Stereocontrol and Morphology

Most cases of radiation-induced polymerizations of crystalline monomers lead to atactic, noncrystallizable polymers. This occurs because of the density difference between the polymer and monomer crystal, which does not allow the monomer enough mobility to orientate sufficiently during the propagation step to produce a stereoregular polymer. 

In actual photochemical polymerizations, on the other hand, sterically pure products can occasionally be produced, since here every propagation step must be photochemically controlled. A prerequisite for this, however, is that the density difference between polymer and monomer crystals be very small. An example of this is the polymerization of cinnamic acid derivatives, which, according to the crystal structure of the monomer, 

If the crystal structures of the initial monomer and the polymer produced are crystallographically related, the polymerization is described as being topochemical or topotactic. Suitably substituted diacetylenes are so arranged in the crystal lattice that the conjugated triple bonds represent the steps of a ladder. Substituents linked by hydrogen bonding represent the runners of the ladder. A suitable substituent is, for example, —CH2—O—CO—NH—C6H5. A kind of shear takes place on polymerization, since the density difference between monomer and polymer crystals is small  

According to Raman spectroscopy data, the polymers consist of alternate conjugated double and triple bonds. The shapes of the monomer crystals are not altered on polymerization. The polymer crystals are highly colored and birefringent they also conduct electricity. 

The conformation of polymers produced by solid-state polymerization often depends strongly on the crystal structure of the monomers. For example, the polymerization of tetroxane, (CH20)4, leads to helix structures, whereas the polymerization of trioxane, (CH20)3, gives a zigzag poly-(oxymethylene). 

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One way to control the blend morphology, as well the filler dispersion and filler/ matrix interaction in polyolefin composites, is to apply a compatibilizer, which acts as an interfacial agent promoting adhesion between the phases. Functionalized copolymers prepared by coordination copolymerization have many advantages over functionalized polyolefins prepared by radical polymerization or grafting. Coordination polymerization gives access to stereocontrol as well as to precise control of composition, crystallinity, molar mass, and their distributions. 

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