Core‐cleavable star polymers were synthesized by the coupling of living anionic poly(alkyl methacrylate) arms with either dicumyl alcohol dimethacrylate (DCDMA) or 2,5‐dimethyl‐2,5‐hexanediol dimethacrylate (DHDMA). This synthetic methodology led to the formation of star polymers that exhibited high molecular weights and relatively narrow molecular weight distributions. The labile tertiary alkyl esters in the DCDMA and DHDMA star polymer cores were readily hydrolyzed under acidic conditions. High‐molecular‐weight star polymer cleavage led to well‐defined arm polymers with lower molecular weights. Hydrolysis was confirmed via 1H NMR spectroscopy and gel permeation chromatography. Thermogravimetric analysis (TGA) of the star polymers demonstrated that the DCDMA and DHDMA star polymer cores also thermally degraded in the absence of acid catalysts at 185 and 220 °C, respectively, and the core‐cleavage temperatures were independent of the arm polymer composition. The difference in the core‐degradation temperatures was attributed to the increased reactivity of the DCDMA‐derived cores. TGA/mass spectrometry detected the evolution of the diene byproduct of the core degradation and confirmed the proposed degradation mechanism. The DCDMA monomer exhibited a higher degradation rate than DHDMA under identical reaction conditions because of the additional resonance stabilization of the liberated byproduct, which made it a more responsive cleavable coupling monomer than DHDMA. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3083–3093, 2003
The living synthesis of poly(1,3-cyclohexadiene) was performed with an initiator adduct that was synthesized from a 1:2 (mol/mol) mixture of N,N,NЈ,NЈtetramethylethylenediamine (TMEDA) and n-butyllithium. This initiator, which was preformed at 65°C, facilitated the synthesis of high-molecular-weight poly(1,3-cyclohexadiene) (number-average molecular weight ϭ 50,000 g/mol) with a narrow molecular weight distribution (weight-average molecular weight/number-average molecular weight ϭ 1.12). A plot of the kinetic chain length versus the time indicated that termination was minimized and chain transfer to the monomer was eliminated when a preformed initiator adduct was used. Chain transfer was determined to occur when the initiator was generated in situ. The polymerization was highly sensitive to both the temperature and the choice of tertiary diamine. The use of the bulky tertiary diamines sparteine and dipiperidinoethane resulted in poor polymerization control and reduced polymerization rates (7.0 ϫ 10 Ϫ5 s Ϫ1 ) in comparison with TMEDA-mediated polymerizations (1.5 ϫ 10 Ϫ4 s Ϫ1 ). A series of poly(1,3-cyclohexadiene-block-isoprene) diblock copolymers were synthesized to determine the molar crossover efficiency of the polymerization. Polymerizations performed at 25°C exhibited improved molar crossover efficiencies (93%) versus polymerizations performed at 40°C (80%). The improved crossover efficiency was attributed to the reduction of termination events at reduced polymerization temperatures. The microstructure of these polymers was determined with 1 H NMR spectroscopy, and the relationship between the molecular weight and glass-transition temperature at an infinite molecular weight was determined for polymers containing 70% 1,2-addition (150°C) and 80% 1,4-addition (138°C).
Self-healing macromolecular structures, submicron capsules and fibers with molecular recognition, stimuliresponsive molecules, solvent-free rheological reversibility, multivalency in rational drug design, and the emergence of new fields of adaptive and evolutive chemistry will require a predictive synergy of tailored non-covalent and covalent bonding in molecular design. Supramolecular chemistry has emerged as a stimulating focal point that will enable these scientific and technological discoveries, and biorecognition and biomolecular organization often serve as the inspiration for the future design of supramolecular assemblies. Linear and branched macromolecules are conventionally prepared using unique combinations of step-growth and chain polymerization strategies wherein the repeating units are irreversibly connected using stable covalent bonds. Moreover, optimum physical properties and commercial success of macromolecules are derived from our ability to prepare exceptionally high molecular weights in a controlled fashion. Although high molecular weight linear macromolecules are desirable for the optimization of physical performance and commercial impact, high molecular weights often compromise future solvent-free manufacturing, melt processability, thermal stability, and recyclability of the final products. Our recent efforts have demonstrated the utility of living anionic polymerization techniques to place functionality at desired positions on the polymer backbone. This control allowed investigation of the relationship between topology and tailored functionality, a fundamental investigation that may lead to interesting adaptive and smart applications. Specifically, the synthesis of polyisoprene homopolymers in a variety of topologies was performed, as well as the introduction of complementary hydrogen bonding to diverse families of hydroxyl containing polymeric and monomeric precursors.
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