Free-radical homopolymerization

The homopolymerization of o)-(4-vinylbenzyl)polystyrene macromonomers was also investigated kinetically by Asami21) under quite different conditions, namely very high amounts of initiator and high overall concentrations. Thus, the molecular weights (even if underestimated) are very low. Under these conditions, the rate of polymerization does not depend on the length of the side chains. However, these particular conditions which favour initiation and termination processes cannot be illustrative of regular polymerizations. 

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We saw in the last chapter that the stationary-state approximation is apphc-able to free-radical homopolymerizations, and the same is true of copolymerizations. Of course, it takes a brief time for the stationary-state radical concentration to be reached, but this period is insignificant compared to the total duration of a polymerization reaction. If the total concentration of radicals is constant, this means that the rate of crossover between the different types of terminal units is also equal, or that R... 

Note that this inquiry into copolymer propagation rates also increases our understanding of the differences in free-radical homopolymerization rates. It will be recalled that in Sec. 6.1 a discussion of this aspect of homopolymerization was deferred until copolymerization was introduced. The trends under consideration enable us to make some sense out of the rate constants for propagation in free-radical homopolymerization as well. For example, in Table 6.4 we see that kp values at 60°C for vinyl acetate and styrene are 2300 and 165 liter mol sec respectively. The relative magnitude of these constants can be understod in terms of the sequence above. 

A new rate model for free radical homopolymerization which accounts for diffusion-controlled termination and propagation, and which gives a limiting conversion, has been developed based on ft ee-volume theory concepts. The model gives excellent agreement with measured rate data for bulk and solution polymerization of MMA over wide ranges of temperature and initiator and solvent concentrations. 

PVPA was prepared by the free-radical homopolymerization of vinyl-phosphonyl dichloride using azobisisobutyronitrile as initiator in a chlorinated solvent. The poly(vinylphosphonyl chloride) formed was then hydrolysed to PVPA (Ellis, 1989). No values are available for the apparent pA s of PVPA, but unpolymerized dibasic phosphonic acids have and values similar to those of orthophosphoric acid, i.e. 2 and 8 (Van Wazer, 1958). They are thus stronger acids than acrylic acid, which as a pK of 4-25, and it is to be expected that PVPA will be a stronger and more reactive acid than poly(acrylic acid). 

Several applications of hyperbranched polymers as precursors for synthesis of crosslinked materials have been reported [91-97] but systematic studies of crosslinking kinetics, gelation, network formation and network properties are still missing. These studies include application of hyperbranched aliphatic polyesters as hydroxy group containing precursors in alkyd resins by which the hardness of alkyd films was improved [94], Several studies involved the modification of hyperbranched polyesters to introduce polymerizable unsaturated C=C double bonds (maleate or acrylic groups). A crosslinked network was formed by free-radical homopolymerization or copolymerization. 

F. Vargas, J. Alvarez, and R. Suarez. Nonlinear study of the periodic operation for free-radical homopolymerization reactors. In IEEE Int. Conf. Control Applications, volume 1, pages 84-89, 1989. 

Compare and contrast the nature of the steady-state assumption used to describe the-kinetics of free radical homopolymerizations and copolymerizations. 

Furthermore, we have reported on the first examples of free radical homopolymerization and copolymerization of 2,3,4,5,6-pentafluorostyrene (3) with styrene (5) and its derivative 4-((V-adamantylamino)-2,3,5,6-tetrafluorostyrene (4) in aqueous solution via the host-guest complexation with Me-p-CD using water-soluble initiators (Fig. 5) [31]. The fluorinated monomers (3 and 4) and styrene (5) were complexed by Me-p-CD in water. The stoichiometries of the host-guest complexes were determined by NMR spectroscopy according to the Job method [32-34], It was clearly shown that styrene (5) forms a defined 1 1 complex while the fluorinated monomers (3 and 4) are encapsulated by two Me-p-CD molecules. In the case of fluorinated monomer (3) this result was not expected since 3 and 5 have the same molecular scaffold and molecular modelling showed that the fluorinated styrene derivative 3 is only slightly bigger than styrene itself (Fig. 6) [35],... 

Furthermore, a polycarboxybetaine with a peptide main chain was synthesized from poly(methyl L-glutamate) [57]. Several zwitterionic monomers based on isobutylene with long alkyl spacers between the polymerizable group and the zwitterionic moiety exhibit surfactant properties. They do not undergo free radical homopolymerization, but are copolymerizable with other monomers [58]. 

Hyperbranched polymers were synthesized by direct free-radical polymerization of ethylene glycol dimethacrylate monomer in the presence of a CCT catalyst. The free-radical homopolymerization of divinyl monomers is thought to selectively yield trimer 96,32i 322 though previous work on oligomer distributions would indicate that this is unlikely. 

Bulk addition multipolymerization kinetics occurs when two monomers are employed. Bulk free-radical homopolymerizations and copolymerizations that are implemented in REX include a) styrene-acrylonitrile, styrene-methyl methacrylate, styrene-acrylamide b) methyl methacrylate-acrylonitrile, ABS c) acrylate ester mixtures d) ethyl acrylate-methacrylic acid and mixtures with other monomers e) methyl methacrylate f) 8-caprolactone and, n-isopropylacrylamide-acrylic... 

Some effort must be made to control simultaneous free-radical homopolymerization of the acrylates. This can be a complicating factor in analyses and has at times led to claims of copolymerization when two homopolymers were formed. Ittel, S. D. Personal observation. 

Case Study 2 Comparison of Mathematical Models FOR Free Radical Homopolymerization of Vinyl Monomers in scCOj In this case study, a comparison of performance of the different kinetic models proposed in the literature for dispersion polymerization of styrene and MMA in SCCO2 is presented. The models used by Quintero-Ortega et al. [43] (models 1 and 2) and those presented by the groups of Kiparissides [47] (model 3) and Morbidelli... 

When a similar polymerization was carried out between — 3 and + 9°C, the polymer that was isolated had no optical activity. Free-radical homopolymerizations of this and related monomers led to inactive polymers, presumably because of the loss of asymmetry during possible hydrogen shifts when free radicals started to propagate. 

Solvents influence the rate of free-radical homopolymerization of acrylic acid and its copolymerization with other monomers. Hydrogen-bonding solvents slow down the reaction rates. Due to the electron-withdrawing nature of the ester groups, acrylic and methacrylic ester polymerize by anionic but not by cationic mechanisms. Lithium alkyls are very effective initiators of a-methyl methacrylate polymerization yielding stereospecific polymers.Isotactic poly(methyl methacrylate) forms in hydrocarbon solvents. Block copolymers of isotactic and syndiotactic poly(methyl methacrylate) form in solvents of medium polarity. Syndiotactic polymers form in polar solvents, like ethylene glycol dimethyl ether, or pyridine. This solvent influence is related to Lewis basicity in the following order ... 

The free radical homopolymerization of maaomonomers can be applied to construa core-shell molecular brushes. When maaomonomers contain a diblock copolymer chain, the polymerization of maaomonomers will simultaneously hold one block (close to the vinyl group) in the core and another block as the shell, rendering a core-shell structured molecular brush. A typical process to prepare amphipolar... 

Scheme 3.2 Basic free-radical homopolymerization mechanism.

The monomeric a-D-fructose vinyl ether has been prepared by esterification between a-o-fructose and ethyl vinyl ether in the presence of traces of p-TsOH, which is free-radical homopolymerized. Its copolymers with vinyl ether A -vinylpyrrolidone have also been prepared (Scheme 5.12) [61],... 

Radical-solvent complexes are more difficult to detect spectroscopically however, they do provide a plausible explanation for many of the solvent effects observed in free-radical homopolymerization—particularly those involving unstable radical intermediates (such as vinyl acetate) where complexation can lead to stabilization. For instance, Kamachi (50) observed that the homopropagation rate of vinyl acetate in a variety of aromatic solvents was correlated with the calculated delocalization stabilization energy for complexes between the radical and solvent. If such solvent effects are detected in the homopolymerization of one or both of the comonomers, then they are likely to be present in the copolymerization systems as well. Indeed, radical-complex models have been invoked to explain solvent effects in the copolymerization of vinyl acetate with acrylic acid (51). Radical-solvent complexes are probably not restricted merely to systems with highly unstable propagating radicals. In fact, radical-solvent complexes have even been proposed to explain the effects of some solvents (such as benzyl alcohol, A7 / 7 -dimethyl for-mamide, and acetonitrile) on the homo- and/or copolymerizations of styrene and methyl methacrylate (52-54). Certainly, radical-solvent complexes should be considered in systems where there is a demonstrable solvent effect in the copolymerizations and/or in the respective homopolymerizations. 

An original monomer derived from fructofuranose, with unsaturation at 5,6-exo-cyclic position was synthesized through a four-step reaction. The reactivity of the corresponding 5,6-exo-fructene in free-radical homopolymerization as well as in copolymerization was studied in some detail. Even if this cyclic vinyl ether behaves poorly in homopolymerization (low yields and low MW), it was shown to react conveniently in copolymerization with allgrl acrylates (Scheme 4), allowing for the... 

The free-radical homopolymerization of vinyl ether monomers can be accomplished using various peroxide [131], azo [132], and redox initiators [133]. Polymerization under free-radical conditions gives only low-molecular-weight oligomers which have reported uses as... 

The copolymerization of a,p-unsaturated ketones has been studied extensively in order to improve the poor chemical and thermal stability exhibited by the homopolymers. The vinyl ketones have been copolymerized with most of the common vinyl and diene monomers. The data are given in Ref. [326]. For initiation, the same reagents could be used as for free-radical homopolymerization. Copolymerization was carried out in bulk [371] and in emulsion systems [372]. In copolymerization with methyl methacrylate, vinyl acetate [373], and styrene [371] it was concluded that the relative reactivities of the vinyl ketones increase with the increasing electron-withdrawing nature of the vinyl ketone substituent. Polar and steric effects are not observed. Most of the work has been directed toward the preparation of oil- and solvent-resistant rubbers to replaee styrene-butadiene rubber. Emulsion eopolymerization of butadiene with methyl isopropenyl ketone yielded rubbers with good solvent resistance and low temperature flexibility, but the products tended to harden on storage and were not compatible with natural rubber [374]. The reactive earbonyl function caused sensitivity to alkine reagents. Copolymers of butylacrylate and methyl vinyl ketone, for example, can be erosslinked by treatment with hydrazine [375]. 

In contrast to MATRIF monomer, 2,2,2-Trifluoroethyl acrylate (ATRIF) and its (co)polymers have not been extensively studied. Free-radical homopolymerization of ATRIF and other fluorinated acrylates was carried out under various conditions and the stereostructures of the obtained polymers were investigated [65]. ATRIF monomer seemed promising to form thin films that have high hydrophobicity and good barrier properties [66]. In 1985, Narita et al. [67] reported the radical copolymerization of ATRIF with styrene, and the assessed reactivity ratios. To study the incorporation... 

Twin Screw Extruders as Polymerization Reactors for a Free-Radical Homopolymerization... 

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