Chain topology, control

The molecular weight (M) dependence of the steady (stationary) primary nucleation rate (I) of polymers has been an important unresolved problem. The purpose of this section is to present a power law of molecular weight of I of PE, I oc M-H, where H is a constant which depends on materials and phases [20,33,34]. It will be shown that the self-diffusion process of chain molecules controls the Mn dependence of I, while the critical nucleation process does not. It will be concluded that a topological process, such as chain sliding diffusion and entanglement, assumes the most important role in nucleation mechanisms of polymers, as was predicted in the chain sliding diffusion theory of Hikosaka [14,15]. 

Iedema, P.D. and Hoefsloot, H.C.J. (2001) Synthesis of branched polymer architectures from molecular weight and branching distributions for radical polymerisation with long-chain branching, accounting for topology-controlled random scission. Macromol. Theor. Simul., 10, 855. 

Other things that are particularly controlled in condensation polymerization are chain topology and cascade reactivity of monomers. In general condensation polymerization, not only linear polymers but also cyclic polymers can be produced because both ends of the polymer are reactive during polymerization. When cascade reactions selectively occur on a monomer, condensation polymerization proceeds in a way different from general polymerization behavior based on the principle of Carothers and Flory. Thus, in condensation polymerization of AA and BB monomers, if AA monomers successively react with both the functional groups on a BB monomer, a poly-... 

Ye and co-workers exploited the shear stability of highly branched, high molecular weight polyethylenes (PEs) as lubricant viscosity index improvers [252, 253]. Viscosity index of a lubricant is a critical parameter which defines its quality and application temperature range. They synthesized PEs with controllable chain topologies ranging from linear to a hyperbranched dendritic structure by chain walking polymerization [254]. The PE samples were blended into a base paraffinic oil (density 0.8659 g mL at 15 °C, kinematic viscosity 30.06 cST at 40 °C) to form lubricants. The lubricants were subjected to the Kurt Orbahn (KO) test to measure the shear stability index, which is expressed by... 

Short polymethylene chains with less than 40 carbon atoms when included in PHTP channels show a NMR shift due to kinks of conformations propagating from the chain ends. In TPP the effect is restricted to shorter (< 20 carbon) chains. This is a clear indication for a topological control exerted by each matrix according to the cross-section (Fig. 17). 

Poor control over chain topology (block copolymers and more advanced topologies are virtnally impossible to synthesize efficiently). 

To have greater control of the chain topology, it was decided to use model networks. PDMS networks were chosen because of the extensive work by Mark and his coworkers on these systems. The-networks were prepared by crosslinking divinyl terminated PDMS chains with a tetra silane in the presence of a platinum catalyst. All networks were prepared in bulk. By choosing the appropriate prepolymers, it was possible to prepare unimodal and bimodal networks. For the bimodal networks, the short chains had M = 770 and the long chains had M = 22,500. In both cases M /M 1.8. 

Successfully prepared Seesaw-type diblock macromonomer (Na-PS- -PCL-N3) and model hyperbranched hetero-subchain copolymer [HB-(PS-f>-PCL)n] with controllable and uniform PS and PCL subchains, and found that such prepared hyperbranched copolymer chains still have a fractal-like structure, and the PCL subchains inside HB-(PS-f>-PCL)n crystallize less as the branching degree increases and the PS subchain becomes longer. This work demonstrates that we can use the chain topology to control the crystallization of PCL subchains inside hyperbranched copolymers to regulate their biodegradation for biomedical applications. 

Simple adjustment of temperature and pressure could easily control polymer chain topology. Brookhart catalysts are used in DuPont s Versipol system. Another class of post-metallocenes are Gmbbs catalysts. These catalysts are good for incorporation of polar monomers and particularly suitable for production of specialty low-molecular-weight polymers with reactive functionalities. 

It is beyond our control how the cross-links are spaced along the polymer chains during the vulcanization process. This extraordinary important fact demands a generalization of the Gibbs formula in statistical mechanics for amorphous materials that have fixed constraints of which the exact topology is unknown. Details of a modified Gibbs formula of polymer networks can be found in the pioneering paper of Deam and Edwards [13]. 

---