Copolymerization alternating

It is also possible to observe alternating co-ordination of monomers at active sites in polymerizations initiated by co-ordination catalysts for other reasons. 

Torigoe, E. Okuya, and N. Oooshima, Japan Kokai, 77 57 2S0I1977. 

With some catalysts having a controlled co-ordination number a diene monomer can be preferentially co-ordinated in the presence of an olefin when the polym terminal unit already involves the olefin (2). A diene terminal unit does not allow bidentate co-ordination because of the -allylic structure. 

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). 

When monomers have a weak electron-accepting capacity, as is the case with acrylic acid and its derivatives, no alternating copolymerization occurs with olefins unless a Lewis acid is present. The latter is thought to embrace complexa-tion as shown in (4) arguably by the formation of a ternary complex between the two monomers and itself. 

The observed monomer reactivity ratios of different monomer pairs vary widely but can be divided into a rather small number of classes. A useful classification (Rudin, 1982 Odian, 1991) is based on the product of ri and T2, such as rir2 0 (with n 1, T2 1), rir2 1,0 r r2 1, and 1 (with n 1, T2 1), representing, respectively, alternating, random (or ideal), random-alternating, and block copolymerizations. 

A zero, or a nearly zero, value for the reactivity ratio means that the monomer is incapable of undergoing homopolymerization and its radical prefers to add exclusively to the other monomer. This leads to alteration of the two monomer units along the copolymer chain. Forri = t2 = 0,Eq. (7.11)reducesto i[Mi]/(i[M2] = 1. Thus, copolymerization of two monomers for which ri 1 and r2 1 will tend to produce an alternating copolymer (in which the two monomer units alternate in a regular fashion along the chain), irrespective of the composition of the monomer feed. 

Copolymerization of the two monomers therefore produces an alternating copolymer (in which the two monomer units alternate in a regular fashion along the chain) irrespective of the composition of the monomer feed. 

A value of unity (or nearly unity) for the monomer reactivity ratio signifies that the rate of reaction of the growing chain radicals towards each of the monomers is the same, i.e. kn ki2 and 22 — A 2i and the copolymerization is entirely random. In other words, both propagating species and M2 have little or no preference for adding either monomer. The copolymer composition is the same as the comonomer feed with a completely random placement of the two monomers along the copolymer chain. Such behavior is referred to as Bemoullian. Free-radical copolymerization of ethylene and vinyl acetate and that of isoprene and butadiene are examples of such a system, but this is not a common case. Random monomer distributions are obtained more generally in a situation where both types of radicals have exactly the same preference for the same type of monomer as represented by the relationship 

Equation (7.13) means that k /ki2 and k2i/k22 wiU be simultaneously either greater or less than unity or in other words, that both radicals prefer to react with the same monomer. Ail copolymers whose Vir2 product equals 1 are therefore called ideal copolymers or random copolymers. Most ionic copolymerizations are characterized by the ideal type of behavior. 

Problem 7.2 Use simple probability concepts to justify the following statement a value of unity for the product rir2 signifies that the likelihood that an Mj unit in the copolymer chain follows an Mi unit is the same as the likelihood that it follows an M2 unit. 

---

Other miscellaneous compounds that have been used as inhibitors are sulfur and certain sulfur compounds (qv), picryUiydrazyl derivatives, carbon black, and a number of soluble transition-metal salts (151). Both inhibition and acceleration have been reported for styrene polymerized in the presence of oxygen. The complexity of this system has been clearly demonstrated (152). The key reaction is the alternating copolymerization of styrene with oxygen to produce a polyperoxide, which at above 100°C decomposes to initiating alkoxy radicals. Therefore, depending on the temperature, oxygen can inhibit or accelerate the rate of polymerization. 

An alternative copolymerization is illustrated by the method of Blasius. In this preparation, a phenol-formaldehyde (novolac) type system is formed. Monobenzo-18-crown-6, for example, is treated with a phenol (or alkylated aromatic like xylene) and formaldehyde in the presence of acid. As expected for this type of reaction, a highly crosslinked resin results. The method is illustrated in Eq. (6.23). It should also be noted that the additional aromatic can be left out and a crown-formaldehyde copolymer can be prepared in analogy to (6.22). ... 

Copolymerization occurred when the olefin had a basicity lower than that of the aldehyde (with respect to the initiator used), but sufficiently high occasionally to displace a molecule of initiator and give rise to an active species this situation produced copolymers with varying proportions of ether units in the chain, depending on the monomers feed ratio and on the olefin used. Isopropenylbenzene gave the best results with alternate copolymerization over a fairly wide range of feed ratios rt = 0.03 0.03, r2 = 0.4 0.1 (2-furaldehyde = Mj) indene produced copolymers with lower 2-furaldehyde contents. 

In this section wc consider systems where the radical formed by propagation can eyclizc to yield a new propagating radical. Certain 1,4-dicncs undergo cyclocopolymerization with suitable olefins. For example, divinyl ether and MAH are proposed to undergo alternating copolymerization as illustrated in Scheme 4.19.167 These cyclo-copolymerizations can he quantitative only for the case of a strictly alternating copolymer. This can be achieved with certain electron donor-electron acceptor pairs, for example divinyl ether-maleic anhydride. 

Most recent work is in accord with mechanism (b). In an effort to distinguish these mechanisms studies on model propagating species have been carried out.IS6 liW For S-MMA polymerization initiated by AIBMe- -13C (Scheme 8.13) it has been established by end group analysis that extremely small amounts of ethyl aluminum sesquichloride (<10 3M with 1.75 M monomers) are sufficient to cause a substantial enhancement in specificity for adding S in the initiation step. This result suggests that complexation of the propagating radical may be sufficient to induce alternating copolymerization but does not rule out other hypotheses. 

The mechanism of this alternating copolymerization was explained by the following equation ... 

In their paper Hill and coworkers discriminate between alternative copolymerization models by fitting the models to composition data and then predicting sequence distributions based on the fitted models. Measured and fitted sequence distributions are then compared. A better approach taken here is to fit the models to the sequence distribution data directly. 

Based on the structure of the monomers in the monomer feed, the systems under study can be classified as acceptor (MA) — acceptor (TASM) systems and the assumption that alternating copolymerization occurs in these systems seems at first sight... 

Alternating copolymerization of trialkylstannyl methacrylates with MA can proceed via several routes, e.g. by sequential addition of free monomers to the macroradical, by the addition of monomer pairs to a complex or with simultaneous contribution of both free and complex-bound monomers. 

To elucidate the chain propagation mechanism of alternating copolymerization of TBSM with MA and to quantitatively estimate the contribution of complex-bound monomers to the propagation reaction, a kinetic approach described in Ref.91) was employed. 

The insertion of unsaturated molecules into metal-carbon bonds is a critically important step in many transition-metal catalyzed organic transformations. The difference in insertion propensity of carbon-carbon and carbon-nitrogen multiple bonds can be attributed to the coordination characteristics of the respective molecules. The difficulty in achieving a to it isomerization may be the reason for the paucity of imine insertions. The synthesis of amides by the insertion of imines into palladium(II)-acyl bonds is the first direct observation of the insertion of imines into bonds between transition metals and carbon (see Scheme 7). The alternating copolymerization of imines with carbon monoxide (in which the insertion of the imine into palladium-acyl bonds would be the key step in the chain growth sequence), if successful, should constitute a new procedure for the synthesis of polypeptides (see Scheme 7).348... 

Palladium(II) complexes possessing bidentate ligands are known to efficiently catalyze the copolymerization of olefins with carbon monoxide to form polyketones.594-596 Sulfur dioxide is an attractive monomer for catalytic copolymerizations with olefins since S02, like CO, is known to undergo facile insertion reactions into a variety of transition metal-alkyl bonds. Indeed, Drent has patented alternating copolymerization of ethylene with S02 using various palladium(II) complexes.597 In 1998, Sen and coworkers also reported that [(dppp)PdMe(NCMe)]BF4 was an effective catalyst for the copolymerization of S02 with ethylene, propylene, and cyclopentene.598 There is a report of the insertion reactions of S02 into PdII-methyl bonds and the attempted spectroscopic detection of the copolymerization of ethylene and S02.599... 

The second part will be devoted to alternating copolymerization of conjugated dienes with olefins. 

Alternating Copolymerization. In the last part of this paper we would like to refer briefly to our findings in connection with the alternating copolymerization of dienes with olefins. The alternating copolymerization of butadiene with propylene was first investigated in 1969 by Furukawa and others (15, 16, 17). They used catalyst systems based on titanium or vanadium compounds. 

The same catalyst system, consisting of dineopentoxyvanadium oxychloride and triisobutylaluminum, is also suitable for the alternating copolymerization of isoprene with ethylene (20). 

The synthesis of polycarbonates from the alternating copolymerization of epoxides with C02 was first reported in 1969 using a ZnEt2/H20 mixture.954 Subsequent studies have focused upon a... 

A related enantiomerically pure zinc amide initiator, (340), has also been described.966 This complex catalyzes the alternating copolymerization of CHO and C02 to yield isotactic material (RR SS = 86 14). Similar enantiomeric excesses have been achieved using a mixture of Et2Zn and the chiral amino alcohol (341).967 Molecular weight distributions are much broader than using catalyst (340), but this protocol is still a convenient way to prepare optically pure diols (Scheme 23). 

PALLADIUM-CATALYZED ALTERNATING COPOLYMERIZATION OF ALKENES AND CARBON MONOXIDE... 

The copolymerization of carbon monoxide and a-olefins is one of the most challenging problems in polymer synthesis. Sen and his coworkers discovered that some cationic palladium compounds catalyze this alternative copolymerization, giving polyketones (Eq. 13). 

Abstract Development in the field of transition metal-catalyzed carbonylation of epoxides is reviewed. The reaction is an efficient method to synthesize a wide range of / -hydroxy carbonyl compounds such as small synthetic synthons and polymeric materials. The reaction modes featured in this chapter are ring-expansion carbonylation, alternating copolymerization, formylation, alkoxycarbonylation, and aminocarbonylation. 

Kinetic resolution of propylene oxide in its alternating copolymerization with CO2 is performed using similar Co-salen complexes. Reaction conditions,... 

---