Influence of tertiary diamines on the synthesis of high-molecular-weight poly(1,3-cyclohexadiene)

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,N′,N′-tetramethylethylenediamine (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 = 50,000 g/mol) with a narrow molecular weight distribution (weight-average molecular weight/number-average molecular weight = 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 × 10 ^-5 s ^-1) in comparison with TMEDA-mediated polymerizations (1.5 × 10 ^-4 s ^-1). 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 ¹H 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).