Macrocycles 25. Cyclic poly(ether sulfone)s derived from 4-tert-butylcatechol
Introduction
It has been demonstrated in previous publications [1], [2], [3] that in kinetically controlled polycondensations cyclization reactions compete with propagation steps at any concentration and at any stage of the polycondensation process. The efficiency of cyclization depends on the flexibility of the polymer chain and on the concentration of active species and limits the chain growth as outlined in Eq. (1). Another aspect of this new theory of polycondensation is that the fraction of cycles rapidly increases at very high conversion together with the generation of high molecular weights. is the average degree of polymerization, p, is the conversion of the functional groups, x, is the constant >1.0 allowing for the adjustment of Eq. (1) to various concentrations, α=Vcg/Vpr, is the ratio of cyclization rate and rate of propagation.
The detection of cyclic polymers by MALDI-TOF mass spectrometry was limited in most previous studies to masses around 10 000–13 000 Da [1], [2], [3]. The technical reasons responsible for the limited detection of high molar mass polymers have been discussed by several authors [4], [5]. An additional problem arises from the frequency distribution of individual oligomers and polymers in polycondensates. Regardless to what extent cyclization takes place, the oligomers form the maximum of the distributions curve which decreases exponentially towards higher molar masses. The large fraction of oligomers saturates the detector before the slowly flying higher masses arrive. Therefore, fractionation of the polycondensate may be beneficial for the detection of individual reaction products with masses above 13 000 Da.
In this context the present work served the following purposes. First, poly(ether sulfone)s should be prepared from silylated 4-tert-butylcatechol (TBC) and 4,4′-difluorodiphenyl-sulfone (DFDPS) (Eq. (2)) with the expectation that the cyclization tendency is higher than in that of the previously studied [3] polycondensations of (silylated) bisphenol-A. Second, the new poly(ether sulfone)s should be soluble in tetrahydrofuran (THF) allowing for a characterization of the molar mass distribution and for a fractionation by SEC. Third, it should be elucidated to what extent the fractionation favors the detection of high-molar mass cycles by MALDI-TOF mass spectrometry. Fourth, it should be checked, if polycondensations of silylated diphenols with DFDPS in NMP are the clean step growth polymerizations suggested by our previous experiments with silylated bisphenol-A. Finally, all syntheses and measurements together should contribute to the confirmation of our polycondensation/cyclization theory [1], [2], [3].
Section snippets
Materials
4-tert-Butylcatechol (TBC), fluorobenzene and chlorosulfonic acid were purchased from Aldrich Co. (Milwaukee, WI, USA) and used as received. 4,4′-Difluorodiphenylsulfone (DFDPS) was prepared according to the literature [6] and recrystallized (mp 98–100 °C, Aldrich gives a mp of 98–99 °C for a product of 99% purity). N-Methylpyrrolidone (NMP) was a gift of BASF AG (Ludwigshafen, Germany). It was twice distilled over P4O10 in vacuo prior to use. K2CO3 (p.a. grade, Merck KG, Darmstadt, Germany)
Syntheses
In a previous study of synthesis and cyclization of poly(ether sulfone)s based on bisphenol-A [3] the highest molecular weights combined with the highest fraction of cycles was obtained by polycondensation of the silylated bisphenol in NMP at 140–145 °C. Therefore, these reaction conditions were also used in the present work. The feed ratio of the monomers were varied from an excess of TBC to an excess of DFDPS for two reasons. First, a sample containing a maximum of cycles should be prepared
Conclusion
The results of this work allow the following conclusions. The polycondensation of silylated TBC with DFDPS in NMP/K2CO3 is in agreement with previous studies [3] a rather clean kinetically controlled step growth polymerization. However, the MWDs of samples prepared with nearly ideal stoichiometry show a tendency towards a bimodal mass distribution with high polydispersities which differ from the classical theory of step growth polymerizations. However, the MALDI-TOF m.s. do not show any bimodal
Acknowledgements
We wish to thank Dr Just (BAM, Berlin) for SEC/light-scattering measurements and Dr C.L. Schultz (Bayer AG, Uerdingen) for the fractionation of sample No. 5, Table 2.
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