Monomers vinyl aromatics

In order to increase the solubiUty parameter of CPD-based resins, vinyl aromatic compounds, as well as other polar monomers, have been copolymerized with CPD. Indene and styrene are two common aromatic streams used to modify cyclodiene-based resins. They may be used as pure monomers or contained in aromatic steam cracked petroleum fractions. Addition of indene at the expense of DCPD in a thermal polymerization has been found to lower the yield and softening point of the resin (55). CompatibiUty of a resin with ethylene—vinyl acetate (EVA) copolymers, which are used in hot melt adhesive appHcations, may be improved by the copolymerization of aromatic monomers with CPD. As with other thermally polymerized CPD-based resins, aromatic modified thermal resins may be hydrogenated. 

Certain monomers may act as inhibitors in some circumstances. Reactivity ratios for VAc-S copolymerization (r< 0.02, rVu -2.3) and rates of cross propagation are such that small amounts of S are an effective inhibitor of VAc polymerization. The propagating chain with a terminal VAc is very active towards S and adds even when S is present in small amounts. The propagating radical with S adds to VAc only slowly. Other vinyl aromatics also inhibit VAc polymerization.174... 

Polymerization Aliphatic, aromatic and oxygenated monomers Vinyl chloride Isoprene Acrylonitrile Catalyst activation... 

A number of ex situ spectroscopic techniques, multinuclear NMR, IR, EXAFS, UV-vis, have contributed to rationalise the overall mechanism of the copolymerisation as well as specific aspects related to the nature of the unsaturated monomer (ethene, 1-alkenes, vinyl aromatics, cyclic alkenes, allenes). Valuable information on the initiation, propagation and termination steps has been provided by end-group analysis of the polyketone products, by labelling experiments of the catalyst precursors and solvents either with deuterated compounds or with easily identifiable functional groups, by X-ray diffraction analysis of precursors, model compounds and products, and by kinetic and thermodynamic studies of model reactions. The structure of some catalysis resting states and several catalyst deactivation paths have been traced. There is little doubt, however, that the most spectacular mechanistic breakthroughs have been obtained from in situ spectroscopic studies. 

Only a limited number of monomer pairs form block copolymers in this manner. Examples are conjugated dienes and vinyl aromatics that have similar Q-e values. The nature of the anionic initiator, i.e., the ionic character of the carbon-metal bond plays an important role in both the amount and sequence of block formation. For instance, when potassium or cesium initiators are used, styrene polymerizes first as can be seen in Figure 12. 

Danusso extended the above work with various substituted vinyl aromatic monomers (304) and discussed the reaction mechanisms based on these results (277). Some of the conclusions which are pertinent to this discussion are summarized below. 

Instead of block copolymers, the use of pseudo-random linear copolymers of an aliphatic a-olefin and a vinyl aromatic monomer has been reported [20], where the styrene content of the polymer must be higher than 40 wt%. Preferred are styrene and ethylene copolymers. These blends may contain, amongst other things, an elastomeric olefinic impact modifier such as homopolymers and copolymers of a-olefins. Presumably the styrene-ethylene copolymer acts as a polymer emulsifier for the olefinic impact modifier. Using 5 wt% of an ethylene-styrene (30 70) copolymer and 20% of an ethylene-octene impact modifier in sPS, a tensile elongation (ASTM D638) of 25 % was obtained. 

Studies of ethylene-vinyl aromatic monomer polymerizations continue to be published. Chung and Lu reported the synthesis of copolymers of ethylene and P-methylstyrene [28] and the same group extended these studies to produce and characterize elastomeric terpolymers which further include propylene and 1-octene as the additional monomers [29,30]. Returning to the subject of alternative molecular architectures for copolymers, Hou et al. [31] has reported the ability of samarium (II) complexes to copolymerize ethylene and styrene into block copolymers. 

We have not commented on the criteria determining the actual polymerisability of a monomer by cationic initiation, since they are well known The present review deals with monomers belonging to the n-donor (olefins, dienes, vinyl aromatics) and n-and n-donor (vinyl ethers, vinyl amines, vinyl sulphides) families, but not with purely n-donor and 6-donor monomers. 

It is of interest to note that within the limit of acoiracy of these experiments, monomer decay curves (Fig. 22) were single exponential, whereas Sdieme 1 predicts dual exponentiality (Eq. 65). The results thus imply that in pdyslyrene reverse dissociation ( feedback ) of the excimer is not of importance. This point is amplified by time-resolved fluorescence spectra which show that late- ted >ecti a (see experimental section) are composed exclusively of excimer emission (Fig. 23). The same is true in poly(a-methylstyrene) In view of more recent work mi other vinyl aromatic ptdymers, it would be of interest to study pdy(styrene) further with more sophisticated techniques. 

We conclude that interpretation of even the most precise experiments on fluorescence decay in synthetic polymers will be difficult, given the complexities of such systems. However, with physically well-defined systems, adequate discrimination between models should be possible. In less well defined systems, such as co-polymers of vinyl aromatic monomers in fluid solution, discrimination between models may be more intractable, but some progress can be made if extensive, and complete, experiments are attempted. 

The role of sulphur-containing compounds in photopolymerization appears to have attracted some interest. Bis(j -methylpyridazinyl)-3,3 -disulphide has been found to initiate the photopolymerization of styrene but inhibits the thermal polymerization. The role of thiyl radicals (PhS-) in photoinitiated polymerization of vinyl monomers by aromatic thio-compounds has been postulated by several workers. In one study, flash photolysis was used to identify the nature of the radical. Sulphur-containing monomers such as 4-methyl-2-(vinylthio)thiazole and thiocyclanes have been photopolymerized and copolymerized with other vinyl monomers. Luca et al. have devised a mathematical model for the photopolymerization of 2,3-dimethylbutadiene and thiourea. 

Ionic polymerization systems of commercial importance employ mostly batch and continuous solution polymerization processes. Suitable monomers for ionic polymerization include conjugated dienes and vinyl aromatic. Among these, the anionic polymerization of styrene-butadiene (SB) and styrene-isoprene (SI) copolymers and the cationic polymerization of styrene are the most commercially important systems. 

More recently, a US Patent has been issued to the Goodyear Tire Rubber Company, which claimed that polar functional monomers could be copolymerized with conjugated dienes and vinyl aromatics to chemically modify the polymer chain. Functional monomers, such as 3-(2-pyrrolidinoethyl) styrene ... 

Cationic polymerizations are started by reaction of electrophilic initiator cations with electron-donating monomer molecules. Catalysts are Lewis acids and Friedel-Crafts catalysts, such as aluminum trichloride (AICI3), and strong acids, such as sulfuric acid (H2SO4). Monomer molecules able to undergo cationic polymerization include electron-rich olefins, such as vinyl aromatics and vinyl ethers, and ring compounds, such as ethylene oxide and tetrahydrofuran. 

Water-white resins are a recent commercial development. They are made by the homo- or copolymerization of pure vinyl aromatic monomers such as styrene or alpha-methylstyrene. Polymerization is carried out in aromatic solvents using BF3 or complexes of BF3 at 15-40°C. A controlled amount of water is added in order to improve initiation efficiency. The molecular weight of the product is low as a result of the extensive chain transfer processes at this temperature. Water-white resins are used where color stability is important. 

Many studies have focused on catalysts that could potentially copolymerize ethylene and vinyl aromatic monomers, together with the associated polymerization chemistry and chemical analyses of the produced polymers. It is evident from a number of references (eg 12-14) that the catalyst structure and polymerization conditions, such as temperature and monomer feed ratios, have major influences on the reaction product in terms of production efficiency, product composition (copolymer, homopolymer contents), and copolsrmer microstructure, including stereoregularity. 

Natta, G Danusso, R Sianesi, D. Stereospecific polymerization and isotactic polymers of vinyl aromatic monomers. Macromol. Chem. 1958, 28, 253-261. 

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