Ring Formation in Step-Growth Polymerizations

Step growth polymerizations can be complicated by cyclization reactions that accompany formations of linear polymers. Such ring formations can occur in reactions of monomers with either the same type of functional groups or with different ones. Following are some illustrations of cyclization reactions:

Similarly, dicarboxylic acids can cyclize into anhydrides. Whether ring formation is likely to take place or not depends upon the size of the ring that can form. If cyclization results in rings with strained bond angles or repulsions due to crowding, the probability of their formation is low. So, small rings, with less than five members, do not form readily. Five-membered rings, however, are essentially free from bond angle distortion and have a greater chance to form. Greater than five-membered rings are not planar and six- and seven-membered rings can also form freely, though not quite as easily as five-membered ones. Six-membered rings are more favored than seven-membered ones. Rings with eight to twelve members are relatively strain-free from bond angle distortion, but they are thermodynamically unstable. This is because substituents (hydrogens or others) are forced into positions of repulsion due to crowding. Also, there is little ring formation with eight to twelve members. Whether cyclization will take place or not during polymerization also depends on the kinetic feasibility to cyclize. This feasibility is a function of the probability that functional end groups on a molecule will approach each other. As the size of the monomer increases, so does the size of the potential ring. An increase in the size of the monomer, however, also means an increase in the number of different configurations that the monomer molecule can assume. Very few of these configurations are such that the two ends become adjacent [4]. With fewer chances of the end groups encountering each other, there is decreased probability of ring formation. From practical considerations, ring formations are mainly a problem when five-, six-, or seven-membered rings can form. Formation of large rings with more than twelve members is seldom encountered [5]. It was reported that at least in one instance, the products of a reaction can be affected by the isomer used [244]. Thus, in the following Knoevenagel condensation, a polymer will form when an ortho isomer is being reacted:

By contrast, the meta isomer forms only ring structures instead [242]:

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مواضيع ذات صلة

Linear Unsaturated Polyesters

Copolyesters

Synthetic Methods

Linear Saturated Polyesters

Techniques of Polymer Preparation

Kinetic Considerations

Reactions of Functional Groups

Poly (vinyl alcohol) and Poly (vinyl acetal) s

Poly (vinyl acetate)

Poly (vinylidine chloride)

Copolymers of Vinyl Chloride

Poly (vinyl chloride)