Conventional free-radical copolymerization

The active centers in this process are free radicals, whose reaction with double bonds of monomers leads to the growth of a polymer chain. In the framework of the ideal kinetic model, the reactivity of a macroradical is exclusively governed by the type of its terminal unit. According to this model, the sequence distribution in macromolecules formed at any moment is described by the Markov chain with elements controlled by the instantaneous composition of the monomer mixture in the reactor as 

The allowance for the short-range effects has been carried out in two types of kinetic models (Kuchanov, 1992). In the first of them, the reactivity of a macroradical is presumed to be dependent on the types of n monomeric units preceding the terminal one. Here the mathematical formalism differs from that used in the case of the ideal model only in one point. The states of the Markov chain are associated in the framework of these models with monomeric units, each supplied by the label specifying the type of sequence U, of units acting upon the reactivity of the active center. 

The second type of nonideal models takes into account the possible formation of donor-acceptor complexes between monomers. Essentially, along with individual entry of these latter into a polymer chain, the possibility arises for their addition to this chain as a binary complex. A theoretical analysis of copolymerization in the framework of this model revealed (Korolev and Kuchanov, 1982) that the statistics of the succession of units in macromolecules is not Markovian even at fixed monomer mixture composition in a reactor. Nevertheless, an approach based on the labeling-erasing procedure has been developed (Kuchanov et al., 1984), enabling the calculation of any statistical characteristics of such non-Markovian copolymers. 

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Today, the majority of all polymeric materials is produced using the free-radical polymerization technique [11-17]. Unfortunately, however, in conventional free-radical copolymerization, control of the incorporation of monomer species into a copolymer chain is practically impossible. Furthermore, in this process, the propagating macroradicals usually attach monomeric units in a random way, governed by the relative reactivities of polymerizing comonomers. This lack of control confines the versatility of the free-radical process, because the microscopic polymer properties, such as chemical composition distribution and tacticity are key parameters that determine the macroscopic behavior of the resultant product. 

A PP macromonomer with a methacryloyl end group was synthesized, and was used to prepare PMMA-g-PP graft copolymers by conventional free radical copolymerization [104]. Vinylidene-terminated PP (Mn = 1000) was converted into terminally hydroxylated PP (PP-OH) by a combination of the hydroboration reaction of the unsaturated group and oxidation reaction. Resulting PP-OH was reacted with methacryloylchloride to synthesize termi-... 

One potential problem with conventional free-radical copolymerization is that the reactivity ratios of the two monomers tend to be different from one another [6]. On one hand this leads to non-random sequences of the monomers on a single chain (usually the product of the reactivity ratios is less than one so that there is a tendency to form alternating sequences) and, on the other, to substantial composition drift if the polymerization is carried out in bulk to high conversions. Random copolymers with a range of compositions as a result of composition drift may however be useful in practice, allowing a compositionally graded interface to be formed. 

Random copolymers of styrene/isoprene and styrene/acrylonitrile have been prepared by stable free radical polymerization. By varying the comonomer mole fractions over the range 0.1-0.9 in low conversion SFRP reactions it has been demonstrated that the incorporation of the two monomers in the copolymer is analogous to that found in conventional free radical copolymerizations. The composition and microstructure of random copolymers prepared by SFRP are not significantly different from those of copolymers synthesized conventionally. These two observations support the conclusion that the presence of nitroxide in the SFR process does not influence the monomer reactivity ratios or the stereoselectivity of the propagating radical chain. Rather, the SFR propagation mechanism is essentially the same as that of the conventional free radical copolymerization process. 

The conventional free radical copolymerization at low conversions yields copolymers that follow a first-order Markoff distribution. According to this model, the sequence distribuHon of a two-component copolymer is completely defined when the four elements Pab> Pba/ Pbb of the probability matrix (P-matrix) are determined. The P-matrix elements vary between 0 and 1, and are related by the following conditions ... 

The conventional free radical copolymerization of 3-pinene and MMA or St, initiated by AIBN, yielded copolymers with Mw of about 11 600 and 25 400, respectively and MWD of 1.5 and 1.7, respectively [56]. When 2,3,4,5,6-pentafluorostyrene (PFS) was used as comonomer, and benzoyl peroxide as the initiator, a PFS-rich copolymer incorporating isolated isomerized 3-pinene units distributed between poly(PFS) segments was obtained [57]. This feature is related to the low reactivity of the 3-pinene free radicalar towards its monomer, that is to a reactivity ratio close to zero. These PFS-[3-pinene copol5miers were shown to combine the typical high water contact angles of perflourinated polymers (hydrophobicity) with the optical activity of polyOPESf) [57]. A recently published study indicated that the radical random copolymerization of both a- and 3-pinene with styrene under microwave irradiation yields materials with Mn values considerably higher than those obtained under conventional conditions, but still with very low conversions [18]. 

For conventional free-radical copolymerizations, polar effects of growing polymer radicals on the approaching monomer is expressed by the Alfrey-Price Q — e scheme, where the copolymerization tendency, i.e., product of monomer reactivity ratios, may be expressed, Eq. (20), in terms of e values. 

Emulsion breakers are made from acrylic acid or methacrylic acid copolymerized with hydrophilic monomers [148]. The acid groups of acrylic acid and methacrylic acid are oxalkylated by a mixture of polyglycols and polyglycol ethers to provide free hydroxy groups on the molecule. The copolymers are made by a conventional method, for example, by free radical copolymerization in solution, emulsion, or suspension. The oxalkylation is performed in the presence of an acid catalyst, the acid being neutralized by an amine when the reaction is complete. 

Fig. 5. Copolymerization of methyl methacrylate and styrene in tetrahydrofuran (O) and in N,N-dimethyl formamide ( ). Solid line represents copolymer composition produced by conventional free-radical initiators...

In general, an alternating eopolymer is formed over a wide range of monomer compositions. It has been reported that little chain transfer occurs, and in some cases, conventional free radical retarders are ineffective. Reaction occurs with some combinations, like styrene-acrylonitrile, when the monomers are mixed with a Lewis acid, but addition of a free-radical source will increase the rate of polymerization without changing the alternating nature of the copolymer. Alternating copolymerizations can also be initialed photochemically and electrochemically. The copolymerization is often accompanied by a cationic polymerization of the donor monomer. 

In contrast to the above situations, parylene polymer deposition has very poor adhesion to a smooth surface substrate but can penetrate deep into small cavities. para-Xylylene prefers to react with another para-xylylene or its derivatives. Although it has the feature of difunctional free radical, it is rather stable and does not initiate polymerization of other monomers for conventional free radical polymerization. In spite of numerous attempts, the polymerization of various vinyl monomers initiated by para-xylylene or copolymerization of vinyl monomers with /7ura-xylylene has been elusive. 

Recent investigations [259] have indicated that the polymerization is not conventional free radical in character but is likely to be coordinated anionic. In support of this view are the reactivity ratio coefficients in copolymerization of vinyl chloride with vinyl acetate and methyl methacrylate, which are different from those found with free radical initiators. 

A mixture of two monomers that can be homopo-lymerized by a metal catalyst can be copolymerized as in conventional radical systems. In fact, various pairs of methacrylates, acrylates, and styrenes have been copolymerized by the metal catalysts in random or statistical fashion, and the copolymerizations appear to also have the characteristics of a living process. The monomer reactivity ratio and sequence distributions of the comonomer units, as discussed already, seem very similar to those in the conventional free radical systems, although the detailed analysis should be awaited as described above. Apart from the mechanistic study (section II.F.3), the metal-catalyzed systems afford random or statistical copolymers of controlled molecular weights and sharp MWDs, where, because of the living nature, there are almost no differences in composition distribution in each copolymer chain in a single sample, in sharp contrast to conventional random copolymers, in which there is a considerable compositional distribution from chain to chain. Figure 26 shows the random copolymers thus prepared by the metal-catalyzed living radical polymerizations. 

Sawamoto et aL used the RuCl2(PPh3)3/Al(OzPr)3 catalyst to prepare St/MMA copolymers [126]. They found that the polymerization proceeded well using 1-phenylethyl bromide as the initiator and that the composition of the copolymer matched the comonomer feed composition, or behaved azeotropically [126]. The polymers were well-defined, with predictable molecular weights and relatively low polydispersities (Mw/Mn<1.5). The reactivity ratios were similar to those determined from conventional free radical processes. Later work used a NiBr2(n-Bu3P)2 catalyst system for the ATRP of a 50/50 mixture of MMA/MA and MMA/nBA [127]. The results indicated that the copolymerization was controlled with copolymer Mn=ll,800 (Mw/Mn=1.47) and 12,500 (Mw/Mn=1.47),respectively. 

If the nitroxide does leave the vicinity of the propagating chain end then the reactivity ratios for the radicals should also be the same as in conventional radical polymerizations. However, if the capping and uncapping rates for the two monomers are different this would lead to different concentrations of the two types of propagating chain ends relative to what would be present in a conventional radical polymerization. To address this issue, stable free radical copolymerizations of styrene-isoprene and styrene-acrylonitrile were studied in detail to compare the low conversion copolymer compositions to those prepared by conventional radical polymerization. The microstructure of the polymers was also examined. 

Copolymers of styrene with DVB obtained by free radical copolymerization in the absence of any solvent represent the most investigated polymers. These basic networks have often been referred to as standard or conventional, and all the other three-dimensional styrene copolymers, including... 

Graft Copolymerization by Conventional Free Radical Reactions... 

This influence by the penultimate chain end can considerably modify the reactivity ratio (Table 22-4). The effect is particularly strong with monomers containing very polar groups near the main chain, and it is probably also responsible for the fact that the product of the conventionally determined reactivity ratios in the free radical copolymerization of ethylene with different monomers is sometimes greater than one (Table 22-5). 

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