MICROSTRUCTURE AND MOLECULAR WEIGHT

The polymerization of norbornene, Eq. (19), is stopped by cooling the reaction mixture to room temperature. The active polymer 11 can be stored for long periods of time. Heating 11 to temperatures above 65 °C in the presence of monomer causes renewed chain propagation. The subsequent addition of different cyclic olefins, such as endo- and exo-dicyclopentadiene, benzonorbomadiene and 6-methylbenzonorbornadiene resulted in the formation of well-defined AB- and ABA-type block copolymers, Eq. (21) [38]. Triblock copolymers 13 with narrow molecular weight distributions (polydispersity = 1.14) were prepared. Thus, the living character enables the preparation of new uniform block copolymers of predictable composition, microstructure and molecular weight. 

Three polybutadiene/poly isoprene diblocks were provided to us by Paul Rempp of CRM, Strasbourg. Details of the molecular weights and the expected diene microstructures were discussed earlier (1,13) and are summarized in Table I. Homopolymers of corresponding microstructures and molecular weights (1,13) are also described in Table I. The molecular-weight distributions of the homopolymers are considerably broader than those of the block polymers, owing to a mastication procedure (13) carried out on these samples to lower their molecular weights. To insure that" the microstructures of the homopolymers were well matched to the constituent blocks in the copolymers, 60-MHz NMR spectra were obtained on a Varian T-60 apparatus for the diblocks, the individual homopolymers, and three equivalent homopolymer blends (13). 

Table III. Microstructure and Molecular Weights of Butadiene and Isoprene Polymers...

Table 3 Influence of alkylaluminum compounds on the microstructure and molecular weight of polybutadiene afforded by NdV/ BuCl [63]...

Ring-opening metathesis polymerization (ROMP) of substituted bicyclo octa-dienes or paracyclophane-enes initiated by Gmbbs molybdenum, tungsten-based carbenes have been used to prepare PPV s [178—181]. The living character of ROMP has been exploited to prepare soluble well-defined precursors, which can be converted into XI. Yu and Turner have used ROMP of tetra octyloxy-substituted paracyclo-phanedienes initiated by reactive ruthenium-based carbenes to prepare monodisperse, soluble yellow fluorescent PPV with an alternating cis-trans microstructure and molecular weights as calculated [178] (Fig. 9.21). 

Figure 8. Luminescence of SBS block copolymer as weU as of PS and PBD homopolymers similar in microstructure and molecular weight to the component polymers of SBS. Samples were irradiated with x-rays at 90 K to 5 eV g K...

The extension of thermodynamics to a blend of elastomers has been discussed by Roland [4], Miscible blends are most commonly formed from elastomers with similar three-dimensional [7] solubility parameters. An example of this is blends from copolymer elastomers (e.g., ethylene-propylene or styrene-butadiene copolymers) from component polymers of different composition, microstructure, and molecular weights. When the forces between the components of the polymer blend are mostly entirely dispersive, miscibiUty is only achieved in neat polymers with a very close match in Hansen s three-dimensional solubility parameter [7]. 

Physical and mechanical properties of filled rubber composites are strongly influenced by chemical nature, microstructure and molecular weight of the... 

Chain microstructure and molecular weight distribution are independent of initiation mechanism and type and function of reaction variables such as temperature and concentration of reactants. 

Polybutadiene samples of different microstructure and molecular weight, as shown in Table I. 

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