Reactions Limiting High Polymer Formation

3 Reactions Limiting High Polymer Formation 10.3.1 Macrocyclization 

Macrocyclization is one of the processes that limits the preparation of high-molecular-weight polymers from both cyclic sulfides and some cyclic amines. The mechanism of cationic macrocyclization is discussed in detail in Chap. 3. 

In the polymerization of cyclic sulfides, cyclization becomes particularly important because of the enhanced basicity (and nucleophilicity) of the linear sulfides in comparison with their parent monomers43). The simplest cyclic oligomer, formed in the polymerization of thiiranes, is a dimer-1,4-dithiane or its derivatives. 1,4-Dithiane was first observed by Bell in the polymerization of ethylene sulfide 441 later Price isolated the styrene sulfide dimer 45) and recently Goethals obtained dimers of isobutylene and cyclohexene sulfides 46 . Structures of these dimers are  

The yields and conditions of macrocyclization are given in Table 3.1 in Chap. 3. 

In the polymerization of propylene sulfide and 1,2-butylene sulfide mainly tetra-mers were observed. Cycles were formed mostly during the slow degradation process that followed rapid polymerizations. Degradation can also be induced by adding cationic initiators to polymer prepared by other mechanisms, e.g. by anionic processes. Thus, poly(trans-2,3-butene sulfide) is rapidly degraded to equimolar amounts of 3,5,6,7-tetramethyl-l,2,5-trithiacycloheptane and trans-butene 47). Poly(cis-2,3-butene sulfide) forms, however, a mixture of tetramer, trithiacycloheptane derivative and cis-butene 47 . If one is forced to use cationic processes for the synthesis of poly-sulfides, the reaction conditions should be controlled to avoid macrocyclization. If cyclic products are desired, the kinetics of their formation should be studied to determine optimum yields. 

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This chapter is largely limited to reactions of hydrocarbons and related compounds. Reactions in which the metal is involved merely as a reducing agent are not included. The alkali-metal-catalyzed reactions leading to the formation of high polymers are not discussed because they have been treated elsewhere (1). 

There is a very wide variety of condensation reactions that, in principle, can be used to form high polymers. However, as explained above, high polymers can be obtained only in high-yield reactions, and this limitation severely restricts the number of condensation reactions having any practical importance. A specific example of an impractical reaction is the formation of poly-1,4-butanediol by reaction of 1,4-dibromobutane with the disodium salt of the diol ... 

Reactions between a diene and a dienophile yielding cyclohexene derivatives through carbon-carbon or carbon-heteroatom bond formation are referred to as Diels-Alder (DA) reactions. The concerted [4 + 2] cycloaddition was first described in 1928 by Diels and Alder, ° and can be adapted for the efficient conjugation of peptides and proteins to synthetic polymers. Despite being recognized as highly selective and additive-free, DA reactions require elevated temperatures to obtain the desired product. Furthermore, the reversibility of the reaction limits its application in materials science. ... 

The irreversible binding of accelerator moieties by desulfuration or by some other reaction limits the formation of zinc accelerator complexes and subsequently hinders desulfuration of both pendent groups and crosslinks. Although direct evidence for this pathway being responsible is lacking, it does appear that with NR at high temperatures, or with butadiene-based rubbers at all temperatures, removal of the accelerator by combination with the polymer gives the system many of the characteristics of an unaccelerated sulfur vulcanization" (see Section 4.7). 

Polyester resins can also be rapidly formed by the reaction of propylene oxide (5) with phthaUc and maleic anhydride. The reaction is initiated with a small fraction of glycol initiator containing a basic catalyst such as lithium carbonate. Molecular weight development is controlled by the concentration of initiator, and the highly exothermic reaction proceeds without the evolution of any condensate water. Although this technique provides many process benefits, the low extent of maleate isomerization achieved during the rapid formation of the polymer limits the reactivity and ultimate performance of these resins. 

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