Styrene macroradicals

The mode of termination varies with monomer and reaction conditions. While styrene macroradicals typically terminate by coupling, methyl methacrylate macroradicals terminate by coupling at temperatures below 60°C, but by disproportionation at higher temperatures. 

Fig. 2. Reactivity of monomers with n-n conjugation (a) with styrene macroradical and (b) with methacrylate macroradical. O, Ethyl vinyl sulphide JV-vinylcarbazole , methyl vinyl sulphide.

Among the copolymerizations of vinylazole complexes, a successful process with ZnCl2-l-vinylbenzimidazole (VBI) or 1-vinylbenztriazole (VBT) adducts (styrene as the comonomer) (Table 4-10) may be mentioned. It is evident that the activity of the VBI and VBT complexes in the interaction with a styrene macroradical (l/ i) is lower than that for an uncoordinated ligand. 

In a similar preparation styrene macroradicals have been reported when styrene was polymerized in viscous solvents and viscous poor solvents(130). When styrene was added to these macroradicals, an increase in relative viscosity was noted. Block like copolymers were formed when either acrylonitrile, methyl methacrylate, ethyl acrylate or methacrylate was added to the macroradicals. The fastest rates of polymerization and greatest yields of copolymer were noted when polystyrene was pol37merized in viscous, poor solvent (131). ... 

In the case of AB diblock copolymers prepared by the RAFT technique, the order of monomer addition must be taken into account. A characteristic example of such a block copolymer synthesis is demonstrated in Scheme 19. Initially, a poly(N, N-dimethylacrylamide) (PDMA) macro-CTA was prepared, followed by the use of PDMA-CTA as an initiator to polymerize successfully the second monomer N,N-dimethyl vinyl benzy-lamide (DMVBA). The final diblock copolymer is not contaminated with homopolymer. It has been discovered that the reverse approach is impossible, probably due to the slow fragmentation of the intermediate radical or due to the slow initiation efficiency of the intermediate radical (styrenic macroradical). 

The radiochemical oxidation of PS in a chloroform solution is accompanied by its destruction and formation of products of styrene oxidation, namely, benzaldehyde and styrene oxide [136]. The radiochemical yield of these products was equal to the radiochemical yield of PS macromolecule cleavages. Butyagin [137] analyzed the products of decomposition of the peroxyl radicals of PS and polyvinyIcyclohexane. Alkyl macroradicals were produced mechano- or photochemically, volatile products were evaporated in vacuum, and alkyl radicals were converted into peroxyl radicals using labeled lsO. Peroxyl radicals were then... 

The resonance stability of the macroradical is an important factor in polymer propagation. Thus, for free radical polymerization, a conjugated monomer such as styrene is at least 30 times as apt to form a resonance-stabilized macroradical as VAc, resulting in a copolymer rich in styrene-derived units when these two are copolymerized. 

Poly (methyl methacrylate) was also subjected to mechanical reaction in a vibrating mill in common solvent for several monomers (ethylene, acrylic acid and its esters, acrylonitrile and styrene) at temperatures from —196 to 20° C (22). The formation of macroradicals and their reactions were followed by EPR (electron paramagnetic resonance). The macroradicals reacted with vinyl monomers at temperatures less than —100° C, while quinones underwent reaction as low as —196° C. The same experiments were performed also with polystyrene and polybutylenedimethacrylate. The radicals from polystyrene were more reactive than those from poly(methyl methacrylate). 

The macroradicals of natural rubber react with those of the styrene elastomer, due to the presence of the very reactive 1,2 pendent vinyl groups in the latter. This mechanism leads to a structure where the styrene rubber forms a gel network with grafted branches of natural rubber. 

Berlin and coworkers (5,56) desired to obtain a material with an increased mechanical strength. They carried out a plasticization of bulk ami emulsion polystyrene molecular weight 80000 and 200000 respectively at 150-160° C, with polyisobutylene, butyl rubber, polychloroprene, polybutadiene, styrene rubber (SKS-30) and nitrile rubber (SKN 18 and SKN 40). The best results were obtained with the blends polystyrene-styrene rubber and polystyrene-nitrile rubber. An increase of rubber content above 20-25% was not useful, as the strength properties were lowered. An increase in the content of the polar comonomer, acrylonitrile, prevents the reaction with polystyrene and decreases the probability of macroradical combination. This feature lowers the strength, see Fig. 14. It was also observed that certain dyes acts as macroradical acceptors, due to the mobile atoms of hydrogen of halogens in the dye, AX ... 

In the poly(methyl methaerylate)-styrene system, less than 7% of the original polymer remained as homopolymer at total conversion (77). Over 85% of the product was non-branched, single-segment block copolymer. The difference for these two systems is in part due to the higher molecular weight of the initial poiy(methyl methycrylate) (2900000 versus 495000) and in part to the preferential scission of the poly(methyl methacrylate) chain. This point was confirmed by running tests on a mixture of the two homopolymers in the presence of a radical acceptor to prevent macroradical recombination, and on the isolated block copolymers. 

The mechanical degradation and production of macroradicals can also be performed by mastication of polymers brought into a rubbery state by admixture with monomer several monomer-polymer systems were examined (10, 11). This technique was for instance studied for the cold mastication of natural rubber or butadiene copolymers in the presence of a vinyl monomer (13, 31, 52). The polymerization of methyl methacrylate or styrene during the mastication of natural rubber has yielded copolymers which remain soluble up to complete polymerization vinyl acetate, which could not produce graft copolymers by the chain transfer technique, failed also in this mastication procedure. Block and graft copolymers were also prepared by cross-addition of the macroradicals generated by the cold milling and mastication of mixtures of various elastomers and polymers, such as natural rubber/polymethyl methacrylate (74), natural rubber/butadiene-styrene rubbers (76) and even phenol-formaldehyde resin/nitrile rubber (125). 

If the diazonium groups result from the diazotation of poly-/>-amino-styrene, the macroradicals will initiate grafting. Contrarily, if >-(N-acetyl) phenylenediamine is diazotized and used as initiator of a first monomer, a polymer is obtained with an acetamino. phenyl end group (-CGH4-NH-Ac). After hydrolysis of this last and diazotation of the free amine group, the polymeric terminal diazonium salt can be used with ferrous ions for the synthesis of block copolymers. 

The macroradicals obtained by the copolymerization of equimolar quantities of maleic anhydride and styrene were also used as initiators to form higher molecular weight copolymers and to prepare block copolymers. These macroradicals were effective as initiators after being stored for 180 hours at —20°C in an oxygen-free atmosphere. However,... 

As shown by the gel permeation chromatograph in Figure 6, the average molecular weight of poly(styrene-co-maleic anhydride) obtained by adding the macroradical to a benzene solution of the monomers was over 250,000. No copolymer was obtained under comparable conditions in the absence of the macroradicals. Attempts to use these macroradicals to produce copolymers in an acetone solution were unsuccessful. 

Macroradicals obtained by the copolymerization of equimolar quantities of styrene and maleic anhydride in benzene or in cumene were also used as initiators to produce block copolymers with methyl methacrylate, ethyl methacrylate, and methyl acrylate. The yields of these block copolymers were less than those obtained with styrene, but as much as 38% of methyl methacrylate present in the benzene solution added to the macroradical to produce a block copolymer. The amount of ethyl methacrylate and methyl acrylate that was abstracted from the solution to form block copolymers was 35 and 20%. 

The formation of block copolymers from styrene-maleic anhydride and acrylic monomers was also indicated by pyrolytic gas chromatography and infrared spectroscopy. A comparison of the pyrograms of the block copolymers in Figure 7 shows peaks comparable with those obtained when mixtures of the acrylate polymers and poly(styrene-co-maleic anhydride) were pyrolyzed. A characteristic infrared spectrum was observed for the product obtained when macroradicals were added to a solution of methyl methacrylate in benzene. The characteristic bands for methyl methacrylate (MM) are noted on this spectogram in Figure 8. 

Macroradicals obtained by the heterogeneous copolymerization of styrene and maleic anhydride in poor solvents such as benzene were used to initiate further polymerization of selected monomers. This technique was used to produce higher molecular weight alternating copolymers of styrene and maleic anhydride and block copolymers. Evidence for the block copolymers was based op molecular weight increase, solubility, differential thermal analysis, pyrolytic gas chromatography, and infrared spectroscopy. 

The analysis of the reaction serum (the continuous phase without polymer particles) at the end of polymerization led to the conclusion that the molecular weight of the soluble oligomers of styrene and PEO macromonomer varied from 200 to 1100 g mol-1. This indicates that the critical degree of polymerization for precipitation of oligomers in this medium is more than ten styrene units and only one macromonomer unit per copolymer chain. Several reasons for the low molecular weight of the soluble copolymers were proposed, such as the thermodynamic repulsion (or compatibility) between the PEO chain of the macromonomer and the polystyrene macroradical, the occurrence of enhanced termination caused by high radical concentration, and, to a lower extent, a transfer reaction to ethanol [75]. 

Compound 35 contains a thermolabile C-C bond which, on thermally induced fragmentation, yields a high proportion of macroradicals. The PDMS diradical also acts as a counter radical and can undergo chain extension at both ends in the presence of vinylic monomers (acrylonitrile, maleic anhydride, diethylfumarate) or styrenic monomers, leading to diblock copolymers in 95% yield according to the following scheme [211, 212] ... 

It appears that within each group, the reactivity of monomers towards the styrene radical increases with both q and e (see Chap. 5, Sect. 5.2). Higher q values correspond to greater resonance stabilization of the newly formed radical growth in e is connected with the interaction of the easily polarizable macroradical with the / carbon of the monomer whose electronegativity is increased. 

Ludwico and Rosen [23] polymerized styrene in solution in the presence of a known amount of polystyrene of known molecular mass. The value of k again reached a maximum its decrease was more or less linear. The effect of the molar mass of macroradicals and of added polystyrene was small in the stage of decreasing kr... 

The expression for u [eqn. (53)] indicates how important it is to consider primary radical dissociation [the term 1fed/(ki[M])] when correct values of kt pr/fkjkp) are to be obtained, even when termination of macroradicals by secondary phenyl radicals is neglected. In Fig. 6, a graphical representation of eqn. (55) for styrene polymerization with dibenzoylperoxide in benzene is shown. When this dependence is measured for two monomer concentrations, fct pr/(kikp) and kj/kj can be calculated from the slope u. To reduce error,... 

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