Polymerization reactions step

In contrast to the relatively slow step-reaction polymerizations discussed in Chapter 4, chain polymerizations are usually rapid, and the initiation species continues to propagate until termination. Thus, in the extreme case, a single initiation species could produce one high-molecular chain, leaving all of the other monomer molecules unchanged. In any case, the concentration of monomer, which is often a substituted vinyl compound, decreases continuously throughout the reaction. In contrast to stepwise polymerization, the first species produced in chain polymerizations is a high-molecular polymer. 

Name an elastomer produced by (a) cationic, (b) anionic, (c) free-radical, and (d) step-reaction polymerization techniques. 

Linear polymers prepared by step reaction polymerization, such as nylon 66, and linear, ordered polymers prepared by the chain polymerization of symmetrical vinylidene monomers, such as polyvinylidene chloride (PVDC), can usually be crystallized because of symmetry and secondary-bonding. Isotactic polymers, such as isotactic polypropylene (PP), usually crystallize as helices. 

The second common method of polymer synthesis involves the stepwise coupling of small molecules which are difunctional by virtue of reactive functional groups. A typical example of step-reaction polymerization would be the synthesis of polyamides by reaction of a diamine with a diacid. In these systems the chain is built up slowly by reaction of any pair of functional groups in the system and it is common for the coupling to involve elimination of a small molecule. Conventionally these polymerizations allow more control over the chain structure but difficulties in reaching very high conversions and problems of reagent purity usually lead to much shorter... 

PolyQj-Phenylene) is probably the most thermally and oxidatively stable conducting polymer known and has been of interest to polymer scientists both for its conducting properties and for its stability. Its synthesis by step-reaction polymerization or by chemical or electrochemical oxidation invariably gives powders or rather poor quality films, so that a precursor route would be very attractive. 

There are two primary polymerization approaches step-reaction polymerization and chain-reaction polymerization.12... 

In contrast to the slow step-reaction polymerizations, chain-reaction polymerizations are fairly rapid.14 Chain-reaction polymerizations (often referred to as addition polymerizations) require the presence of an initiator for polymerization to occur. Initiation can occur by a free radical, an anionic, or a cationic species, which open the double bond of a vinyl monomer and the reaction proceeds as shown in Fig. 15.11 where may be a radical,... 

This simple equation demonstrates one fundamental aspect of step-reaction polymerizations—that very high conversions are necessary to achieve practical molecular weight. At 98 percent conversion, for example, DP is only 50. For DP= 100, the monomer conversion must be 99 percent. 

For step-reaction polymerization, the NACL is also sometimes referred to as the degree of polymerimtion. It is the average munber of structural units per chain and can also be calculated from... 

Crystallization During Polymerization of Monomers Capable of Step Reaction Polymerization a) Organic Polymers... 

In summary, there are many examples of crystallization during polymerization of organic monomers which are capable of step reaction polymerization in the dissolved or molten state. These polymerizations allow initial step reactions to form larger concentrations of oligomers which... 

The asterisk denotes the active species and M stands for a monomer unit. The monomer is unreactive except towards the initiated active polymer chain end, in contrast to the step reaction polymerization where reaction between all species, monomer, oligomer, and polymer is possible at any time. Step reactions can be speeded up by catalysts or enzymes which, however, act only during one reaction step and do not lead to a series of successive reactions of the same molecule as the initiator does. After a short reaction time a step reaction has produced many oligomer molecul which slowly increase in molecular weight and decrease in... 

This reaction is a termination and limits the molecular weight of the final molecule. In solution or melt polymerization the level of free radical concentration must for this reason not be raised over 10 to 10 M to 3deld reasonably high molecular weight (5). In case a ci3 tal surface would contain higher numbers of mobile free radicals, termination would be unavoidable. Also, crystal nucleation by active intermediate oligomers, as is observed in crystallization during step reaction polymerization, would most likely lead to termination. 

This kind of polymerization, each step of which consumes a reactive particle and produces another, similar particle, is an example of chain-reaction polymerization. In Chap. 32, we shall encounter chain-reaction polymerization that takes place, not by way of free radicals, but by way of organic ions. We shall also encounter step-reaction polymerization, which involves a series of reactions each of which is essentially independent of the others. 

There is an alternative, somewhat less meaningful system of classification addition polymerization, in which molecules of monomer are simply added together and con-iensation polymerization, in which monomer molecules combine with loss of some simple nolecules like water. As it happens, the two systems almost exactly coincide nearly all ases of chain-reaction polymerization involve addition polymerization nearly all cases pf step-reaction polymerization involve condensation polymerization. Indeed, some hemists use the term addition polymerization to mean polymerization via chain reactions. 

These are examples of step-reaction polymerization (Sec. 32.2). Here, reaction does not depend on chain-carrying free radicals or ions. Instead, the steps are essentially independent of each other they just happen to involve more than one functional group in a monomer molecule. 

Treatment of jS-propiolactone with base gives a polymer. Give a likely structure for this polymer, and show a likely mechanism for the process. Is this an example of chain-reaction or step-reaction polymerization ... 

Figure 2.3.1. Variation of DP with p for a step-reaction polymerization.

Characteristic Condensation (step-reaction) polymerization Chain reaction (vinyl-type) polymerization... 

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