Article pubs.acs.org/Macromolecules
Precision Polyketones by Ring-Opening Metathesis Polymerization: Effects of Regular and Irregular Ketone Spacing Kyle J. Arrington, Clifton B. Murray, Emily C. Smith, Hervé Marand,* and John B. Matson* Department of Chemistry and Macromolecules Innovation Institute, Virginia Tech, Blacksburg, Virginia 24061, United States S Supporting Information *
ABSTRACT: The synthesis and characterization of regioregular aliphatic polyketones is reported. Poly(1-oxoheptamethylene), a semicrystalline polyketone, was prepared via ruthenium-catalyzed ring-opening metathesis polymerization (ROMP) of a ketal-protected 7-membered cyclic ketone followed by subsequent hydrogenation and deprotection. Temperature and catalyst studies of the ROMP reaction guided the preparation of polyketones with high monomer conversions, molecular weights as high as 30 kDa, and dispersities as low as 1.4. Because of the symmetric nature of the monomer, the polymer has ketones spaced every six methylene units apart. The thermal properties of this polyketone were investigated by differential scanning calorimetry, revealing a peak melting range of 160−165 °C. A related polymer, poly(1-oxooctamethylene), was also prepared in a similar fashion, and a peak melting range of only 130−133 °C was observed. This difference in melting range is attributed to the lack of the regioregularity in poly(1-oxooctamethylene), which was derived from an asymmetric 8-membered ring monomer and has ketones spaced every 6, 7, or 8 methylene units apart.
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INTRODUCTION Crystallization plays a considerable role in polymer applications because the degree of crystallinity affects thermal, mechanical, optical, and other pertinent properties.1 The crystallizability of a particular polymer depends on both the primary polymer structure (chain rigidity, persistence length, degree of branching, etc.) and the magnitude of the secondary attractive forces among the polymer chains. Polyethylene (PE) is one of the largest volume commodity polymers and remains one of the simplest and best-studied linear semicrystalline polymers. Even though PE has weak intermolecular forces, it readily crystallizes due to its linear topology and chain flexibility.2 PE’s performance is controlled industrially by changing the mode of polymerization, catalyst type, pressure, and temperature. These reaction conditions alter the amount and distribution of short and long chain branches, which affect crystallinity and melting temperature and allow for control over tensile strength, stiffness, and processability.3 Functionalized PE derivatives have been prepared to study the effects that different functional groups exert on crystallization and physical properties. The conventional method to develop new PE derivatives is acyclic diene metathesis polymerization (ADMET) followed by postpolymerization hydrogenation. ADMET is a step-growth polymerization technique that relies on olefin cross-metathesis and affords perfectly linear polymers.4 It has been used to build model systems to study correlations between a polymer’s semicrystalline morphology and its associated physical properties. Initial studies using Mo-based catalysts explored the precise placement of specific alkyl groups along a PE backbone.5 More © XXXX American Chemical Society
recently, ruthenium metathesis catalysts have allowed for the placement of many different polar functionalities on a PE backbone, including amides,6 carboxylic acids,7,8 phosphonic acids,9 sulfonates,10 sulfites,11 halides,12 and others.13 Ringopening metathesis polymerization (ROMP) of strained cyclic olefins is a chain-growth alternative to ADMET that relies on ring strain to drive the reaction rather than removal of ethylene. By utilizing symmetrical or unsymmetrical monomers, regioregular or regioirregular linear polymers can be prepared by ROMP to produce PE analogues.14−20 We became interested in aliphatic polyketones as analogues to polyethylene because of their similarity to poly(εcaprolactone) (PCL), which we have studied previously.21,22 Additionally, polyketones have interesting properties themselves and have recently gained attention.23,24 Originally prepared as perfectly alternating copolymers from ethylene and carbon monoxide at high pressures with a metal catalyst, polyketones were first studied as a method to use carbon monoxide as a cheap comonomer in the development of new ethylene−carbon monoxide (E−CO) copolymers.25 Commercially available polyketones are random copolymers typically synthesized with
