Vinyl polymers, polymerization mechanism

Vinyl copolymers contain mers from two or more vinyl monomers. Most common are random copolymers that are formed when the monomers polymerize simultaneously. They can be made by most polymerization mechanisms. Block copolymers are formed by reacting one monomer to completion and then replacing it with a different monomer that continues to add to the same polymer chain. The polymerization of a diblock copolymer stops at this point. Triblock and multiblock polymers continue the polymerization with additional monomer depletion and replenishment steps. The polymer chain must retain its ability to grow throughout the process. This is possible for a few polymerization mechanisms that give living polymers. 

When many molecules combine the macromolecule is termed a polymer. Polymerization can be initiated by ionic or free-radical mechanisms to produce molecules of very high molecular weight. Examples are the formation of PVC (polyvinyl chloride) from vinyl chloride (the monomer), polyethylene from ethylene, or SBR synthetic rubber from styrene and butadiene. 

The difficulties of devising a basis for conveniently classifying various polymerizations in an appropriate manner have been discussed earlier in this chapter and several borderline examples which offer particular difficulty have been mentioned. One of these, the polymerization of the N-carboxyanhydrides, falls within the definition of a condensation polymerization, proceeds by a mechanism resembling a vinyl addition polymerization, and yields a product which possesses the structure of a typical condensation polymer. Definitions have been... 

Radical polymerization is the most useful method for a large-scale preparation of various kinds of vinyl polymers. More than 70 % of vinyl polymers (i. e. more than 50 % of all plastics) are produced by the radical polymerization process industrially, because this method has a large number of advantages arising from the characteristics of intermediate free-radicals for vinyl polymer synthesis beyond ionic and coordination polymerizations, e.g., high polymerization and copolymerization reactivities of many varieties of vinyl monomers, especially of the monomers with polar and unprotected functional groups, a simple procedure for polymerizations, excellent reproducibility of the polymerization reaction due to tolerance to impurities, facile prediction of the polymerization reactions from the accumulated data of the elementary reaction mechanisms and of the monomer structure-reactivity relationships, utilization of water as a reaction medium, and so on. 

The living radical polymerization of some derivatives of St was carried out. The polymerizations of 4-bromostyrene [254], 4-chloromethylstyrene [255, 256], and other derivatives [257] proceed by a living radical polymerization mechanism to give polymers with well-controlled structures and block copolymers with poly(St). The random copolymerization of St with other vinyl... 

Before the mechanism of vinyl polymerization was understood, the question of the structure of vinyl polymers was of considerable interest. Staudinger had written these polymers as having a head-to-tail arrangement of recurring units, but he had not really furnished evidence of the structure. As Carothers once said, Staudinger had assigned the structure by pronouncement. He was as usual correct, and chemical evidence was developed to establish such structures. For example, when monovinyl methyl ketone polymerized, it could produce by head-to-head, tail-to-tail reaction a 1,4-diketone. By head-to-tail polymerization it would give a 1,5-diketone. These two types have different reactions. The study of the polymer proper showed that the polymer was a 1,5-diketone. In the case of polyvinyl chloride, a head-to-head, tail-to-tail polymerization would lead to a 1,2-dihalide compound, and a head-to-tail polymerization would lead to a 1,3-dihalide. 

Possibly the most convincing evidence for positive ion-molecule reactions in polymers is the high rate of decay of vinyl unsaturation during the radiolysis of polyethylene, as recently discussed by Dole, Fallgatter, and Katsuura (13). The ideas of these authors with respect to the carbonium ion mechanism for vinyl decay by means of a dimerization reaction were largely suggested by the mechanisms proposed by Col-linson, Dainton, and Walker (5) for vinyl decay (polymerization) in the radiolysis of n-hexa-l-decene, Reactions 3 and 4 of Table I. 

Another mechanism, conceivable with most monomers, and believed to occur in vinyl acetate polymerization (see Section 10), is transfer with the monomer forming a polymer with an unsaturated end-group capable of copolymerizing, e.g. ... 

In contrast to coordination polymerization, formation of vinyl polymers by radical chain mechanisms is reasonably well understood —at least for the kinds... 

This discussion is not intended to be an exhaustive review of the wood-polymer literature, but rather an overview of the processing procedures used today. In general, the free radicals used for the polymerization reaction come from two sources, temperature sensitive catalysts and Cobalt-60 gamma radiation. In each case a free radical is generated by the process, but from that point the vinyl polymerization mechanism is the same. Each... 

Recently Vandenberg (189) published additional information on vinyl ether polymerizations with Ziegler type catalysts. Details of the catalyst preparations, polymerization conditions and polymer characterization were presented together with an excellent discussion of mechanism. 

T his paper presents a polymerization reaction, as yet unreported in the literature, wherein block polymerization of a free radical type can be caused to take place onto an actively growing chain which had proceeded by an anionic mechanism. Specifically, a Ziegler type of polymerization, such as that of propylene or ethylene, can be interrupted by adding vinylic monomers and an organic peroxide, and a vinyl polymer grown on the end of the polyolefin. For simplicity we will refer to these types as anionic free radical (AFR) polymerizations. 

Similar materials but with different topologies and mechanical behavior have been made by combining epoxy and vinyl or acrylic polymers by conventional graft polymerization. A vinyl or acrylic polymer with or without (3-60) active graft sites is added to an epoxy resin prepolymer, and the epoxy is cured with a greater or lesser extent of grafting to the vinyl polymer. 

Relative to the initiator/activator mechanism shown in Scheme 5, it is interesting to compare vinyl ether polymerizations initiated with the HI/I2 system and with iodine alone. The former system provides living polymers of controlled molecular weights and very narrow MWD [58], whereas the latter has been known for more than a century but fails to give such controlled polymerizations (cf., Sections IV.A) [49,55]. In the iodine-mediated polymerization, iodine serves as both the initiator and activator one molecule of iodine first slowly adds across the vinyl ether double bond to give an adduct. The a-carbon-iodine bond is activated by another molecule of iodine [34,95]. Thus, both systems would in fact form the identical growing chain end [ CH2CH(OR)+.I3 ], and the ob-... 

Chiral polymers have been applied in many areas of research, including chiral separation of organic molecules, asymmetric induction in organic synthesis, and wave guiding in non-linear optics [ 146,147]. Two distinct classes of polymers represent these optically active materials those with induced chirality based on the catalyst and polymerization mechanism and those produced from chiral monomers. Achiral monomers like propylene have been polymerized stereoselectively using chiral initiators or catalysts yielding isotactic, helical polymers [148-150]. On the other hand, polymerization of chiral monomers such as diepoxides, dimethacrylates, diisocyanides, and vinyl ethers yields chiral polymers by incorporation of chirality into the main chain of the polymer or as a pedant side group [151-155]. A number of chiral metathesis catalysts have been made, and they have proven useful in asymmetric ROM as well as in stereospecific polymerization of norbornene and norbornadiene [ 156-159]. This section of the review will focus on the ADMET polymerization of chiral monomers as a method of chiral polymer synthesis. 

When a monomer unit adds to a growing chain it usually does so in a preferred direction (Figure 2-8). Polystyrene, poly(methyl methacrylate) and poly(vinyl chloride) are only a few examples of common polymers where addition is almost exclusively what we call head-to-tail. To illustrate what we mean by this, consider a polymer chain during polymerization. If the mechanism of polymerization is something called chain polymerization, then there will be an active site at the end of this chain to which the next unit will add. We have shown a vinyl polymer with the general structural formula CH2=CXY (Figure 2-9). If the X = H and the Y = Cl, then this would be vinyl chloride. If we label the CI part the tail (T) and the CXY part the head (H), then it is easy to see that this monomer can add to the chain in either of two ways, TH or HT. As mentioned above, in many common polymers, such as polystyrene, addition occurs almost exclusively in a head-to-tail fashion. Obviously, steric fac-... 

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