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. 

While there is clear evidence for complex formation between certain electron donor and electron acceptor monomers, the evidence for participation of such complexes in copolymerization is often less compelling. One of the most studied systems is S-.V1 Al I copolymerization/8 75 However, the models have been applied to many copolymerizations of donor-acceptor pairs. Acceptor monomers have substituents such as carboxy, anhydride, ester, amide, imide or nitrile on the double bond. Donor monomers have substituents such as alkyl, vinyl, aryl, ether, sulfide and silane. A partial list of donor and acceptor monomers is provided in Table 7.6.65.-... 

The formation of w-donor complexes (IV). This involves stronger forces than the previous two types because the lone pair of the hetero-atom is involved. It is clear that the polymerisations of some of the favourite monomers, such as the alkyl vinyl ethers, 4-MeO-styrene, and N-vinylcarbazole may be dominated by this phenomenon. A corollary of the complexation by an w-donor monomer is that the hetero-atoms in the corresponding polymers will also interact with the growing carbenium ions. The authors who have proposed this include Stannett (alkyl vinyl ethers) [11], Boelke (dimethoxyethene) [12], and Sauvet (4-MeO-St) [7]. 

Further, if we follow these ideas through, we may find at least one reason why the monomers containing hetero-atoms appear to polymerise so much more rapidly than the hydrocarbons under the most common conditions. The reason may well be that because for these n-donor monomers the KM is much greater than for the 7t-donors, the fraction of the Pn+M chain-carriers is much greater than that of the much more slowly growing Pn+A. In other words, it is a question of the composition of the ionic population rather than of rate-constants, although these may, of course, contribute to the effect. This idea is open to test by measurements of KM and kp+, kp+M, and kp. ... 

Models (Hi) and (iv). Strictly, the only way of finding out definitely whether there is any complexation between the growing cation and the monomer or the polymer, or both, is to investigate whether (and if so, how) the apparent kp+ depends on monomer concentration [16, 17]. We have such evidence only for ACN and styrene and for these the value of kp does not depend on m. This is in accord with the prediction [15,17] that in a highly polar solvent the complexation of Pn+ by a Jt-donor monomer or its polymer is likely to be negligible. The likely behaviour of the w-donor vinyl ethers and their polymers is less clear, but a consideration of the dipole moments and concentrations involved makes it extremely unlikely that these monomers or their polymers could compete successfully for a place in the solvation shell of the growing cations. 

The Kus can be estimated as follows an extrapolation of the Kus for the CT complex formed by any one donog such as mesitylene or hexamethylbenzene, with 1,3,5-trinitrobenzene and 1,4-dinitrobenzene to PhN02, and an extrapolation from solvent CC14 to one of DC > ca. 10 (Foster, 1969) shows that for our system Kus is very unlikely to be greater than 0.01 hmol"1. Therefore, with m = 1 mold"1, and [Sv] = 10 mold"1, [MSv] < 0.1 mold 1. This means that for styrene and other 7t-donors effectively all the monomer is free. For n-donor monomers such as the VE, however, the fraction of uncomplexed monomer may be somewhat smaller. Therefore it appears that the formation of CT complexes probably did not affect significantly at least the results for the three hydrocarbon monomers. 

Characterization of the donor bound polymers follows from their spectroscopic (ir and uv-vis KBr) properties in comparison with the starting donor monomers, and from elemental analyses. That the donors are covalently bound to the polymer and not present as unreacted monomers can be seen by the absence of the characteristic monomer functional group absorption (i.e. -OH, COzH) in the donor bound polymer. For example in Figure 1, the comparative ir spectra of p-hydroxyphenyl-TTF monomer and this donor covalently bound to linear and to cross-linked polysytrene are given. Except for the presence of the hydroxyl absorption in the monomer, all three spectra are essentially identical, indicating a rather clean polymer attachment reaction. 

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. 

Trivalent carbenium ions play a key role, not only in the acid-catalyzed polymerization of alkenes [Eq. (5.348)] but also in the polycondensation of arenes (7r-bonded monomers) as well as in the cationic polymerization of ethers, sulfides, and nitrogen compounds (nonbonded electron-pair donor monomers). On the other hand, penta-coordinated carbonium ions play the key role in the electrophilic reactions of cr-bonds (single bonds), including the oligocondensation of alkanes and alkenes (Section 5.1.5). 

Notable work in the area of photopolymerizations of donor monomers initiated by acceptor initiators was done by Shirota [5-7] in his study of polymerization of N-vinylcarbazole (VCZ) and by Hayashi and Irie [8] on the polymerization of a-methylstyrene (a-MSt). The initiation mechanism was proposed to proceed via the charge-transfer complex between VCZ (or a-MSt) and the acceptor, which then yields two kinds of ion-radicals D and A ... 

The photo-induced charge-transfer copolymerization entity consists of a donor monomer and an acceptor monomer, without initiator. Reported combinations usually consist of an aryl vinyl monomer as the donor component and a 1,2-disubstituted vinyl monomer as the acceptor component. 

Acrylonitrile (AN), a weak electron-accepting but very polar monomer, was also reported to photocopolymerize with donor monomers, such as St [28], isobutyl vinyl ether [29], or butadiene [30]. 

For the strong donor monomer VCZ, the photoreaction depends on the solvent basicity and the molar ratio of the donor and the acceptor. In strongly basic solvents such as dimethyl formamide (DMF), the radical homopolymerization of VCZ occurs in the presence of catalytic amounts of FN or diethyl fumarate (DEF), but it is replaced by radical copolymerization in an equimolar amount of the monomers. The cationic homopolymerization of VCZ, which proceeds in less basic solvents, e.g., benzene, and the cyclodimerization of VCZ, which proceeds in moderately basic solvents, e.g., acetone, is accompanied by the radical copolymerization of VCZ with FN or DEF [6],... 

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). 

In contrast, the donor monomer-acceptor monomer interaction involves a one-electron transfer from the donor monomer to the acceptor monomer to form a charge transfer complex. The latter undergoes homopolymerization through a radical mechanism to give an alternating copolymer. 

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. 

Among the systems with chemical different donor and acceptor molecules, the photocopolymerization between maleic anhydride (MSA), which functions as an acceptor, and electron-rich monomers has been widely investigated. As donor monomers such compounds as styrene (Sty) [19-29], cyclohexene [30], N-vinylcarbazole [31], 2-vinyl naphthalene [32], vinyl acetate [33], 2.4.8.10-tetra-oxaspiro[5.5]undecan [34] and phenyl glycidyl ether (2,3-epoxypropyl phenyl ether, PGE) [35] have been used. In all the above cases, using high concentrations of both monomers, the absorption of the CT has been obtained in various solvents. Thus, with spectroscopic methods the complex formation constant Kct can be calculated (e.g., MSA-cyclohexene Kcl = 0.0681 mol -1 [33], MSA-tetrahydrofuran Kct = 0.331 mol-1 [36]), and a selective excitation of the CT is possible in many cases. 

Complexes of monomers with acceptors can initiate polymerizations of the respective monomers even without photoexcitation. The properties of DA complexes and the conditions of the thermal activation are determined by the type of donor (monomer), acceptor, and solvent [300,303]. Ion radicals from the monomer, dipolar intermediates (zwitterions), as well as the relatively... 

The library synthetic scheme is reported in Fig. 10.53. The synthesis was performed in 96-well plates using an automated liquid dispensing unit for sampling liquid aliquots. Bergenin was first submitted to 11-regioselective acylation with a mixture of four immobilized lipases (step a) and the acyl donor monomer set Mi (12 representatives, Fig. 10.54) in organic solvents, using 168 reaction wells. Purification of the... 

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