Vinyl polymerization, illustration

Fig. 9.35 Schematic illustration of the self-condensing vinyl polymerization ATRSIP on planar silica substrates resulting in hyper-branched surface-bonded polymer layers.

Figure 1. Schematic illustration of ring-opening and vinyl polymerization.

Since the initiator concentration remains fairly unchanged in the course of vinyl polymerization, if the initiator efficiency is independent of monomer concentration, first-order kinetics with respect to the monomer is expected. This is indeed observed over a wide extent of reaction for the polymerization of styrene in toluene solution with benzoyl peroxide as initiator (Figure 7.3). The polymerization of certain monomers, either undiluted or in concentrated solution, shows a marked deviation Irom such first-order kinetics. At a certain stage in the polymerization process, there is a considerable increase in both the reaction rate and the molecular weight. This observation is referred to as autoacceleration or gel effect and is illustrated in Figure 7.4 for polymerization of methyl methacrylate at various concentrations of the monomer in benzene. 

A chain reaction without termination produces so-called living polymers. Even if on polymerization the initial monomer is used up, a new monomer can be added in a second step and the polymerization restarted as long as the active sites are not destroyed. The reaction became possible when initiation of vinyl polymerization with anionic mechanism was discovered by Szwarc in 1956 [19]. The process is easy to understand. A fixed number of initiator molecules, N, is added to the monomer under conditions that eliminate termination (i.e., in the absence of water and oxygen). Figure 3.31 illustrates living polymerization with 10 initiator and 42 monomer molecules. Without termination, the reaction stops when all monomers are used up. 

Vinyl polymerization, as illustrated by the above reactions, involves a three-part process, namely initiation, in which is formed an active species capable of starting polymerization of the otherwise unreactive vinyl compound propagation, in which high molecular weight polymer is formed termination, in which deactivation occurs to produce the final stable polymer. The active species in vinyl polymerizations may be of three different types, namely free radicals, anions and cations and these possibilities give rise to three distinct methods of accomplishing polymerization. 

Good amdaticHi of reaction rate coeffidents over the very wide temperature range represented by Table 5 imposes a severe demand on the e qierimental data. Results obtained for other vinyl polymerizations have, in general, not been analyzed over so broad a temperature range, and attempts to assign a definite cause to the discrepancies illustrated in Table 5 would therefore be premature. 

The configuration about an asymmetric carbon atom in a polymer is determined at the stage of monomer addition. This is illustrated in the following representation of the propagation reaction in a free radical vinyl polymerization, wherein tacticity is determined by the mode of presentation of monomer units ... 

Photopolymerization reactions of monolayers have become of interest (note Chapter XV). Lando and co-workers have studied the UV polymerization of 16-heptadecenoic acid [311] and vinyl stearate [312] monolayers. Particularly interesting is the UV polymerization of long-chain diacetylenes. As illustrated in Fig. IV-30, a zipperlike process can occur if the molecular orientation in the film is just right (e.g., polymerization does not occur readily in the neat liquid) (see Refs. 313-315). 

Positional isomerism is conveniently illustrated by considering the polymerization of a vinyl monomer. In such a reaction, the adding monomer may become attached to the growing chain in either of two orientations ... 

VEs do not readily enter into copolymerization by simple cationic polymerization techniques instead, they can be mixed randomly or in blocks with the aid of living polymerization methods. This is on account of the differences in reactivity, resulting in significant rate differentials. Consequendy, reactivity ratios must be taken into account if random copolymers, instead of mixtures of homopolymers, are to be obtained by standard cationic polymeriza tion (50,51). Table 5 illustrates this situation for butyl vinyl ether (BVE) copolymerized with other VEs. The rate constants of polymerization (kp) can differ by one or two orders of magnitude, resulting in homopolymerization of each monomer or incorporation of the faster monomer, followed by the slower (assuming no chain transfer). 

Scheme 12. Schematic structure of vinyl-eb-PDMS chains (dashed line) crosslinked with polymeric TMS-eb-PHMS through the hydrosilylation cure reaction. For illustration purpose the PDMS chains in this scheme are shorter and less abundant relative to PHMS than in real system.

In this paper, the pseudo-kinetic rate constant method in which the kinetic treatment of a multicomponent polymerization reduces to that of a hcmopolymerization is extensively applied for the statistical copolymerization of vinyl/divinyl monomers and applications to the pre- and post-gelation periods are illustrated. 

The polymerization of vinyl monomers by transition metal sigma complexes has been shown by Ballard and van Lienden (25,28) to be catalyzed by white light which has been filtered through pyrex glass. The effect is best illustrated by the following experiment ... 

During polymerization, a polymeric radical with a perfluoro(alkyl vinyl ether)-derived active center can have one of two fates it can cross-propagate to tetrafluoroethylene or it can undergo P-scission to yield an acid-fluoride-terminated polymer chain and generate a peduoroalkyl radical capable of initiating further polymerization (ie., chain transfer to monomer). These scenarios are illustrated in Scheme 3. 

The initiation reaction in the polymerization of vinyl ethers by BF3R20 (R20 = various dialkyl ethers and tetrahydrofuran) was shown by Eley to involve an alkyl ion from the dialkyl ether, which therefore acts as a (necessary) co-catalyst [35, 67]. This initiation by an alkyl ion from a BF3-ether complex means that the alkyl vinyl ethers are so much more basic than the mono-olefins, that they can abstract alkylium ions from the boron fluoride etherate. This difference in basicity is also illustrated by the observations that triethoxonium fluoroborate, Et30+BF4", will not polymerise isobutene [68] but polymerises w-butyl vinyl ether instantaneously [69]. It was also shown [67] that in an extremely dry system boron fluoride will not catalyse the polymerization of alkyl vinyl ethers in hydrocarbons thus, an earlier suggestion that an alkyl vinyl ether might act as its own co-catalyst [30] was shown to be invalid, at least under these conditions. 

Initiation of a free radical chain takes place by addition of a free radical (R ) to a vinyl monomer (Equation 6.8). Polystyrene (PS) will be used to illustrate the typical reaction sequences. (Styrene, like many aromatic compounds, is toxic, and concentrations that come into contact with us should be severely limited.) It is important to note that the free radical (R ) is a companion of all polymerizing species and is part of the polymer chain acting as an end group and hence should not be called a catalyst even though it is often referred to as such. It is most properly referred to as an initiator. 

Polymerization of N-vinylimidazole deviates from the polymerization of conventional vinyl polymers such as styrene or methyl methacrylate. The process leads usually to low molecular weight products, which is due to degradative addition. This process can be illustrated by the reaction ... 

Figure 4.3 Scheme illustrating the development of immobilized ionic liquids by thermally induced free radical polymerization of vinyl-substituted imidazolium-based monocationic and dicationic monomers. 

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