Linear Viscoelastic Properties of Putative Cyclic Polymers Synthesized by Reversible Radical Recombination Polymerization (R3P)

Linear viscoelastic properties in both melt and solution states are reported for a series of poly(3,6-dioxa-1,8-octanedithiol) (polyDODT) made by reversible radical recombination polymerization (R3P) under conditions designed to produce linear (LDODT), cyclic (RDODT), and linear–cyclic mixtures (LRDODT). PolyDODT is amorphous (Tg < −50 °C) and highly flexible (entanglement molecular weight Me,lin ≈ 1850 g/mol for LDODT). PolyDODT’s low Tg and low Me,lin enable characterization over a wide dynamic range and a wide range of dimensionless weight-average molecular weight Zw = Mw/Me,lin. Measurements at temperatures from −57 to 100 °C provide up to 18 decades of reduced frequency, which is necessary to characterize RDODT melts with Zw from 23 to 300. The two highest-molecular-weight polymers in the present RDODT series have such high Mw (406k and 556k g/mol) that mass spectrometry, NMR spectroscopy, and even chemical assays for chain ends are unable to rule out up to 2 mol % of linear contaminant. By studying the samples in solution (using dilution to reduce Zw), we could compare their dynamics with those of previously established high-purity polystyrene (PS) rings (limited to Zw ≤ 13.6). RDODT solutions with Zw < 15 (concentrations G* that accord with LCCC-purified PS rings in terms of the frequency dependence (including the absence of a plateau), the progression of shapes of G* as a function of Zw, and the linear scaling of their zero-shear viscosity η0 with Mw. The shape of G* as a function of Zw for solutions of RDODT-406k and -556k also accords with lower Mw RDODT melts (which have ≤1.3 mol % of linear contaminant). Thus, the measurement of the linear viscoelastic properties of appropriate concentrations of high Mw (>200k g/mol) putative cyclic polymers, in which linear chains evade spectroscopic detection, may provide an alternative means (though not fully proven) of validation of sample purity. When Zw > 15 (including all seven RDODT melts and eight of their solutions), G* has a rubbery plateau. This suggests that the onset of entanglement-like behavior in rings requires 4–5-fold greater Zw than is required for linear chains. Further, the plateau moduli of RDODT samples are indistinguishable from GNo of the corresponding LDODT (melt or matched-concentration solutions). In entangled linear polymers, the observation that GNo is independent of Zw follows from limitations on lateral fluctuations due to neighboring chains becoming independent of position along a given chain. The present results for RDODT suggest that this holds for sufficiently long endless chains, too. While the RDODTs have the same GNo as entangled LDODTs, when Zw > 60, the terminal relaxation, if reached at all, of RDODT extends to orders of magnitude lower frequency than an entangled linear polymer of the same Zw. Consequently, the viscosity of RDODT with Zw > 60 increases with Zw much more strongly than the 3.4 power observed for entangled linear polymers. Finally, these novel polymers, with a disulfide-linked backbone and broad relaxation time distribution, may prove important in relation to biodegradable elastomers and materials with exceptional low-frequency dissipation, extending at least 12 decades below the onset of the rubbery plateau