They were
1446 Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990
rt;
=
L
2
102.
44
96t
140
.
0 0
a"
g
~
Solvent Reservoir -
I
_ _ i
24
Reactor
c (
m
Sample
i 56
114
78,
72 1
a
66 i 0
200
400
600
800
1000
1200
1400
Elapsed Time, min
326
Figure 4. Apparatus used for determination of the molecular weight distributions of the polybutadiene and hydroformylated products.
1
'z P a-
m
cu
a
.-
-12200 01
322
l
318 3141 310[
Reactor
a
12120
g 7
11960
a
0
286:
Reservoir I
282: 278
-
0
200
400
600
800
1000
1200
primarily toluene, methanol, triphenylphosphine, and the rhodium catalyst. By decanting the solution and adding additional toluene until the polymer became completely soluble again, the above precipitation process was repeated several times. This was found to be necessary to reduce the catalyst and excess triphenylphosphine concentrations in the polymer to less than 1-5 ppm as determined by elemental analysis using atomic absorption. For applications where a hydroformylation product having a residual quantity of solvent was needed, vacuum evaporation at 1 Torr and T = 313 K (40 C) was found to be effective for reducing the solvent content to less than ca. 0.5 wt 7'0.
1400
Elapsed Time, min
Figure 3. Typical pressure and temperature data collected during the course of a polybutadiene hydroformylation kinetics experiment. (a, top) Reactor and reservoir pressures versus reaction time. (b, bottom). Reactor and reservoir temperatures versus reaction time. Reaction conditions: C , = 1.809 mol/L, ,C , = 3.54 X lo4 mol/L, PPh3/Rh (molar) = 165/1, P = 2066 kPa (gauge), CO:H2 = 1:1, T = 360 K,solvent is toluene.
obtained from Aldrich with a stated minimum purity of 99 mol % and used as received with no further treatment. Kinetic Study Procedure. In a typical kinetic study, a premixed solution containing the toluene reaction solvent, the mixed polybutadiene, the rhodium catalyst precursor, and excess triphenylphosphine was charged from a sealed glass bottle to the autoclave in the presence of an argon gas purge. The autoclave head was then installed and subjected to several purge and vent cycles at 345 kPa total pressure of CO/H2 to displace air and saturate the reaction mixture with the gas. The reactor was then heated up to the desired temperature and allowed to stabilize. The stirring was momentarily stopped, the reactor pressure was quickly increased to the desired value by using the Tescom regulator, and both the stirring and data acquisition were initiated corresponding to t = 0. Previous test experiments had shown that a negligible amount of reaction had occurred during the reactor heating period. After a predetermined amount of blend gas was consumed, corresponding to an overall degree of olefin conversion, the reaction was terminated by directing cooling water to the internal autoclave coils. An aliquot of the crude hydroformylated reaction product was collected, recovered as explained below, and analyzed by GPC, 13C NMR, and 'H NMR. Polymer Product Recovery. The polymer product was recovered from the crude reaction mixture by precipitation using an approximately equal volume of methanol. The net result was a two-phase mixture containing a solid polymer as one phase and a liquid phase containing
Analytical Section Both the mixed polybutadiene and the hydroformylated products were analyzed by using NMR spectroscopy and gel permeation chromatography (GPC). During the initial stages, IR spectroscopy was also used for qualitative characterization. NMR Spectroscopy. A Varian 300-MHz Fourier transform instrument was routinely used to obtain both the 'H and 13C NMR spectrums of the polymer reactants and products. The sample concentration was 4-5 w t % in deuterochloroform (CDCI,) as the solvent. The instrumental operating parameters were optimized through a series of preliminary tuning type experiments. Assignment of suspected unknown peaks was obtained by comparisons to known model olefin and aldehyde monomer species. GPC Analysis. The polymer molecular weight distributions were determined by using a size exclusion chromatography system assembled from commerciallyavailable components. A diagram of the system is given in Figure 4. It consisted of a solvent reservoir, a high-pressure injection valve (Rheodyne), a solvent pump (Waters Associates Model 590), three styrene-divinylbenzene columns connected in series (Polymer Laboratories), a low-angle laser-light-scattering (LALLS) detector (LDC/Milton-Roy Model CMX-loo), a differential refractive index (DRI) detector (Waters Associates Model R 410),and an ultraviolet (UV) detector (Kratos Model 757). For the range of polymer molecular weights encountered here, mean column packing permeabilities of 1O00,100, and 50 A were found to yield satisfactory results. Prior to performing any chromatographic measurements, the system was allowed to stabilize with UV-grade tetrahydrofuran (THF) a t a flow rate of 1 mL/min. Polymer concentrations from 4 to 6 mg/mL in a solvent mixture containing 95 w t % THF and 5 w t % methanol were found to yield optimal column loadings. Methanol was used to
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1447
Internal 4C=HC
INERNAL
- CH=CH /
Branched -CH.
BRANCHED
,CHO
BRANCHED
1-
C,HO
li
,
Branched zCH2
I
I
Figure 6. I3C NMR spectrum of the LX-16 polybutadiene reactant in CDCl,. Major peaks: (S) branched sp2 C 142.6 ppm; (S)intemal sp2 C 129.4 ppm; (S) branched sp2 C 114.25 ppm; (S)sp3 C 32.68 ppm; (S) sp3 C 27.39 ppm.
IS0
INTERNAL
1
I
\
INTERNAL
LK 10
9
8
7
6
5
4
3
2
1
OPPM
Figure 5. 'H NMR spectra of the LX-16 polybutadiene polymer reactant (a) and a typical hydroformylated product (b). Reaction conditions: C, = 8.05 mol/L, C, = 4.402 X lo4 mol/L, PPh3/Rh (molar) = 163/1, T = 353 K, P = 2066 kPa (gauge), CO:H2 = 1:1, solvent is toluene. Major peak assignments: CHO singlets 9.4,9.5, and 9.67 ppm; sp2 CH singlet 5 3 3 ppm; sp3 CH singlet 2.35 ppm.
inhibit the dimerization of the aldehyde groups present in the functionalized polymer based upon the findings of Ramp et al. (1966). Samples were typically heated to 318 K (40 "C) for 1 h prior to injection to ensure solution homogeneity. A sample injection volume of 100 pL was used. Calibration of the system was performed by using a series of monodisperse polystyrene standards having molecular weights in the 2000-20 000 range. The output voltages of all three detectors were connected to a three-channel analog-to-digital converter that was interfaced to a dedicated microcomputer. A special-niirnose comnutm m o m a m W A R routinelv used t o
calculations, such as the number-, weight-, and z-average molecular weight averages. A more detailed description of these calculations is given in Appendix A.
Results and Discussion Hydroformylation Product NMR Analysis. Figure 5 gives the 'H NMR spectra for the LX-16 polybutadiene polymer reactant and a typical hydroformylated reaction product. Details related to the experimental conditions are given in the figure caption, along with the major peak assignments. Both spectra show that paraffinic protons yield peaks in the 1-2.4 ppm range, while the peaks associated with the olefinic protons occur in the 4.7-5.6 ppm range. Protons in the internal olefin corresponding to the CH=CH group of the cis/trans-l,4-polybutadienesyield a large singlet at 5.33 ppm, while protons associated with the CH=CH2 group in the 1,2-polybutadiene yield multiplets in the 4.7-5.0 ppm range. Figure 5b shows that the magnitude of the singlet a t 5.33 ppm assigned to the CH=CH group is reduced when compared to that shown in Figure 5a, since it has been partially converted to the internal aldehyde product. Similarly, the peak associated with the CH==CH2 group is present in Figure 5a but absent in Figure 5b since it has been completely converted to either the branched or iso-branched aldehyde product. Peaks associated with the internal, iso-branched, and
.
204.4 ppm
220
I
3
/j
Internal CHO
Sec-branched
t i Internal -CH=CK
Terminal CHO 202.4 ppm
200
180
160
140
120
100
Figure 7. NMR spectra of the LX-16polybutadiene reactant (a, top) and a typical hydroformylated product (b, bottom) in the olefin and aldehyde carbon region. Major peaks: singlet, secondary branched CHO 204.6 ppm; singlet, internal CHO 204.4 ppm; singlet, terminal branched CHO 202.4 ppm; (S) branched sp2 C 142.6 ppm; (S) internal sp2 C 129.99 ppm; (S) internal sp2 C 129.4 ppm; (S) branched sp2 C 114.25 ppm. Reaction conditions: same as in Figure 5.
branched aldehydes are observed in Figure 5b at 9.4,9.5, and 9.67 ppm, respectively. Figure 6 shows the 13C NMR spectrum of the LX-16 polybutadiene reactant. This provides additional information on the polymer microstructure and a basis for later comparison to the 13C NMR spectrum of the hydroformylated product. The 22-44 ppm range is associated with the carbons on the CH2group, while the 110-150 ppm range corresponds to the olefinic carbons on the terminal =CH, methylene and =CH methine groups, as well as the internal CH=CH group. The assignments of the various key peaks are given in the figure caption and are in excellent agreement with model monomeric olefins. Quantitative 13CNMR of the peaks associated with the intemaland terminal-branched carbons showed that the polymer consisted of ca. 88 w t 70cis- and trans-r,4-polybutadiene
1448 Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 Table I. Model Aldehydes for 13C NMR Assignments of the LX-16Polybutadiene Hydroformylation Products CHO 13C NMR hydroformylation compd shift, ppm model type butyraldehyde 201.9 terminal branched 2-methylvaleraldehyde 204.6, 13.3 (CH3) secondary branched 2-ethylhexanal 204.4 (no CH3) internal
and 12 wt ?% 1,2-polybutadiene. Figure 7 compares the 13C NMR spectra of LX-16 polybutadiene reactant and the hydroformylated product in the 100-220 ppm range corresponding to the olefin and aldehyde carbon region. The hydroformylation reaction conditions are identical with those given earlier in Figure 5 and yield an overall olefin conversion of 32% to the terminal and branched aldehyde products. This latter result was based upon the overall CO:H2gas uptake (Appendix B) and was verified within 0.15% by quantitative 'H NMR. Inspection of Figure 7 and the peak assignments given in the caption show that the terminal branched and secondary aldehydes yield 13C NMR shifts a t 202.4 and 204.6 ppm, respectively, while the internal aldehyde produces a 13C NMR shift at 204.4 ppm. 13C NMR spectra of hydroformylated polymers having overall aldehyde concentrations in the 5-80 wt ?% range resulted in 13C NMR shifts that differed by no more than 0.18 ppm from the above values. Verification of the 13C NMR shifts cited above was performed by comparison with selected model compounds. Table I lists the 13C NMR shifts in the aldehyde region for butyraldehyde, 2-methylvaleraldehyde, and 2-ethylhexanal, which are models for terminal-branched, secondary or iso-branched, and internal hydroformylation reaction products. As explained earlier by reference to Figure 7b, the amount of iso-branched polybutadiene hydroformylation product was negligible compared to the terminal-branched and internal products for the high PPh3/Rh molar ratio (ca. 160-165) used here. Earlier experiments at PPh3/Rh molar ratios in the 1-10 range resulted in greater iso/ (terminal + internal) aldehyde ratios, so this model was useful in discriminating between these products. The results in Table I show that the 13C NMR shift of 201.9 ppm for the butyraldehyde-branched aldehyde model is in good agreement with the value of 202.4 ppm cited above for the terminal-branched polybutadiene hydroformylation product. Similarly, the 204.4 ppm 13CNMR shift for the internal aldehyde is in perfect agreement with the 204.4 ppm value obtained for the corresponding internally branched polybutadiene hydroformylation product. The same statement applies to the iso-branched model for the polymer products also. Additional inspection of Figure 7b shows a multiplet at ca. 100 ppm that was identified as an acetal group. As explained by Ramp et al. (1966), acetals can be formed when the aldehyde groups on the polymer react with certain reagents that might be added to inhibit the hydroformylation reaction and to prevent or minimize gelation due to aldol condensation. Methanol, which was used as a solvent to precipitate the hydroformylated polymer as described above, was demonstrated by Ramp et al. (1966) to produce acetal a t a slow rate. Hence, the presence of the minor acetal peak in the 13C NMR product spectrum shown here is speculated to be due to the interaction of methanol and aldehyde during the polymer recovery process. Hydroformylation Reaction Pathway. The above results obtained from both 'H and 13C NMR analysis of the LX- 16 polybutadiene reactant and the hydroformylated product provide the basis for inferring the
reaction pathways. It is apparent from the NMR results that the 1,2-polybutadiene chains lead to a terminal aldehyde as the primary product and an iso-branched aldehyde as the secondary product. The cis/trans-1,4polybutadiene chains yield an internal aldehyde across the olefinic bond. Saturation of either the 1,2- or 1,Colefinic bonds through direct homogeneous hydrogenation was either absent or so small that it could not be detected for the reaction conditions used in this study. On the basis of these results, the following reactions are most likely occurring on an arbitrary 1,2- or 1,4-polybutadiene chain unit whose indices are m and n, respectively.
+
fCH&H(CH=CHZ)+j 1,2-PBD unit CO
k
+CHzCH(CH(CHO)CHJh (2)
+ Hz
k3
11 k-3
+CHZCH(CHZCH2CHO)+,,, (3) +CHzCH=CHCHz+m 1,4-PBD unit
+ CO + Hz
+
fCH2CHzCH(CHO)CHz+,,,
(4)
A mixed polybutadiene system, such as LX-16, actually consists of a distribution of polymer molecular weights whose individual chains contain a random ordering of the 1,2 and 1,4 monomeric olefin structures. The reactions given by eqs 2-4 represent the hydroformylation of a given chain member within this distribution. A more precise description of the reaction would consider the molecular transformation of the LX-16 reactant distribution to a corresponding product distribution. This lies outside the scope of this work, however, and the above method of describing the hydroformylation reaction will be used for purposes of this work. Confirmation that the reactions given by eqs 2-4 occur as written is provided in a related study (Tremont et al., 1990). In this study, hydroformylation of syndiotactic 1,2-polybutadiene and cis-1,4-polybutadiene was performed in separate experiments using the same reaction conditions that were used for the mixed LX-16 polybutadiene system. It was shown that hydroformylation of a syndiotactic 1,2-polybutadiene results in the exclusive formation of the terminal-branched aldehyde as the primary product and the iso-branched aldehyde as the secondary product, as shown by eqs 2 and 3. Similarly, hydroformylation of a cis-1,4-polybutadiene produced the internally branched aldehyde as shown by eq 4. One notable difference was that the initial reaction rate for hydroformylation of the syndiotactic 1,2-polybutadiene was ca. 6 times greater than the one for hydroformylation of cis-1,4-polybutadiene. This result correlates with the reaction rate data for the hydroformylation of terminal and internal olefins as summarized in Table 1.32 of Cornils ( 1980). The 'H NMR given in Figure 7b shows that the hydroformylated polybutadiene product contains a negligible amount of iso-branched aldehyde in comparison with the terminal-branched aldehyde. As shown by eqs 2 and 3, these products are apparently formed from the 1,2-polybutadiene units in the LX- 16 via series-parallel reactions. In addition, no hydrogenation products are detected. Brown and Wilkinson (1970) have shown that the addition of excess triphenylphosphine ligand in olefin hydroformylation using a HRhCO(PPh3)3precursor increases the n-aldehyde to isoaldehyde product ratio, reduces or eliminates olefin hydrogenation and isomerization, and inhibits dimerization reactions. As mentioned above, a PPh3/Rh molar ratio of ca. 165 was used here in an attempt to achieve similar results. The NMR results show that the mechanistic pathways that lead to high n-aldehyde
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1449 70
I$ b
g
A
140
f
50
CHO CHO CHO CHO
20
37.8 wt% CHO
120
a 0 40
g
10 O19
20 19
20
21
22
23
24
25
26
27
28
29
30
31
lime, min
lime, min
Figure 8. Gel permeation chromatograms of LX-16 mixed polybutadiene and various hydroformylation products based upon the differential refractive index (DRI) detector. Reaction conditions: C , = 8.05 mol/L, C,, = 4.402 X lo-' mol/L, PPh3/Rh (molar) = 163/1, T = 535 K, P = 2066 kPa (gauge), CO:H2 = 1:1, solvent is toluene.
Figure 9. Gel permeation chromatograms of LX-16 mixed polybutadiene and various hydroformylation products based upon the ultraviolet (UV) detector. Reaction conditions and reaction times are the same as in Figure 8.
Table 11. Molecular Weight Averages for LX-16and Various Hydroformyfated ProductsoVb polymer 10-3~, 10-3Mw 10-3Mz p = MJM,, LX-16 5.2 7.2 8.8 1.385 10.1 14.4 1.263 10 wt % CHO 8.0 16.7 89.4 1.704 37.8 wt % CHO 9.8 78.3 w t % CHO 21.8 103.3 762.8 4.739 "Reaction conditions: C , = 8.05 mol/L, C,, = 4.402 X lo4 mol/L, T = 353 K, P = 2066 kPa of 1:l CO/H2, PPh3/Rh (molar) = 163. bAll values are the average of three trials except for LX-16, which is the average of two trials.
to isoaldehyde product ratios in olefin hydroformylation must also be operative in polybutadiene hydroformylation as well. The primary reaction pathways would then appear to be given by eqs 3 and 4 with eq 2 being a secondary pathway. Hydroformylation Product GPC Analysis. The molecular weight distributions of LX-16 mixed polybutadiene and selected hydroformylated products having various degrees of aldehyde functionalization were characterized by gel permeation chromatographyto gain insight into the polymer microstructure. Figure 8 compares GPC chromatograms obtained from the differential refractive index detector for the LX-16 polybutadiene and three hydroformylated polymer products having various increasing degrees of aldehyde functionalization. The particular chromatograms shown here are based on polybutadiene in which 10,37.8, and 78.3 wt % of the original terminal-branched and internal olefin units were converted to the corresponding aldehydes. These particular percentages were determined from both quantitative 'H NMR of the hydroformylated polymers and from calculations based upon the overall CO/H2 gas uptake data measurements. The absolute differences between the two methods were 1.7% at the 78.3 wt % level and ranged between 0.1% and 0.9% at the remaining levels. The percentages cited here are based upon the 'H NMR results. The corresponding molecular weight averages derived from the chromatograms are compared in Table 11. The chromatograms and the results in Table I1 show that the LX-16 polybutadiene has a relatively narrow unimodal molecular weight distribution with a polydispersity of about 1.4. Addition of aldehyde to the polybutadiene, even at the 10 wt 9% level, results in bimodal distributions that show a shift toward smaller elution times, corresponding to increasing molecular weight. The shift in molecular weight from the low molecular weight components to higher molecular weight components is consistent with the conversion of an olefinic (-C4H8)- unit to an aldehyde (-C5H,,O)- unit with a subsequent mo-
lecular weight increase of 30 g/mol per chain unit. Inspection of Table I1 shows that the various molecular averages do not increase linearly with respect to the percentages of olefin units converted to aldehydes. This nonlinear behavior, when viewed with the bimodal molecular weight distributions, suggest that the polymers have either undergone a certain degree of cross-linking or are associated in solution to form aggregates of individual polymer molecules. Neither the chromatograms in Figure 8 nor the derived molecular weight averages in Table I1 permit the precise reason to be determined, although some reasonable speculation is possible, which is given below. It is well-known in oxo hydroformylation processes for olefin conversion that the aldehyde product can undergo self-aldolization to form condensation products, especially in basic media (Cornils, 1980). Addition of excess triphenylphosphine for control of product selectivity also increases the basicity of the reaction solution and can promote the formation of condensation products. Since reaction rates for aldol condensation reactions have been successfully described in at least one case (Weiss et al., 1977) by a reaction rate form that is first order in the aldehyde and second order overall, the rate of condensation would be more pronounced with increasing aldehyde concentration. The chromatograms shown in Figure 8 show that the large narrow peaks corresponding to the low molecular weight components undergo a significant decrease with increased aldehyde functionalization with a corresponding increase in the high molecular weight components. This behavior is consistent with what might be expected for aldol condensation reactions where the reactants and products have a distribution of molecular weights. It is interesting to note that, in a few preliminary experiments, addition of a nucleophile, such as methanol, to the polymer would result in a polymer having a unimodal distribution. This is consistent with the previously cited work of Ramp et al. (1966),who found that methanol and other similar compounds were effective depolymerization reagents for polyaldehydes. GPC analysis of the LX-16 and the hydroformylated products using the ultraviolet absorption detector provides additional insight into the polymer microstructure. At a wavelength of 310 nm, the UV detector exhibited maximum sensitivity to the aldehyde functionality on the polymer and negligible sensitivity to the olefinic structure. Figure 9 shows the GPC chromatograms obtained when the LX-16 polybutadiene and hydroformylated products shown above in Figure 8 were analyzed by using the UV detector and the resulting raw detector output voltages were normalized for mass differences. These show that the LX-16 polybutadiene polymer without any aldehyde functionality results in negligible response, but the remaining functionalized polymers yield responses whose
1450 Ind. Eng. Chem. Res., Vol. 29, No. 7 , 1990
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24
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30
31
Elution Time, min E
f
60 50
L
z: 4o
r
0 Q
K
Table 111. Experimental Variables Used in t h e LX-16 Polymer Hydroformylation Kinetics Experiments parameter conditions LX-16 Lithene (12 wt % substrate 1,2-PBD, 88 wt % 1,4-PBD, = 5200 g/mol) solvent toluene catalyst HRhCO(PPh& with excess PPh3 PPh,/Rh, M (160-165)/1 substrate concn, mol/L 1.605-7.054 catalyst concn, mol/L (3.53-56.7) X lo4 temp, K 350-390 partial pressure of CO + H2, kPa 2172 (315 psia), CO:HZ = 1:l 2.5
= 4.0 x 1 0 ' molesrliter C,,, C,, 0.55 moleelliter PPh,:Rh E 16511 T -361 C P = 21.41 atm CO'H, 1'1 R,,.R,, 5.11
30 r
L
p
20 i-
Y
L
27
28
29
30
31
Elution Time, min Figure 10. Comparison of GPC data obtained from the DRI and UV detectors after applying mass and delay time corrections. (a, top) LX-16 with 10 wt % olefin functionalization. (b, bottom) LX-16 with 80 wt % olefin functionalization. Reaction conditions are the same as in Figure 8.
areas increase with increasing aldehyde addition. These chromatograms are indicative of the molecular weight distribution of aldehyde groups as compared to the previous chromatograms based upon the DRI detector, which correspond to the total polymer. Molecular weight distributions of the aldehyde relative to that of the total polymer are compared in Figure 10 for LX-16 polybutadienes with 10 and 80 w t % olefin functionalization. Before this could be performed, however, it was necessary to account for the differences in the sample transport time between the DRI and UV detectors and to normalize each chromatogram to unit area. The results show that the responses of both detectors produce chromatograms that overlap nearly perfectly over the indicated range of elution times. If the aldehyde addition was biased to a particular fraction of the molecular weight distribution corresponding to a range of chain lengths, then perfect overlap would not be obtained. This result suggests that the coordination of the active rhodium catalyst species with the olefin units along the polybutadiene chains, at least for the specific conditions examined here, does not depend upon the chain length to any noticeable degree. Hence, the mechanism of polybutadiene hydroformylation can be viewed as randomly occurring on a given polymer unit in a chain containing n units. One can envision, however, a situation where the configuration of the polymer in solution may interfere in the coordination of the active forms of the rhodium catalyst with the olefinic units in the polymer. The resulting polymer may not be uniformly functionalized over the entire range of molecular weight. Experimental evidence to support this situation has not yet been set forth, however. Reaction Kinetics. The hydroformylation kinetics of the LX-16 polybutadiene were investigated to identify the key differences or similarities between previous literature results involving the hydroformylation of olefin monomer substrates. The experimental variables selected for study included the concentrations of the homogeneous catalyst HRhCO(PPh3)3and the LX-16 polybutadiene and the reaction temperature. The effects of total gas pressure,
0.01" 0
'
I
200
"
"
400
"
"
600
"
"
"
800
'
'
'
1000
'
1200
1400
Batch Reaction Time, min.
Figure 11. Comparison of reaction rate versus time data for hydroformylation of syndiotactic 1,2-polybutadiene (solid line) and cis-1,4-polybutadiene (dashed line) in toluene.
CO to H2 molar ratio, and triphenylphosphine concentration were not examined but were maintained at constant values which had been previously determined to be reasonable for use in a pilot-plant or commercial-scale facility. A summary of the experimental variables used is given in Table 111. In a typical sequence of experiments, the effect of a selected variable (e.g., initial polymer concentration) on the rate was examined while the remaining ones were kept constant. Preliminary experiments were performed in which the stirring speed was varied to ensure that the gas-liquid mass-transfer resistance was negligible in comparison with the intrinsic kinetic resistance (Ramachandran and Chaudhari, 1983). For this purpose, the upper limits on the polymer and catalyst concentration as well as the temperature were selected corresponding to where the intrinsic rate was expected to be a maximum. No detectable effect of stirring speed on the initial rate over the range from 500 to 2500 rpm was detected, so subsequent experiments were conducted at 1000 rpm. Before the kinetic experiments using the LX-16 polybutadiene were conducted, a few runs were performed using commercially available syndiotactic 1,2-p0lybutadiene and a cis-1,4-polybutadiene in separarate experiments. These are described in detail elsewhere (Tremont et al., 1990), but are analogous in concept to those given here for the LX-16. Figure 11compares the reaction rate versus time data for these two substrates where the rates have been derived from the concentration versus time data given in Figure 10 of the above-cited paper. The details of the data reduction procedure are described in Appendix B. In both cases, the same reaction conditions were used. The results show that the initial hydro-
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1451
- 5.4
- 6.15
n
c., = 3.13 x 10" moksfliter 1:l
-5.6 E: 0
= 1.518
C,
t
/ I
L
/
COIH,
C,, = 1.605
-
-6.25 I
log
-5.7
Rp(t=O) -6.30 C$(t=O)
t
/
- 6.35 R, = k, , :C C c = 0.078 k, = 3.092 x l o 3
