Cyclic Macromolecules: Dynamics and Nonlinear Rheology, Final Report DOE Award # DE-FG02-05ER46218, Texas Tech University

Abstract

Introduction: The work described in the present report had the original goal to produce large, entangled, ring polymers that were uncontaminated by linear chains and to characterize by rheological methods the dynamics of these rings. While the work fell short of this specific goal, the outcomes of the research performed under support from this grant provided novel macromolecular synthesis methods, new separation methods for ring and linear chains, and novel rheological data on bottle brush polymers, wedge polymers and dendron-based ring molecules. The grant funded a total of 8 archival manuscripts and one patent, all of which are attached to the present report. In the following paragraphs we highlight the major accomplishments. Polymer Synthesis Cyclic polymer topologies remain a synthetic challenge for polymer chemistry, and a region of untapped potential for material engineering. The cyclic macromolecule synthesis related to the present grant was based on a robust platform for cyclic polymer synthesis that had several perceived advantages. 1,2 These derive from the cyclic nature of all monomers, catalysts and intermediates that participate, leading to a ring-expansion metathesis polymerization (REMP). The technology utilizes ruthenium-based olefin metathesis catalysts with bidentate ligands bearing N-heterocyclic carbene (NHC) and alkylidene moieties that are based on commercially available ruthenium catalysts (Figure 1).Two series of REMP catalysts were synthesized and characterized. 3 The first class contains N-mesityl, Nalkyl imidazol-2-ylidene ligands (denoted UC-#; the number indicates the length of the alkylidene chain), while the second class contains the dihydro analogs (SC-#). The alkylidene-linked chains are varied from four to seven carbons atoms long (Figure 2). The polymerization kinetics for these catalysts were also investigated, with the aim to determine the factors that influence catalyst activity and molecular weight control (Figure 3). 4 The model that we have developed involves four steps (Figure 3, depicted with a generic REMP catalyst and cyclooctene monomer). As with all living polymerizations, the process contains an initiation step and a propagation step (controlled by rate constants k i and k p , respectively). In an initiation kinetics study, the relative observed initiation rates were found to fall in the order SC-5>3>UC-5>SC-6>UC-6. In general, it appears that linker length is the dominant variable in initiation, where the five-carbon linker promotes faster initiation than the six-carbon linker. The propagation rates were found to fall in the order SC-6>UC-7>SC-5>>UC-6>UC-5>>UC-4. Here, the electronic characteristics of the NHC ligand