Electron donor monomers

Acrylonitrile copolymeri2es readily with many electron-donor monomers other than styrene. Hundreds of acrylonitrile copolymers have been reported, and a comprehensive listing of reactivity ratios for acrylonitrile copolymeri2ations is readily available (34,102). Copolymeri2ation mitigates the undesirable properties of acrylonitrile homopolymer, such as poor thermal stabiUty and poor processabiUty. At the same time, desirable attributes such as rigidity, chemical resistance, and excellent barrier properties are iacorporated iato melt-processable resias. 

A substantial number of photo-induced charge transfer polymerizations have been known to proceed through N-vinylcarbazole (VCZ) as an electron-donor monomer, but much less attention was paid to the polymerization of acrylic monomer as an electron receptor in the presence of amine as donor. The photo-induced charge-transfer polymerization of electron-attracting monomers, such as methyl acrylate(MA) and acrylonitrile (AN), have been recently studied [4]. In this paper, some results of our research on the reaction mechanism of vinyl polymerization with amine in redox and photo-induced charge transfer initiation systems are reviewed. 

This section describes polymerizations of monomer(s) where the initiating radicals are formed from the monomer(s) by a purely thermal reaction (/.e. no other reagents are involved). The adjectives, thermal, self-initialed and spontaneous, are used interchangeably to describe these polymerizations which have been reported for many monomers and monomer combinations. While homopolymerizations of this class typically require above ambient temperatures, copolymerizations involving certain electron-acceptor-electron-donor monomer pairs can occur at or below ambient temperature. 

Two mechanisms have been proposed to explain the strong alternation tendency between electron-acceptor and electron-donor monomers. The polar effect mechanism (analogous to the polar effect in chain transfer—Sec. 3-6c-2) considers that interaction between an electron-acceptor radical and an electron-donor monomer or an electron-donor radical and... 

Acrylonitrile copolymerizes readily with many electron-donor monomers oilier than styrene. Hundreds of acrylonitnle copolymers have been reported, and a comprehensive listing of reactivity ratios for acrylonitrile copolymerizations is readily available. 

In contrast to the radical-monomer interaction in the transition state proposed by Mayo and Walling (62, 63), the formation of a molecular complex between the electron donor monomer and the electron acceptor monomer—i.e., monomer-monomer interaction—has been proposed as the contributing factor in the free radical alternating copolymerization of styrene and maleic anhydride (8) as well as sulfur dioxide and mono-or diolefins (6, 9, 12, 13, 25, 41, 42, 43, 44, 61, 79, 80, 88). Walling and co-workers (83, 84) did note a relationship between the tendency to form molecular complexes and the alternating tendency and considered the possibility that alternation involved the attack of a radical on a molecular complex. However, it was the presence in the transition state of polar resonance forms resembling those in the colored molecular complexes which led to alternation in copolymerization (84). 

Iwatsuki and Yamashita (46, 48, 50, 52) have provided evidence for the participation of a charge transfer complex in the formation of alternating copolymers from the free radical copolymerization of p-dioxene or vinyl ethers with maleic anhydride. Terpolymerization of the monomer pairs which form alternating copolymers with a third monomer which had little interaction with either monomer of the pair, indicated that the polymerization was actually a copolymerization of the third monomer with the complex (45, 47, 51, 52). Similarly, copolymerization kinetics have been found to be applicable to the free radical polymerization of ternary mixtures of sulfur dioxide, an electron donor monomer, and an electron acceptor monomer (25, 44, 61, 88), as well as sulfur dioxide and two electron donor monomers (42, 80). 

The charge transfer complex resulting from the one-electron transfer from the electron donor monomer to the electron acceptor monomer has a stability which varies as a function of the internal resonance stabilization. The degree of stabilization apparently determines the ease with which the diradical complex opens, and consequently the stability of the complex determines whether the copolymerization occurs spontaneously or under the influence of heat, light, or free radical attack. 

Increasing the electron-accepting character of an electron acceptor monomer would result in a greater separation in the donor—acceptor relationship with a given electron donor monomer. As a result there would be an increased tendency for alternation in the copolymerization. 

Photoactivated Copolymerization. Although polymerization and copolymerization generally involve the addition of a monomer to a reactive chain end, the ground state charge transfer complex generated by the interaction of an electron donor monomer and a strong electron acceptor monomer, acts as a single unit and, upon excitation of the complex, both monomers enter the chain. 

In the last year some attention has been paid to the electropolymerizations in which the electrodic depolarizer is a complex between the monomer and some Lewis acids (see Ref. 7, p. 650). These researches, pioneered by Funt, were oriented towards the formation of alternating copolymers, in reactions in which the Lewis acid gives a monomer pair charge transfer complex. This research is connected to the discovery that certain polar monomers, containing nitrile or carbonyl groups, (A), can complex with Lewis acids such as zinc halides. These complexes I can undergo a thermal homopolymerization by themselves, or can react with some electron donor monomers (D), giving rise to complexes like II,... 

The rate of polymerization of polar monomers, for example, maleic anhydride, acrylonitrile, or methyl methacrylate, can be enhanced by coraplexing them with a metal halide (zinc or vanadium chloride) or an organoaluminum halide (ethyl aluminum sesqui-chloride). These complexed monomers participate in a one-electron transfer reaction with either an uncomplexed monomer or another electron-donor monomer, for example, olefin, diene, or styrene, and thus form alternating copolymers (11) with free-radical initiators. An alternating styrene/acrylonitrile copolymer (12) has been prepared by free-radical initiation of equimolar mixtures of the monomers in the presence of nitrile-coraplexing agents such as aluminum alkyls. 

The difficulties in forming copolymers of certain olefins with SO2 have been discussed in a number of papers [61d]. The highest copolymerization rates were found in systems containing an electron-donor monomer with low resonance stabilization and gave alternating copolymer compositions. Monomers with high resonance stabilization with elechon-acceptor groups result mainly in homopolymerization under normal conditions. For example, styrene forms only a... 

The tendency for alternation increases as the difference in polarity between two monomers increases. Styrene is an electron donor monomer, while maleic anhydride is an electron acceptor. Consequently, alternating copolymerization between both monomers is facilitated. Notice that r,r2 for the monomers is 0.0006. 

The contribution of complex formation to MCM copolymerization is more substantial than it is to homopolymerization. The coordinational unsaturation of central metal atoms, for example the pentacoordination state of Sn(IV), plays a definite part. The transfer of an electron from an MCM (electron-donor monomer) to a multiple bond of the comonomer is comparatively easily carried out in the transition state [112] as shown in Eq. 4-36, for example for maleic aldehyde the complex formation constant. 1= 0.17 0.002 L mol". ... 

When, however, initiations take place by one-electron transposition, they occur as a direct result of oxidation of free radicals. They can also take place through electron transfer interactions involving electron donor monomers. The carbon cations can form from olefins in a variety of ways. One way is through direct additions of cations, like protons, or other positively chaiged species to the olefins. The products are secondary or tertiary carbon cations ... 

Copolymerization. Acrylonitrile copolymerizes readily with electron-donor monomers, and >800 acrylonitrile copolymers have been registered with Chemical Abstracts. A comprehensive hsting of reactivity ratios for acrylonitrile copolymerizations is available (95). Copolymerization is carried out by bulk emulsion, slurry, or suspension processes. The arrangement of monomer imits in acrylonitrile copolymers is most commonly random. Special techniques can be used to achieve specific arrangements. 

Other Copolymers. Acrylonitrile copolymerizes readily with many electron-donor monomers other than the copolymers mentioned above. More than 800 acrylonitrile copolymers have been registered with Chemical Abstract and a comprehensive listing of reativity ratios for acrylonitrile copolymerizations is readily available (174). Some of the other interesting acrylonitrile copolymers follows acrylonitrile-methyl acrylate-indene terpolymers, by themselves, or in blends with acrylonitrile-methyl acrylate copolymers, exhibit even lower oxygen and water permeation rates than the indene-free copolymers (175,176). Terpolymers of acrylonitrile with indene and isobutylene also exhibit excellent barrier... 

The higher MWs of copolymers obtained from vinylether (Table 2 M > 12000 g mol ) did not allow their detailed analysis hy ESI-MS. However, thanks to HSQC NMR analysis, molar fractions of electron-donor monomer were determined and eompared to values obtained by FTTR (Table 4). For copolymers synthesized from vinylotgr compounds (DEF-VIG and DEF-HVE), the molar fraction of donor monomer was equal to 0.5 as expected for a perfectly alternating eopolymerization, hence contrasting with allylether copolymers (DEF-AHE and DEF-AIG), for which the molar fraction in donor monomer was elose to 0.3, confirming the easier insertion of the fumarate comonomer during propagation. 

The involvement of 1 1 complexes in propagation can occur in many ways. Combinations of an electron donor monomer, such as a vinyl ether or an olefin, and an electron-accepting monomer, such as maleic anhydride, carbon dioxide or sulphur dioxide, are thought to give rise, sometimes spontaneously, to alternating copolymers via a binary charge-transfer complex (CT) intermediate (3). 

A-substituted maleimides as electron acceptor monomers copolymerize alternatingly with a variety of electron donor monomers like styrene [1091-1098], a-methylstyrene [1099,1100], alkyl (2-chloroethyl) vinyl ethers [1093,1101], cyclohexyl vinyl ketone and its derivatives [1102,1103], isobutylene [1095], 1,3-butadiene [1104] and 2-vinylpyridine [1095]. Maleimides can also be polymerized by means of anionic initiators, such as sodium methoxide, lithium or potassium er -butoxide, and w-butyllithium [1083,1105-1107]. Anionic polymerizations proceed at low temperatures (e.g., at —72 °C) and give high yields [1107]. The molecular weights are in general lower than those obtained by radical initiation and increase with the monomer/initiator ratio. 

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