Homopolymerizations acrylonitrile

Polyacrylonitrile, which is a semicrystalline polymer, can be used for many engineering applications, such as fiber spinning or for housing and package applications. A peculiarity of polyacrylonitrile is that it is insoluble in its monomer. This makes it very difficult to homopolymerize acrylonitrile in an emulsion polymerization process since nucleated polymer particles cannot grow by monomer swelling. 

Fig. 22 DTA-curves of commercial PAN fibres in air (A) and nitrogen (B) (heating rate 10 oc/min), a) Dry spun homopolymeric acrylonitrile (DRALON T) b) wet spun co-polymeric acrylonitrile (DOLAN 6% methylacrylate) (31).

Several radical copolymerizations of vinyl 2-furoate with well-known monomers (50 50) were also studied. Complete inhibition was obtained with vinyl acetate, very strong retardation with styrene, vinyl chloride and acrylonitrile methyl methacrylate homopolymerized without appreciable decrease in rate. It is evident that the degree of retardation that vinyl 2-furoate imposes upon the other monomer depends on the stability of the latter s free radical. With styrene and vinyl chloride the small amounts of fairly low molecular-weight products contained units from vinyl 2-furoate which had entered the chain both through the vinyl bond and through the ring (infrared band at 1640 cm-1). 

Various mechanisms have been proposed to explain the initiation processes. The self-initiated copolymerizations of the monomer pairs S-MMA and S-AN proceed at substantially faster rates than pure S polymerization. For S-AN333 and S-MAHJJ the mechanism of initiation was proposed to be analogous to that of S homopolymerization (Scheme 3.62) but with acrylonitrile acting as the dicnophile in the formation of the Diels-Alder adduct (Scheme 3.66). 

Propen-l-ol. See Allyl alcohol 2-Propenal. See Acrolein 2-Propenamide. See Acrylamide Propene, copolymerizations of, 16 111 Propene homopolymerization, 16 104-110 Propene polymerization, 16 94, 99 2-Propenenitrile. See Acrylonitrile (AN) Propenoic acid, physical properties, 5 31t Propenoic acid nitrile. See Acrylonitrile (AN)... 

Bell, 1989 Rhee and Bell, 1991), random copolymers of methyl acrylate and acrylonitrile were directly polymerized onto the carbon fiber surface. Dimethyl formamide, dimethyl sulfoxide and distilled water proved to be useful as solvents for this process. Polymerization can take place on the carbon fiber electrode, with initial wetting of the fiber surface leading to better adhesion of the polymer formed. The structure and properties of the polymer can be varied by employing different vinyl and cyclic monomers in homopolymerization. Chemical bond can also be formed, such as polymer grafting to the carbon fiber surface. 

Although the reactivity of 1,2-disubstituted ethylenes in copolymerization is low, it is still much greater than their reactivity in homopolymerization. It was observed in Sec. 3-9b-3 that the steric hinderance between a P-substituent on the attacking radical and a substituent on the monomer is responsible for the inability of 1,2-disubstituted ethylenes to homopolymerize. The reactivity of 1,2-disubstituted ethylenes toward copolymerization is due to the lack of P-substituents on the attacking radicals (e.g., the styrene, acrylonitrile, and vinyl acetate radicals). 

Many reactions have been performed in the presence of a solvent. However, the solvent must be chosen carefully to avoid reaction with polymer. For example, the low yield for grafts of acrylonitrile on polyamides in the presence of methanol has been shown to be due to the methanolysis (18,31). Generally speaking, the grafted products are principally obtained however minor amounts and homopolymers can also result. The homopolymerization proceeds by an intramolecular transfer reaction between macroradicals and monomers. The amount of homopolymer depends on the system. Details on systems already investigated will be described in the next section. 

Homopolymerization. The free-radical polymerization of VDC has been carried out by solution, slurry, suspension, and emulsion methods. Slurry polymerizations are usually used only in the laboratory. The heterogeneity of the reaction makes stirring and heat transfer difficult consequently, these reactions cannot be easily controlled on a large scale. Aqueous emulsion or suspension reactions are preferred for large-scale operations. The spontaneous polymerization of VDC, so often observed when the monomer is stored at room temperature, is caused by peroxides formed from the reaction of VDC with oxygen, fery pure monomer does not polymerize under these conditions. Heterogeneous polymerization is characteristic of a number of monomers, including vinyl chloride and acrylonitrile. 

The copolymerization equation is valid if all propagation steps are irreversible. If reversibility occurs, a more complex equation can be derived. If the equilibrium constants depend on the length of the monomer sequence (penultimate effect), further changes must be introduced into the equations. Where the polymerization is subjected to an equilibrium, a-methylstyrene was chosen as monomer. The polymerization was carried out by radical initiation. With methyl methacrylate as comonomer the equilibrium constants are found to be independent of the sequence length. Between 100° and 150°C the reversibilities of the homopolymerization step of methyl methacrylate and of the alternating steps are taken into account. With acrylonitrile as comonomer the dependence of equilibrium constants on the length of sequence must be considered. 

In addition to the polymerization of dienes the versatility of NdP-based catalysts is exceptional regarding the number of different non-diene monomers which can be polymerized with these catalysts. Acetylene is polymerized by the binary catalyst system NdP/AlEt3 [253,254]. Lactides are polymerized by the ternary system NdP/AlEt3/H20 [255,256]. NdP/TIBA systems are applied in the copolymerization of carbon dioxide and epichlorhy-drine [257] as well as for the block copolymerization of IP and epichloro-hydrin [258]. The ternary catalyst system NdP/MgBu2/TMEDA allows for the homopolymerization of polar monomers such as acrylonitrile [259] and methylmethacrylate [260]. The quaternary system NdP/MgBu2/AlEt3/HMPTA is used for the polymerization of styrene [261]. 

Various patents on the homopolymerization of BD in the presence of styrene are available [581-590]. According to these patents, St is used as a solvent in which BD is selectively polymerized by the application of NdV/DIBAH/EASC. At the end of the polymerization a solution of BR in St is obtained. In subsequent reaction steps the unreacted styrene monomer is either polymerized radically, or acrylonitrile is added prior to radical initiation. During the subsequent radical polymerization styrene or styrene/acrylonitrile, respectively, are polymerized and ris-l,4-BR is grafted and partially crosslinked. In this way BR modified (or impact modified) thermoplast blends are obtained. In these blends BR particles are dispersed either in poly(styrene) (yielding HIPS = high impact poly(styrene) or in styrene-acrylonitrile-copolymers (yielding ABS = acrylonitrile/butadiene/ styrene-terpolymers). In comparison with the classical bulk processes for HIPS and ABS, this new technology allows for considerable cost reductions... 

Table 5. Initiated radical homopolymerization of captodative acrylonitriles CH2 = C(d)CN...

The free radical initiated homopolymerization of acrylonitrile and methyl methacrylate in the presence of zinc chloride was characterized by an increase in the reaction rate and the molecular weight with increas-... 

The free radical copolymerization of methyl methacrylate or acrylonitrile in the presence of zinc chloride with allylic compounds such as allyl alcohol, allyl acetate, and allyl chloride or butene isomers such as isobutylene, 1-butene, and 2-butene is characterized by the incorporation of greater amounts of comonomer than is noted in the absence of zinc chloride (35). Analogous to the radical homopolymerization of allylic monomers in the presence of zince chloride, the increase in the electron-accepting capability of the methyl methacrylate or acrylonitrile as a result of complexation results in the formation of a charge transfer complex which undergoes homopolymerization and/or copolymerization with a polar monomer-complexed polar monomer complex. 

Homopolymerization of complexed acrylonitrile initiated by a one-electron transfer from isoprene monomer ... 

Spontaneous homopolymerization of the (isoprene-complexed acrylonitrile) complex ... 

The addition of propylene to the complex suspended in toluene or the addition of ethylaluminum dichloride to a mixture of propylene and acrylonitrile in the presence or absence of solvent at —78°C. resulted in a rapid reaction to yield a high molecular weight 1 1 copolymer, irrespective of the initial ratio of monomers. Neither propylene nor acrylonitrile undergo homopolymerization under the same conditions. 

Alkylaluminium chlorides form complexes with methyl acrylate which can react with styrene yielding alternating copolymers [164], The donor-acceptor complex of acrylonitrile (donor) and potassium persulphate (acceptor) makes possible the homopolymerization of acrylonitrile in polar media (in water, dimethylformamide, dimethylsuphoxide, dioxan) at low temperatures [165]. The initiating radicals are formed according to the scheme... 

The radical III would be the most stable (kt k2). Post-polymerization and copolymerization of acrylamide- or acrylonitrile-enriched solutions show higher tendency to homopolymerize rather than to copolymerize. 

Copolymers may be produced by step reaction or by chain reaction polymerization in similar mechanisms to those of homopolymerization. The most widely used synthetic rubber (SBR) is a copolymer of styrene (S) and butadiene (B). Also, ABS, a widely used plastic, is a copolymer or blend of polymers of acrylonitrile, butadiene, and styrene. A special... 

Jagodic et al. (1975, 1976) also studied the effect of the HLB of emulsifying agents in the emulsion homopolymerization of methyl methacrylate, ethyl acrylate, and acrylonitrile and in the emulsion copolymerization of methyl methacrylate and ethyl acrylate, methyl methacrylate and... 

An examination of reported reactivity ratios (Table 6) shows that the behaviour rj > 1, r2 1 or vice versa is a common feature of anionic copolymerization. Only in copolymerizations involving the monomers 1,1-diphenylethylene and stilbene, which cannot homopolymerize, do we find <1, r2 <1 [212—215], and hence the alternating tendency so characteristic of many free radical initiated copolymerizations. Normally one monomer is much more reactive to either type of active centre in the order acrylonitrile > methylmethacrylate > styrene > butadiene > isoprene. This is the order of electron affinities of the monomers as measured polarographically in polar solvents [216, 217]. In other words, the reactivity correlates well with the overall thermodynamic stability of the product. Variations of reactivity ratio occur with different solvents and counter-ions but the gross order is predictable. 

Reactivity of itaconic add in copolymerization is dependent upon pH and degrees of ionization of the add. Acid reactivity has been studied most carefully in acrylonitrile copolymerization 33, 37). Under acidic conditions an increase in itaconic concentration greatly decreases the polymerization rate, while at pH s of 7—9.8 moderate increases of itaconate do not reduce the rates so strongly. Monomer reactivity ratios and Q and e values have been calculated for the various states of ionization of the acid as reported in Table 5. As the pH rises, drops from 1.57 to 0.1 suggesting, as stated earlier, that the dianion undergoes little homopolymerization. The change in is less than 2-fold which indicates appreciable copolymerization of the dianion. The much greater decrease... 

Adding a homopolymer solvent—N,N -dimethylformamide—to the reaction, or increasing the concentration of acrylonitrile to equal that of starch, led to homopolymerization (sixth and seventh experiments. Table I). 

If the excited ethylene dimer exists as an ion radical pair, propagation may incorporate both monomeric units of the dimer, analogous to the behavior of comonomer charge transfer complexes such as butadiene-maleic anhydride, or only one unit, as noted in the homopolymerization of N-vinylcarbazole in the presence of electron accepting monomers such as acrylonitrile and maleic anhydride. 

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