Units of 2-ureido-4-pyrimidone that dimerize strongly in a self-complementary array of four cooperative hydrogen bonds were used as the associating end group in reversible self-assembling polymer systems. The unidirectional design of the binding sites prevents uncontrolled multidirectional association or gelation. Linear polymers and reversible networks were formed from monomers with two and three binding sites, respectively. The thermal and environmental control over lifetime and bond strength makes many properties, such as viscosity, chain length, and composition, tunable in a way not accessible to traditional polymers. Hence, polymer networks with thermodynamically controlled architectures can be formed, for use in, for example, coatings and hot melts, where a reversible, strongly temperature-dependent rheology is highly advantageous.

No abstract

Two xanthate end-functional poly(ethylene glycol)s (PEGs) were tested as macromolecular chaintransfer agents (macroCTA) in the reversible addition-fragmentation transfer-mediated polymerization of vinyl acetate (VAc) and N-vinylpyrrolidone. The macroCTA leaving group played a determining role in the preparation of the block copolymers. PEG-b-PVAc and PEG-b-PVP diblock copolymers were obtained when the macroCTA had a propionyl ester leaving group, whereas under the same experimental conditions the macroCTA with a phenylacetyl ester leaving group inhibited the polymerization. In situ 1 H NMR spectroscopy polymerizations were performed with low molecular weight xanthate analogues to investigate the cause of inhibition. Block copolymers were prepared with the macroCTA which did not inhibit the polymerization and were characterized via size exclusion chromatography, high-performance liquid chromatography, and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. The ability to produce narrowly distributed (PDI < 1.4) block copolymers end capped with a xanthate moiety with little to no homopolymer contaminant is presented. IntroductionPoly(ethylene glycol) (PEG) is widely used in the pharmaceutical and biomedical fields. It is a nonionic polymer, soluble in water and most common organic solvents. The incorporation of a PEG segment in a macromolecule modulates its solution properties. Many synthetic pathways are available for the preparation of block copolymers comprising a PEG block. Each block can be prepared separately and connected by postpolymerization coupling of functional end groups. 1 Commercially available PEGs prepared via anionic polymerization can be found with one or two hydroxyl end functionalities, which enable almost unlimited chemical modification 2 and the preparation of di-, tri-, or multiblock copolymers. The main prerequisite for this approach is that the second polymeric block must be quantitatively end functionalized. Thus, this method has been used mostly to prepare biodegradable block copolymers of PEG with a second block obtained via polycondensation or ring opening polymerization. 3 Poly(N-vinylpyrrolidone) (PVP) and poly(vinyl acetate) (PVAc) are typical examples of widely used industrial polymers that can only be prepared via free-radical polymerization. Conventional free-radical polymerization does not normally provide end functionality because of transfer and termination reactions. By taking advantage of transfer reactions, however, Ranucci et al. synthesized a variety of low molecular weight PVPs bearing chain-end functionality. 4 Another synthetic approach consists of growing a second block from an endfunctional PEG precursor. By selecting a macromolecular precursor bearing a functional group capable of controlling the polymerization of the second comonomer, it is possible to not only obtain block copolymers but also control the molecular weight distribution of the blocks. The recent development of