Polymerization of cyclic monomers

2 epoxides will not be included since the homopolymerization of these monomers e.g. ethylene oxide, is best carried out by anionic means [114]. 

Detailed analysis of the kinetics of polymerization of cyclic monomers has only relatively recently reached the sophistication of analogous vinyl systems. Despite this an ever increasing volume of relevant data is emerging, and, indeed many reactions are proving to be more amenable to investigation in an absolute manner, than the more extensively studied vinyl polymerizations. This situation has arisen partly as a result of the inherent lower reactivity of these monomers, and also from the fact that, in general, the polymerizations are less susceptible to impurities and side reactions. Even monomer transfer is absent in most of these systems, and a situation approaching that found in anionic vinyl polymerization prevails. 

Generalized methods of initiating the polymerization of these monomers have recently been reviewed in detail [9], and were also mentioned briefly earlier in this Chapter. As with vinyl monomers initiation can be efficient and rapid, with the production of a fixed number of active centres. Propagation appears to be much slower, however, and rates of polymerization are comparable to those in free radical addition polymerizations. Techniques such as dilatometry, spectrophotometry etc. are therefore convenient for kinetic investigation of this type of cationic reaction. 

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Polymerization equilibria frequently observed in the polymerization of cyclic monomers may become important in copolymerization systems. The four propagation reactions assumed to be irreversible in the derivation of the Mayo-Lewis equation must be modified to include reversible processes. Lowry114,11S first derived a copolymer composition equation for the case in which some of the propagation reactions are reversible and it was applied to ring-opening copalymerization systems1 16, m. In the case of equilibrium copolymerization with complete reversibility, the following reactions must be considered. 

Ring-opening polymerization of cyclic monomers, usually by anionic or cationic catalysts, is another route to elastomers. These include the polymerization of octamethylcyclotetrasiloxane... 

Lipase-Catalyzed Ring-Opening Polymerization of Cyclic Monomers... 

The ring-opening polymerizations of cyclic monomers such as propylene oxide... 

The ionic chain polymerization of unsaturated linkages is considered in this chapter, primarily the polymerization of the carbon-carbon double bond by cationic and anionic initiators (Secs. 5-2 and 5-3). The last part of the chapter considers the polymerization of other unsaturated linkages. Polymerizations initiated by coordination and metal oxide initiators are usually also ionic in nature. These are called coordination polymerizations and are considered separately in Chap. 8. Ionic polymerizations of cyclic monomers is discussed in Chap. 7. The polymerization of conjugated dienes is considered in Chap. 8. Cyclopolymerization of nonconjugated dienes is discussed in Chap. 6. 

The ring opening polymerization of cyclic monomers that yield thermoplastic polymers of interest in composite processing is reviewed. In addition, the chemistry, kinetics, and rheology of the ring opening polymerization of caprolactam to nylon 6 are presented. Finally, the rheo-kinetics modelsfor polycaprolactam are applied to the composite process of reaction injection pultrusion. 

Ring opening polymerization of cyclic monomers to yield thermoplastic polymers has been studied by a number of investigators [1-19] over the years. A variety of cyclic monomers ranging in structures from the more commonly encountered olefins, ethers, formals, lactones,... 

Polycarbonates, both aliphatic and aromatic, have been prepared by the ring opening polymerization of cyclic monomers or oligomers [22], Cyclic monomeric precursors are more common in aliphatic polycarbonates, but because of steric reasons aromatic polycarbonates can only be prepared from cyclic oligomers. Both cationic and anionic initiators have been examined and anionic initiators appear to be more efficient. 

Figure 2.2 Polymerization of Cyclic Monomers (9-11). (RO) ROMP, (VA) Vinyl Addition Polymerization, (R/I) Radical/Ionic Polymerization...

Some work has been done on the electrolytically initiated polymerization of cyclic monomers. 

More recently, methods of polymerization of cyclic monomers containing phosphorus became available 3>, leading directly to structures like 7. 

This type of termination may be quite common in some polymerizations of cyclic monomers. 

The ring-opening polymerization of cyclic monomers can be performed by ionic chain polymerization, as is the case of epoxy monomers. Anionic polymerization of ethylene oxide propylene oxide, and caprolactone can be initiated by alkoxides ... 

Many works on the synthesis of cyclic polymers and block copolymers using kinetically controlled ring-expansion polymerizations of cyclic monomers, such as lactones and lactides with various types of cyclic tin initiators, were reviewed by Kricheldorf [147,148]. Kricheldorfs group continued the synthesis of cyclic polymers, and their recent works have focused on the following. Polycondensations of 4,4/-difluorodiphenylsulfone with tris(4-hydroxy phenyl)ethane were performed in DMSO to give multi-cyclic poly(ether sulfone)s derived from tris(4-hydroxyphenyl)ethane [149]. 

In polymerizations of cyclic monomers, the product is isotactic. This is not necessarily so when optically active alkenes are polymerized. 

Most polymerizations of cyclic monomers are ionic processes. Coordination catalysts are effective only for some heterocycles (oxirane and its derivatives, lactones). Ziegler-Natta catalysts can only be used for cycloalkene polymerization by metathesis heterocycles act as a catalytic poison. Smooth radical polymerization of hydrocarbon monomers with ring strain is unsuccessful [304], The deep-rooted faith that ring strain represents a major contribution to the driving force in ring opening (polymerization) has to be revised [305, 306]. 

These salts are most efficient in the polymerization of cyclic monomers [33, 34] though some use in vinyl systems has been reported [34]. Protonic acids, e.g., HCIO4 (and H2SO4) also fall within the definition of pre-formed initiators. As already indicated a considerable amount of work has been carried out on the polymerization of styrene [10—22] by these acids, where an additional question concerning the role of covalent perchlorate ester species is raised. This problem is dealt with in more detail later (see Section 6.2). 

The accumulation of a great number of small particles into one polymer chain is an aggregation process resulting in a decrease in the translational entropy of the system. In the polymerization of cyclic monomers, the decrease of translational entropy is partially counterbalanced by the increase in rotational and vibrational entropy resulting from the conversion of a more or less rigid cyclic monomer into a flexible monomer unit inside a polymer chain. Thus the net entropy of polymerization of lactams is more positive (e.g. —3 eu for seven-membered lactams) than the entropy of polymerization of vinyl monomers (—25 to —30 eu). 

In addition to step and chain polymerizations, another mode of polymerization is of importance. This is the ring-opening polymerization of cyclic monomers such as cyclic ethers, esters (lactones), amides (lactams), and siloxanes. Examples of commercially important types are given in Table 10.1. Of those listed, only the polyalkenes are composed solely of carbon chains. Those that have enjoyed the longest history of commercial exploitation are polyethers prepared from three-membered ring cyclic ethers (epoxides), polyamides from cyclic amides (lactams), and polysiloxanes from cyclic siloxanes. 

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