Chapter II Opening the Way to Very Long Telechelic Polymers
2.4 Discussion
2.4.1 Aspects of the CTA Design
polymer chain ends, the apparent Mw of polymer decreased from 375 kg/mol (PDI = 1.71) to 221 kg/mol (PDI = 1.66).
During the early stage of the present study, we used a small amount (ca. 15 wt%
of polymer) of BHT in TFA hydrolysis of polymer end groups for the purpose of protecting polymer backbones from crosslinking by residual dissolved oxygen (in case of incomplete deoxygenation of reaction mixtures). We were surprised by the unexpected results: The combination of BHT and TFA crosslinked 1,4-PB backbones. Figure 2.9 shows the GPC results of TFA hydrolysis of telechelic 1,4-PB of Mw = 143 kg/mol with 2 tert-butyl ester groups at each chain end (referred to as 143K di-DE 1,4-PB hereinafter) with and without the use of BHT. When 15 wt% of BHT was used in combination with TFA (1.0 mL for 1 g of polymer), Mw of the polymer increased from 143 to 329 kg/mol, whereas the use of TFA only (1.5 mL for 1 g of polymer) resulted in a slightly increased Mw =183 kg/mol. Consequently, all of the carboxyl-terminated telechelic polymers used in the present study, regardless of the number of acid groups and the backbone length, were prepared by treating the corresponding tert-butyl ester prepolymers with TFA in DCM only.
The CTAs employed in ROMP are required to be symmetric, acyclic olefins with terminal functional groups compatible with the catalysts.46 Most CTAs reported in the literature are based on cis-2-butenediol derivatives for the following reasons: (1) they are commercially available and inexpensive; (2) the modification with desired functional groups is straightforwardl; (3) the incorporation of CTAs is not an issue when small terminal groups are used.5,7,27,47-49 These advantages are the reasons why we used cis-1,4- dichloro-2-butene as the core olefin unit to construct the entire family of bis-dendritic CTAs in the present study. We found that even though the spacer between the C=C double bond and the dendrons was a short methylene oxide unit (-CH2-O-), all four CTAs we prepared were able to be completely incorporated into polymers during the first stage of ROMP of COD (Scheme 2.2). We originally expected that when the bulky CTA 10 was used, we would have observed the presence of unreacted 10 in the end of the first stage of ROMP. To our surprise, complete consumption of 10 was still observed, which suggests that the central olefin unit is sufficiently accessible to Grubbs II. We believe the comparatively easy access to the core olefinic unit of 10 is attributed to the cascade structure of the third-generation poly(benzyl ether) dendron, which leaves the core unhindered (Scheme 2.1).
The influence of steric hindrance from bulky dendrons on the reactivity of the central C=C double bond in ROMP can be reduced by having a long spacer between the C=C double bond and bulky terminal groups, as suggested by Higley and co-workers.46 In their study a nonylalkyl spacer was used. In a similar fashion, Sill and Emrick used a succinic ester linkage between the central cis-2-butene unit and the third-generation poly(benzyl ether) dendrons.42 The problem is, however, that none of these functional
olefins are commercially available. Given that CTA 10, which was synthesized from an inexpensive reagent cis-1,4-dichloro-2-butene, could be completely incorporated into polymers in two-stage ROMP of COD, the gain in reactivity of CTAs in ROMP from using a long spacer seems marginal. The extra cost and effort required to synthesize and purify starting olefins with long spacers render this approach comparatively less cost- effective, and inevitably counterbalance the benefit of higher reactivity of CTA provided by the use of long spacers.
In the literature, two distinct synthetic methodologies have been reported for the preparation of dendrimers: (i) The divergent approach, in which construction of dendrimers starts at the core and proceeds radially outward toward the dendrimer periphery, and (ii) the convergent approach, in which peripheral dendrons are built first and then attached to a suitable core to afford a complete dendrimer.50 We chose the divergent approach because it is better suited to large-scale synthesis.51 As shown in Scheme 2.1, the divergent synthesis of the n+1th (n = 1,2) generation dendrimer in our work began with LAH reduction of terminal ester groups of the nth generation. Then, the resultant alcohols were coupled with 2 or 2’ to form the n+1th generation dendrimer via Mitsunobu reaction. Alternatively, halogenation of bis-dendritic alcohols 6 and 9 followed by Williamson etherification with 2 under basic conditions may be used to synthesize CTAs 8 and 10. The problem is, however, that Williamson etherification is not as a clean chemistry as Mitsunobu reaction. When Williamson etherification is used, oxygen-alkylation of the phenolate anion of 2 with the nth generation dendritic halide affords the n+1th generation dendrimer, however the same phenolate anion of 2 can also undergo a resonance transformation and then undergo an ortho-carbon-alkylation (~1.5%
of oxygen-alkylation) with the nth generation dendritic halide as well,52,53 resulting in the formation of polymeric byproducts. Consequently, the issue of over-alkylation compromises the precise structure control of polymer chain ends. On the contrary, Mitsunobu etherification only gives negligible amount of carbon-alkylation when reactive alcohols (such as 6 and 9) are used.52,54 In our parallel study of synthesis of CTA 8 using both routes, by means of GPC analysis we found that Mitsunobu coupling of 6 with 2 afforded a monodisperse dendrimer, whereas Williamson etherification of the chloride of 6 with 2 gave a mixture of 8 and polymeric impurities that could not be removed by reprecipitation (Figure 2.3). Similar success of Mitsunobu reaction was also observed in the synthesis of CTA 10 (Figure 2.3). Therefore, direct etherification of 6 and 9 with 2 via Mitsunobu reaction is a better synthetic route for CTAs 8 and 10.
Among the numerous types of dendrons reported in the literature, poly(benzyl ether) dendrons were chosen in the present study to construct the family of CTAs due to the following reasons: (i) The hyper-branched poly(benzyl ether) skeleton is highly hydrophobic, but still soluble in common organic solvents, which renders the resultant tert-butyl ester terminated CTAs soluble in solvents for ROMP of COD; (ii) poly(benzyl ether) dendrons are solid at room temperature even in the case of first generation. This property gives a significant advantage in purification: We found that CTAs prepared in this study could be purified by either recrystallization (CTAs 3 and 5) or reprecipitation (CTAs 8 and 10) to achieve reasonable purities without resorting to chromatographic separations, which is crucial for production on a large scale. (iii) The poly(benzyl ether) skeleton does not contain any moiety that interferes self-association of carboxyl groups.
Neither the ether linkages nor the phenyl rings are capable of forming hydrogen bonds
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.