2 CHAPTER TWO
2.3 RESULTS AND DISCUSSION
2.3.6 Preparation of (S)-6 and Polymerizations of Chiral Olefins Using (S)-6
(S)-6 was prepared in an analogous manner to (S)-2, (S)-3, and (S)-4 (Scheme 2.6). The (S)-LiENCp was treated with iPr2CpSiMe2Cl to produce the singly bridged ligand. This product was then deprotonated with 2 equiv of base and doubly linked with SiMe2Cl2. The ligand was refluxed in xylenes with Zr(NMe2)4 to obtain (S)- ENThpZr(NMe2)2. The zirconium amide was treated with TMSCl to obtain the final dichloride precatalyst, (S)-ENThpZrCl2.
MeMe2Si2Si Zr Cl Cl S S
1. [1,3-(CHMe2)2C5H3]SiMe2Cl 2. 2 n-BuLi
3. SiMe2Cl2 4. Zr(NMe2)4 5. 2 SiMe3Cl
(S)-6
"S-ethylneopentyl-ThpZrCl2"
Li+ _
41
Scheme 2.6. Preparation of (S)-6.
When activated with MAO, (S)-6 catalyzes the polymerization of 3-methyl-1- pentene (3M1P), 3,4-dimethyl-1-pentene (34DM1P), and 3,5,5-trimethyl-1-hexene (355TM1H) with moderate to good kinetic resolution. The polymerization set-up and method were similar to those using (S)-2.13 The polymerizations were carried out in tetradecane. The reaction conversion was determined using GC. Once the polymerizations reached 30–60% conversion, all volatiles were vacuum transferred out of the reaction flask. The recovered unreacted olefin was oxidized to the acid, and then to the methyl ester.13 The ee was determined from injection of the methyl ester onto a chiral
GC. The selectivity factor (s) was calculated from the experimentally measured conversion and ee. A discussion of selectivity error as a result of experimental errors can be found in Section 1.3.3. One modification to the procedure is the relative amount of toluene and tetradecane used. For polymerizations using (S)-2, 1.5 mL of tetradecane and 0.5 mL of toluene were used for each polymerization reaction. Subsequent to these experiments, it was observed that a greater amount of toluene increased the reaction rate.
Henceforth, the relative amount of toluene used was increased. For polymerizations using (S)-6, 1.5 mL of toluene and 0.5 mL of tetradecane were used.
In Table 2.4, the polymerization results of (S)-6 are compared to the results of (S)- 2. An increase in activity was observed with all olefins tested, most likely due to the
increase in relative amount of toluene. Although s = 20.5 for 34DM1P is the highest s value obtained for this monomer with any catalyst, the value is still within the margin of experimental error. In the region of high s value, small changes in conversion and ee make significant changes in s values. Using the values obtained with (S)-2 (%c = 42.4,
%ee = 58.6), a one percent drop in conversion would raise the s value from 15.9 to 20.5.
One percent error in conversion is experimentally feasible. Hence, it is difficult to conclude if the increase in s value for 34DM1P is a real increase or not. Although the increase in s value for 3M1P (2.4 to 3.2) does not appear to be a significant increase, it can be considered experimentally significant. In order for the s value to increase from 2.4 to 3.2, the conversion has to drop 7% or the ee has to increase 7%. It is unlikely that an experimental error that great was made. The most significant increase in selectivity was observed with 355TM1H (2.1 to 8.5).
Table 2.4. Kinetic resolution of chiral 3-methyl substituted olefins using (S)-6.
47
34
37
• Zr •Cl Cl
• Zr •Cl Cl
TOF (h-1) s TOF (h-1) s
2.4
15.9
2.1
3.2
20.5
8.5 280
75
988
The insignificant increase in s of 34DM1P suggests that the monomer insertion rate is on the same order as the rate of site epimerization, meaning that most of the insertions were already taking place on the right side of the wedge. Hence, a further increase in site epimerization rate does not significantly change the insertion pattern of 34DM1P.
On the other hand, the increase in s for 3M1P and 355TM1H suggests that the rate of insertion for these monomers is faster than the rate of site epimerization and that the rate of site epimerization of (S)-6 is faster than that of (S)-2. Hence, the monomer encounters the right side of the catalyst more often with (S)-6 than it does with (S)-2.
Enchainment on the right side of the catalyst favors the S monomer, while enchainment on the left side of the catalyst favors the R monomer. One can envision the polymerization of 3M1P and 355TM1H to entail consecutive insertions of the S monomer on the right side of the catalyst with occasional misinsertions of the R monomer on the
left side of the catalyst. The increase in s value suggests that increase in site epimerization rate has reduced the opportunity for misinsertion on the left side of the catalyst.
The relative sizes of 3M1P and 355TM1H may be a factor as to why 355TM1H experienced a greater increase in s value than 3M1P. It is proposed that the discriminating interaction between the chiral wedge of the catalyst and the larger 355TM1H monomer is greater than that between 3M1P and the chiral metallocene wedge. Also, frequent misinsertions on the left side of the wedge acts as a counteracting force that reduce the s value. Once the counteracting effect is mitigated as a result of increasing the site epimerization rate and reducing the frequency of misinsertions, the discriminating interaction between the catalyst and 355TM1H is realized to a greater extent, as seen by an increase in s. 3M1P, due to its smaller size, does not interact with the catalyst to the degree of 355TM1H. Hence, even when the counteracting effect of misinsertions is reduced, the s value does not increase to the level of 355TM1H.