Chapter II Opening the Way to Very Long Telechelic Polymers
2.4 Discussion
2.4.2 Molecular-Weight Control
with the resultant carboxyl groups on polymer chain ends after deprotection. Therefore, the end-association of polymers in this study is primarily controlled by the number of carboxyl groups on chain ends.
its poor solubility in common organic solvents, as mentioned earlier in Section 2.1, renders it an inappropriate backbone for telechelic associative polymers as fuel additives.56 Therefore, the presence of pathways in ROMP allowing growing polymer chains to lose molecular weight renders molecular-weight control of telechelic polymers comparatively complicated, especially when target molecular weight is > 200 kg/mol (which will be discussed later).
Previous studies on syntheses of telechelic polymers via ROMP of COD or COE in conjunction with cis-acylic olefin CTAs using Grubbs II were focused only on varying the ratio of monomer to CTA ([M]:[CTA]) to achieve desired molecular weights (provided the amount of Grubbs II << CTA), and a linear relationship between Mn and [M]:[CTA] has been reported up to a 2000:1 [M]:[CTA] ratio by Ji and coworkers.10,38,42,55 However, the influences of other key aspects of ROMP (e.g., backbiting, effective turnover number (TON) of Grubbs II, and catalyst load) on molecular-weight control, were not addressed in these reports. As indicated by Dinger and Mol, ruthenium-based metathesis catalysts can only perform a finite number of metathesis reactions with substrates, i.e., they have finite effective turnover numbers (TONs).57 In a separate report, Dinger and Mol also pointed out that impurities such as primary alcohols, water and oxygen can cause Grubbs catalysts to degrade.40 Nickel and coworkers further reported that the presence of peroxides in substrates for metathesis reactions has a dramatic effect on TONs of Grubbs catalysts, and removal of peroxides from substrates can lead to a 80-fold increase in effective TONs, allowing metathesis reactions to take place readily with extremely low catalyst load (on molar ppm level).39 As mentioned earlier, the protocol developed by the Macosko group to remove VCH
from COD does not address the issue of peroxides, and it also inevitably introduces n- butanol into the monomer (Section 2.2.2). Therefore, we expect that the purity of COD, similar to the [M]:[CTA] ratio, has a crucial role in molecular-weight control in the synthesis of telechelic 1,4-PBs via ROMP of COD, since it directly affects the effective TONs of Grubbs II, which subsequently determines the proceeding of both primary and secondary metatheses.
The results of polymer synthesis in Table 2.2 and Appendix A illustrate the effects of the purity of COD on ROMP using Grubbs II. When VCH-free COD purified according to the Macosko protocol was used, we found that the conditions of entry 4 in Table 2.2 led to a conversion of COD of 85%, a Mw of 264 kg/mol (PDI = 1.58), and a cis/trans ratio of 2.20 (see Appendix A). It is worth noting that the same conditions gave very similar results to Ji and coworkers’ report, despite the fact that a different CTA was used.38 In the case of the rigorously purified, VCH-free COD (see Section 2.2.2), the same conditions resulted in a nearly quantitative conversion of COD (97.6%), a lower Mw
of 142 kg/mol, and a lower cis/trans ratio of 1.73 (entry 4 in Table 2.2). Similar nearly- quantitative conversions of COD and cis/trans ratios <2 were also achieved (entries 2 and 6 in Table 2.2) as rigorously purified COD was vacuum-distilled again immediately prior to use. These results suggest that removal of n-butanol and peroxides from VCH-free COD boosts the effective TON of Grubbs II, and consequently a conversion of COD ≥ 95% can be achieved. In addition, the results of cis/trans ratios and Mw reveal that as the majority of rigorously purified COD (>90%) was consumed, ruthenium benzylidene units on polymer chain ends could still possess sufficient activity to participate metathesis reactions thanks to the boosted effective TON of Grubbs II. Since the abundant species in
this circumstance were formed 1,4-PB backbones, they could undergo metathesis reaction with chain-end ruthenium benzylidene units, leading to the occurrence of backbiting and the subsequent decreases in cis/trans ratio and Mw. In other words, a delicate balance among factors that can affect the molecular weight of telechelic 1,4-PBs (e.g., [M]:[CTA], the amount of Grubbs II, reaction time, etc.) is needed for molecular-weight control as rigorously purified COD is used as the monomer. As for the control (i.e., n-butanol- and peroxides-containing VCH-free COD), Grubbs II may have lost its metathetical activity as the conversion of COD reached ~85% due to the influences of n-butanol and peroxides on the number of metathesis reactions it could perform; consequently, the loss of Mw due to backbiting was not as significant, and the aforementioned linear relationship between Mw and [M]:[CTA] still apparently held.
Although the use of rigorously purified COD seems to complicate the molecular- weight control when a [M]:[CTA] of 2000:1 is used, it provides invaluable access to telechelic 1,4-PBs with Mw useful in mist-control applications (entries 7 and 8 in Table 2.2). The implication of the results from rigorously purified COD at a 2000:1 [M]:[CTA]
ratio is that when a very large excess of such monomer is used to suppress secondary metatheses that cause decrease in molecular weight, Grubbs II would be able to continuously react with COD until it reaches its maximum TON, leading the formation of telechelic chains with Mw >> 200 kg/mol. Therefore, we tested a [M]:[CTA] of 10,000:1 using monomers purified according to the Macosko protocol and the protocol of rigorous purification of COD we developed in Section 2.2.2, respectively. We found that the monomer purified according to the Macosko protocol contained sufficient amount of n- butanol and peroxides to completely quench the metathetical activity of the ruthenium
benzylidene units on chain ends of macro CTAs (Scheme 2.2), leading no formation of any high-molecular-weight polymer chains. On the contrary, the second-stage addition of 10,000 eq of rigorously purified COD and required amount of deoxygenated DCM (Scheme 2.2) led to a rapid increase in viscosity of the reaction mixture, which is a sign of the rapid formation of telechelic chains with very high molecular weights. Relatively low conversions of COD (< 65%) and high cis/trans ratios (≥ 3) were observed in both entries 7 and 8 in Table 2.2, suggesting that the extent of backbiting, which lowers the cis content of 1,4-PB backbone, was insignificant due to the presence of large excess of monomer. Fluctuation in reaction rate (in terms of the ratio of conversion of COD to reaction time) was observed between entries 7 and 8, nevertheless the values of Mw were found virtually the same. We believe that the aforementioned fluctuation may come from variation in the level of trace impurities (e.g., oxygen) introduced into the reaction mixture during syringe transfer of monomer, solvent and catalyst solution, and we expect a better consistency when the polymer synthesis is carried out at a larger scale. The success observed in entries 7 and 8 also implies that an even higher Mw might be achieved if a higher [M]:[CTA], say 20,000:1, is used.
Another intriguing property COD concerning molecular-weight control in the synthesis of telechelic 1,4-PBs via ROMP of COD is that it needs to be freshly vacuum- distilled immediately prior to use, even after all impurities capable of interfering with ROMP are removed. We found that COD underwent side-reactions which generated wax- like substances on a time scale ~ 1 day even when stored at -30oC, and the byproduct could interfere with ROMP as well. Examples are entries 1, 3, and 5 in Table 2.2, in which rigorously purified COD stored at -30oC for 1 day or longer after vacuum-
distillation was used. Conversion, cis/trans ratio and Mw data suggest a lower effective TON of Grubbs II compared to the cases in which freshly vacuum-distilled, rigorously purified COD was used. The composition of the wax-like byproducts and the mechanism accounting for their formation remain unclear, and currently investigation is underway.