Copolymerization initiators

The composition curve of the copolymers of MMA with St by the copolymerization initiated with PSS-Na was shown in Fig. 11. 

Second example was obtained from the copolymerization initiated with starch. The results were shown in Fig. 12. The copolymer isolated from the monomer phase was produced by the thermal polymerization and the composition curve was completely similar to the ordinary curve of the radical copolymerization product. The copolymer isolated from the water phase differed from the usual copolymer. The upper curve indicated that the HA formed by starch were soft, and soft MMA was much more easily incorporated than hard St. 

Figure 12. Composition curves of the copolymers of MMA and styrene by the copolymerization initiated with starch on standing (starch 0.1 g, CuCh 2HtO 0.5 mg, HsO 10 cm3, (MMA styrene) 3 cm 85°C,3hr)...

Some ROPs proceed with the simultaneous operation of two different mechanisms, for example, NCA copolymerizations initiated by some secondary amines proceed with both the amine and activated monomer mechanisms. The monomer reactivity ratios for any comonomer pair are unlikely to be the same for the two different propagations. Any experimentally determined r values are each composites of two different r values. 

These superabsorbents are synthesized via free radical polymerization of acrylic acid or its salts in presence of a crosslinker (crosslinking copolymerization). Initiators are commonly used, water-soluble compounds (e.g., peroxodi-sulfates, redox systems). As crosslinking comonomers bis-methacrylates or N,hT-methylenebis-(acrylamide) are mostly applied. The copolymerization can be carried out in aqueous solution (see Example 5-11 or as dispersion of aqueous drops in a hydrocarbon (inverse emulsion polymerization, see Sect. 2.2.4.2). 

For low radiation doses, peroxides accumulate almost linearly with dose. However, after a certain dose has been reached, their concentration tends to level off. This conclusion can be derived from the observed change in the rate of graft copolymerization initiated by polymers subjected to increasing doses of preirradiation in air. Figure 2 illustrates this effect in the case of grafting acrylonitrile onto polyethylene (2). The drop in the yield of peroxide production presumably results from the efficient radiation-induced decomposition of these peroxides. Peroxides are known to decompose under free radical attack, and selective destruction of peroxides under irradiation has been established experimentally (8). This decomposition can become autocatalytic, and sometimes the concentration of peroxides may reach a maximum at a certain dose and decrease on further irradiation. Such an effect was observed in the case of poly (vinyl chloride). Figure 3 shows the influence of preirradiation dose on the grafting ratio obtained with poly (vinyl chlo-... 

Copolymerizations initiated by lithium metal should give the same product as produced from lithium alkyls. Usually the radical ends produced by electron transfer initiation have so short a lifetime they can have no influence on the copolymerization. This is true for instance in the copolymerization of isoprene and styrene (50). The product is identical if initiated by lithium metal or by butyllithium. With the styrene-methylmethacrylate system, however, differences are observed (79,80,82). Whereas the butyllithium initiated copolymer contains no styrene at low conversions, the one initiated by lithium metal has a high styrene content if the reaction is carried out in bulk and a moderate one even in tetrahydrofuran. These facts led O Driscoll and Tobolsky (80) to suggest that initiation with lithium occurs by electron exchange and that in this case the radical ends are sufficiently long-lived to produce simultaneous radical and anionic reactions at opposite ends of the chain. Only in certain rather exceptional circumstances would the free radical reaction be of importance. Some of the conditions required have been discussed by Tobolsky and Hartley (111). The anionic reaction should be slow. This is normally true for lithium based catalysts in hydrocarbon solvents. No evidence of appreciable radical participation is observed for initiation by sodium and potassium. The monomers should show a fast radical reaction. If styrene is replaced by isoprene, no isoprene is found in the copolymer for isoprene polymerizes slowly by free radical initiation. Most important of all, initiation should be slow to produce a low steady concentration of radical-anions. An initiator which produces an almost instantaneous and complete electron transfer to monomer produces a high radical concentration which will ensure their rapid mutual termination. 

Table 2. Reactivity Ratios in copolymerization initiated by lithium compounds...

Copolymerization Initiators. The copolyineri/ution of styrene and dienes in hydrocarbon solution wilh alkyllithium initiators produces a tapered block copolymer structure because of the large differences in monomer reactivity ratios lor styrene (r, < 0.11 and dienes (rj > 10). In urder to obtain random copolymers of styrene and dienes, it is necessary to either add small amounts of a Lewis base such us tetrahydrofuran or an alkali metal alkoxide (MtOR. where Ml = Na, K. Rb, or Cs>. 

Unfortunately there seems to be no supplementary evidence on copolymerization initiated by phosphines. In a typical experiment, a solution of acrylonitrile in petroleum ether is treated with a little triethyl phosphine. A greenish-brown product settles on the walls of the flask. It is soluble in acetone, furfural, methyl ethyl ketone and DMF. Polymer precipitated from acetone by toluene is of low molecular weight and contains perhaps 0.5% combined phosphorus. In similar experiments it is possible to obtain pale yellow polymers with number average molecular weights of around 30,000 and with about one atom of P per polymer chain. On the basis of these results Horner and coworkers proposed the scheme (72) ... 

The course of this reaction was proved by Hilt et al.42> 54 > who used as copolymerization initiators 14C-labelled sodium and potassium benzoate. The activity of the prepared copolymer is due to the labelled and chemically bound initiator anion. This reaction is analogous to the analytic determination of epoxides by hydrogen halides 59 but instead of inactive halogen hydrine generaled according to Eq. (12), an ionic particle capable of initiating copolymerization is formed. 

Another mechanism of copolymerization was reported by Feltzin et al.73). These authors also considered activation of tertiary amine by a compound with active hydrogen but with the formation of a quaternary ammonium base (Eq. (45)), being regarded as the real initiator of copolymerization. Initiation, propagation and termination reactions according to Feltzin are illustrated by schemes (46)-(49). 

For a copolymerization initiated by a tertiary amine in the presence of a proton donor the previous mechanism is modified 74) by introducing a direct interaction of the donor proton with the tertiary amine yielding an active centre. If the proton donor is an acid, the active centre is formed according to... 

At present, we can say that copolymerization initiated by various salts proceeds by an anionic mechanism, after dissociation of the initiators in the reaction medium. The primary step is the addition of the initiator anion to the epoxide. In the initiation by Lewis bases, i.e. by tertiary amines, initiation involves formation of a primary active centre of an anionic character. This active centre is probably generated by interaction of the tertiary amine with the anhydride and an allyl alcohol. The allyl alcohol can be formed by a base-catalyzed isomerization of the epoxide. In the presence of a proton donor, the formation of active centres is possible through interaction of tertiary amine, anhydride and proton donor without epoxide isomerization. Another way of initiation consists in a direct reaction of epoxide with tertiary amine yielding an anionic primary active centre. We believe that in both kinds of initiation in the strict absence of proton donors, the growing chain end has the character of a living polymer. The presence of proton donors, however, gives rise to transfer reactions. 

Benzoyl peroxide, dicumvl peroxide, ferf-butyl peroxypivalate and potassium persulfate were efficient as graft copolymerization initiators. Azobisisobutylronitrile (AIBN), however, was not an efficient initiator. 

Table 2.25 Anhydride-epoxide copolymerization initiated by tertiary amines... 

The heat evolved should be the same for copolymerizations initiated by both carbonyl compounds and conventional peroxides. The marked difference in the exotherm can be explained by assuming that there is no... 

As previously noticed, butyl rubber (HR), poly(methylpropene-co-2-methyl-1,3-butadiene), is a random copolymer of isobutene and 0.7-2.2 mol% of isoprene. The industrial slurry process used all over the world consists in a low-temperature copolymerization initiated by A1C13 in meth-ylchloride. In contrast to 1,3-butadiene, isoprene copolymerizes readily with the more reactive isobutene. Reactivity ratios of the pair isobutene-isoprene, ri = 2.5 0.5 and r2 = 0.4 0.1, measured at the conditions of industrial process [10], show that the copolymerization behaves ideally (ri-r2 = 1), and, at the used low concentration of isoprene, isolated units of this latter comonomer are randomly distributed along the chain with 90% M-p-aiw-enchainment [52,53] ... 

In this review reactmties of mechano-radicals will be classifted into four categories radic conversion, reaction with oxygen, photoreaction, and copolymerization initiated by mechano-radicals. 

Commercial products based on copolymers of ethylene and TEE are made by addition copolymerization initiated by free radicals [89]. Small amounts (1 to 10 mol%) of modifying comonomers are added to eliminate a rapid embrittlement of the product at exposure to elevated temperatures. Examples of the modifying comonomers are perfluorobutylethylene, hexafluoropropylene, perfluorovinyl ether, andhexafluor-oisobutylene [90]. Additional information on the methods to prepare ETEE copolymers are discussed in [88]. ETFE resins are essentially alternating copolymers [90], and in the molecular formula they are isomeric with PVDF with a head-to-head, tail-to-tail structure. However, in many important physical properties, the modified ETFE copolymers are superior to PVDF with the exception of the latter s remarkable piezoelectric and pyroelectric characteristics. 

Figure 10.8. Instantaneous copolymer composition in cationic, free-radical, and anionic styrene/methyl methacrylate copolymerization initiated by SnCl4, benzoyl peroxide, and Na in liquid NH3, respectively (from Pepper [128]).

A plot of the mole fraction of isoprene in the SFR prepared copolymers as a function of the mole fraction of isoprene in the feed is shown in Figure 2. The data points are the results for the SFRP process initiated with BPO at 125 C in the presence of TEMPO the curve represents data reported by Wiley and Davis (6) for a conventional styrene/isoprene copolymerization initiated with peroxide at 100 C. The... 

Copolymerization initiated by A proceeds readily at low temperatures and gives isobutene—isoprene copolymers structurally identical to those prepared commercially utilizing a conventional Lewis-acid initiator. That is to say, there is no incorporation of isoprene in a 1,2- or 3,4-fashion, as would be anticipated at least in part for a Ziegler—Natta process. As with polyisobutene, lower temperatures result in higher molecular weights (polydispersities 2) while materials with high M values and a low polydisper-sity index could be obtained only at very low contents of isoprene. consistent with observations that chain-transfer processes are extremely facile following isoprene incorporation. - ... 

Tip 17 Termination in homopolymerization and copolymerization, initiation rate in homopolymerization, and copolymerization. Cope with the statement Termination reactions are almost always diffusion-controlled right from the outset of polymerization. Food for thought (and additional investigation). How important is the chain length dependence of the termination rate constant in polymer reactor modeling ... 

This work was directed to a demonstration of a new process developed in our laboratories( 1) for graft polymerization and copolymerization initiated by energetic electrons. 

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