Controlled molecular architecture

The crosslinking must be sufficiently infrequent (about one crosslink per hundred repeat units) as to allow the polymer to adopt a random coil configuration between crosslink sites and so exhibit entropic recovery when deformed. The chemistry of rubber crosslinking is discussed later. 

The discussion in the previous sections has focussed on the properties of a linear homopolymer chain. Attention has been paid to the way the conformation of the chain and the molar mass affect the properties in the melt and the development of the solid state on cooling the melt. The linear chain is an idealization of the real polymer and different architectures may be introduced by 

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The discovery and development of polypropylene, the one genuinely new large tonnage thermoplastics material developed since World War II, forms part of what is arguably the most important episode in the history of polymer science. For many years it had been recognised that natural polymers were far more regular in their structure than synthetic polymers. Whilst there had been some improvement in controlling molecular architecture, the man-made materials, relative to the natural materials, were structurally crude. 

A chemical property of silicones is the possibility of building reactivity on the polymer [1,32,33]. This allows the building of cured silicone networks of controlled molecular architectures with specific adhesion properties while maintaining the inherent physical properties of the PDMS chains. The combination of the unique bulk characteristics of the silicone networks, the surface properties of the PDMS segments, and the specificity and controllability of the reactive groups, produces unique materials useful as adhesives, protective encapsulants, coatings and sealants. 

C. J. Hawker and J. M. J. Frechet, Preparation of polymers with controlled molecular architecture. A new convergent approach to dendritic macromolecules, J. Am. Chem. Soc., 112 (1990) 7638-7647. 

Intramolecular interaction is a powerful factor that controls molecular architecture, particularly in the case of geometrically flexible molecular systems. The existence and energies of intramolecular classical hydrogen bonds and their role in chemistry and biochemistry are well known. They stabilize molecular conformations, promote short- and long-range proton transfers, participate in the creation of three-dimensional structures of large molecules and play a fundamental role in the phenomenon of molecular recognition. 

For living ROP with the ability to control molecular architecture and weight, aluminum alkoxides can be used, the propagation being characterized by the almost total absence of side-reactions. The reaction is usually performed in solution at low temperatures. The sensitivity towards hydrolysis is however a limitation. 

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