Synthesis of New Homopolymers and Block Copolymers
Of course, none of our studies would be possible without specially-synthesized materials. In many cases, these materials can be synthesized by well-established methods, such as “living” anionic polymerization of styrene and dienes followed by catalytic hydrogenation. But in other cases, we need to develop synthetic methods appropriate to our new macromolecular targets. Some examples are given below.
Block copolymers containing linear (high-density) polyethylene blocks have been of interest for at least 40 years, and many investigators have pursued this elusive goal. While polybutadiene obtained by anionic polymerization yields a moderately-crystalline material (30-40% by weight) with a melting point of 100-110°C, linear polyethylene exhibits a crystallinity closer to 80%, melting at 135°C. We have shown that narrow-distribution linear polyethylene can be obtained by ring-opening methathesis polymerization (ROMP) of cyclopentene, followed by catalytic hydrogenation. Well-defined diblock copolymers containing a linear polyethylene block can also be synthesized by this method, using (for example) ethylidene norbornene as the other block; poly(ethylidene norbornene) yields an amorphous rubber after hydrogenation. The synthetic route to these materials is schematized below:
Another ROMP-derived high-crystallinity polymer is hydrogenated polynorbornene (hPN)… despite the fact that hPN is atactic! Norbornene is the canonical monomer for ROMP, and so is easily combined with a wide variety of other monomers which lead to either glassy, rubbery, or crystalline blocks, providing us with the full complement of material structures and properties, with previously unstudied building blocks. We have elucidated the room-temperature crystal structure of hPN, as shown in the figure below. Interestingly, hPN undergoes a transition to a rotationally-disordered crystal structure some 30oC below its melting point; this rotationally-disordered crystal shows essentially no resistance to plastic deformation (near-zero yield stress), which should facilitate solid-state shaping operations.
Right: x-ray diffraction pattern of a drawn fiber of hydrogenated polynorbornene (hPN) at room temperature. All reflections can be indexed with a monoclinic unit cell having a = 6.94Å, b = 9.60Å, c = 12.42Å, and b = 130.7°. Left: perspective view of an approximate representation of the average unit cell in atactic hPN, having a trans zig-zag backbone conformation (approaching space group C2/c symmetry).
Attaching any alkyl group to the 5-position of the norbornene ring yields an amorphous polymer, due to both endo/exo isomerism in the monomer and regioirregularity (random head/tail placement) in the polymer. If the alkyl substituent is lengthened, polymers of low glass transition temperature (Tg) are obtained, which can be hydrogenated to thermooxidatively stable products. This lowering of Tg is an example of the long-recognized idea of “internal plasticization”, and represents a robust strategy for the design of materials of controlled Tg. The fractional depression in these poly(alkylnorbornene)s and their hydrogenated derivatives (PaNs and hPaNs) compares well with that for classical homologous series of polymers, such as the atactic polyolefins and poly(alkylmethacrylate)s, as shown in the figure below.
Besides ROMP, we are also investigating nitroxide-mediated controlled radical polymerization as a route to well-defined block copolymers. Our present synthetic targets are block copolymers derived from styrene and acetoxystyrene (where the blocks could be homopolymers or statistical copolymers of these two monomers); hydrazinolysis of the acetoxy groups yields the pH-responsive vinylphenol unit, so that such polymer are truly amphiphilic.
Supported by the National Science Foundation, Polymers Program and the Princeton Center for Complex Materials
Recent/Current People and Projects:
Sasha Myers GS – Polyethylene Block Copolymers
Jeff Quinn GS – Block Copolymers Synthesized by Nitroxide-Mediated Radical Polymerization
John Bishop GS – Structuring Polymer Crystals Through Macromolecular Architecture
Li-Bong Lee *04 – Polymer Crystalline Texture Controlled Through Block Copolymerization
Selected Recent Publications:
J.D. Hatjopoulos and R.A. Register, “Synthesis and Properties of Well-Defined Elastomeric Poly(alkylnorbornene)s and Their Hydrogenated Derivatives”, Macromolecules, 38, 10320-10322 (2005).
L.-B.W. Lee and R.A. Register, “Hydrogenated Ring-Opened Polynorbornene: A Highly Crystalline Atactic Polymer”, Macromolecules, 38, 1216-1222 (2005).
L.-B.W. Lee and R.A. Register, “Acyclic Metathesis During Ring-Opening Metathesis Polymerization of Cyclopentene”, Polymer, 45, 6479-6485 (2004).