Chain-growth polymers, definition

An oligomer is a very low molecular weight polymer. It consists of only a anall number of mers. The definition of a telomer is that of a chain-growth polymer that is composed of molecules with end groups consisting of different species from the monomer units. Telomers can form by either free-radical or by ionic chain-growth polymerization mechanism. 

Polymers made by chain-growth reactions are often addition polymers by Carothers s definition. The most common polymers made by these processes have only carbon-carbon links in their backbones. 

Although these definitions were perfectly adequate at the time, it soon became obvious that notable exceptions existed and that a fundamentally sounder classification should be based on a description of the chain-growth mechanism. It is preferable to replace the term condensation with step-growth or step-reaction. Reclassification as step-growth polymerization now logically includes polymers such as polyurethanes, which grow by a step-reaction mechanism without elimination of a small molecule. 

The instantaneous MMD can be found from a knowledge of the rate coefficients for chain-stopping events, which are transfer (to monomer, polymer and chain-transfer agent) and termination, and for chain growth, which is propagation. In addition, since in a zero-one system entry into a particle which already contains a growing chain results (by definition) in instantaneous termination, entry is also a chain-stopping event in a zero-one system. 

This section has briefly reviewed some of the sites thatantibacterial agents are known to attack. Numerous and varied, these sites show that a well planned combination of drugs within the same polymer chain have a definite possibility of intersecting. They also show that there are many combinations within which organotin compounds may inhibit bacterial growth. 

One problem associated with polymers made in this way is that they contain phenolic and aromatic-nitro end groups, 4 and 5, which adversely affect their thermo-oxidative stability. In addition, one constraint on this synthetic approach is the critical requirement for extremely low levels of water in the polymerization reaction mixtures. Any water present under these conditions rapidly converts a nitro-imide to the corresponding nitro-amic-acid salt, 6, from which the nitro-group cannot be displaced. This limits chain growth and presents definite reproducibility problems. 

While these extremely simplified cases are met in practice, some very important exceptions occur that show the need for caution in making generalizations. For example, nylon 6 would normally be considered as a step-growth polymer because it is a polyamide. However, it can also be produced by a ring-opening polymerization that is a kind of chain-growth polymerization. Thus, these definitions are useful but can sometimes be misleading. 

Secondly, I wish to counteract anticipated despondency which some of the complexities on the present theoretical scene may perhaps provoke. For this purpose, I wish to invoke the decisive simplicity and definiteness of some of the experimental effects observed within the confines of the above, near ideal systems. This, as I often pointed out elsewhere, is unmatched in the field of crystal growth of simple substances. Complicated as polymers may seem, and subtle as some of the currently relevant theoretical issues, this should not obscure the essential simplicity and reproducibility of the core material. To be specific, the appropriate chains seem to want to fold and know when and how, and it is hardly possible to deflect them from it. Clearly, such purposeful drive towards a predetermined end state should continue to give encouragement to theorists for finding out why Those who are resolved to persevere or those who are newly setting out should find the present review a most welcome source and companion. 

With decreasing temperature, the density oscillation becomes very pronounced and grows into a deeper melt region. At 300 K, for example, we can see at least 5 layers after 1.28 ns. Within the layers, as will be shown later, definite order in chain orientation and chain packing is observed suggesting the growth of polymer crystals. 

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