Copolymers, alternating

Copolymers of carbon monoxide, carbon dioxide, sulfur dioxide or carbon disulfide are frequently formed in combination with oxiranes, thiiranes or aziridines. Copolymerization of carbon monoxide and ethylenimine was carried out under radiation and the formation of 3-nylon was observed238.  

In the ethylene atmosphere, carbon monoxide and ethylenimine copolymerized with a radical initiator to form a terpolymer239. The following radical mechanism may be proposed  

The formation of polyesters from carbon monoxide and propylene oxide using a cobalt catalyst may involve an alternate coordination on the metal and an insertion of monomers into the carbon-transition metal bond240.  

Incorporation of carbon dioxide as a reactive comonomer has been studied by several groups. Inoue et al. were the first to succeed in preparing high molecular weight polycarbonates by the copolymerization of carbon dioxide and propylene oxide241,242  

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

In an alternating copolymer, the backbone chain is produced by the systematic alternation of two monomers in the main chain (Fig. 3.5). For example, vinyl acetate and maleic anhydride form a copolymer with such an alternating arrangement. 

Properties of an alternating AB copolymer tend to be what would be predicted from a homopolymer with repeating unit C, where C = AB. These copolymers are quite unusual, since they can be produced only if the reaction rate for a growing polymer chain ending in unit A is much faster with monomer B than with monomer A, and the reaction rate for a growing polymer chain ending in unit B is much faster with monomer A than with monomer B. Only in this way can random collisions result in a predominantly alternating structure. 

Alternating copolymers have regular structures (AB)n with the randomness parameter = 2 (cf. Chap. 2). They may also be considered to be homopolymers of the new structural unit AB. These copolymers can be formed in two ways  

from e.g. strong donor — acceptor complex between comonomers or even a betaine structure. This intermediate undergoes faster ion-coupling than the corresponding ion-molecule reaction. 

Second, by more rapid crosspropagation than homopropagation. This happens when the more reactive monomer cannot homopolymerize due to unfavorable thermodynamics e.g. the system being above the ceiling or below the floor temperature. 

Developments in this area have their roots in the alternating radical copolymerization 60,61,62), particularly when a donor-acceptor monomer pair forms a charge 

For small values of K (K 0.1) random copolymer may be formed, whereas for higher values alternating copolymer is produced. With still higher K values (K 10) spontaneous homopolymerization of both monomers can be observed 63). 

Alternating copolymers have been previously synthesized via metathesis polymerization by Grubbs et al. using ring-opening insertion metathesis polymerization (ROIMP) [120,121]. Here, a fast ROMP polymerization of a cyclic olefin 

The factors that control the entry of monomers into a chain in a strictly alternating sequence are combinations of strong polar and steric effects. Thus, a powerful electron donor such as SOj can react spontaneously with an electron acceptor such as bicyclo(2.2.1)hept-2-ene even at temperatures as low as 230 K to form a (1 1) alternating copolymer, i.e.. 

In most cases, the spontaneity is absent but in the presence of a radical initiator, maleic anhydride, which is a powerful electron acceptor, reacts readily with a wide range of donor molecules (e.g., styrene, vinyl aeetate, vinyl eihras) to produce copolymers with a strong tendency to form alternating structures. The perfection of the alternating sequaice will depend on the relative strengths of the donor-acceptor pairs, and as this becomes weaker, statistical-copolymer formation becomes more likely. 

The acceptor-Lewis acid complex reaets with conjugated donor molecules (e.g., styrene) under quite mild conditions to produce highly alternating structures. However, much stronger conditions are required for nonconjugated donor molecules (ethylene, propylene, vinyl acetate, etc.), where it is necessary to use alkyl aluminum sesquichloride as the Lewis acid at a temperature of 195 K. It has been postulated that a ternary molecular complex is formed 

A more speeialized reaetion involves the use of a Ziegler eatalyst, formed from vanadium and titanium halides eomplexed with alkyl aluminum compounds, to synthesize alternating copolymers from propylene and dienes, but the meehanism is now different. 

A novel type of spontaneous alternating copolymerization, developed by Sae-gusa, leads to copolymer formation via a zwitterion, in whieh both the propagating ion and the gegen ion are situated at opposite ends of the ehain. In general, an electrophilic monomer (Mg) internets with a nueleophilie monomer (M ) in the absenee of eatalyst to form a dimerie dipolar speeies. 

There are a number of catalysts which produce alternating copolymers, e.g., vanadium salts and aluminium alkyls for butadiene/propene and 

EtAlCl2 [209] or VOCl3/ZnCl2/benzoyl peroxide [210] are effective with polar monomers such as the acrylic esters. With the latter catalysts light as well as peroxide acts as an accelerator but the mechanism is not a simple free radical polymerization. 

CoCla/AKi-BulaCl/BujS (Al/Co = 1.4, BujS/Al = 3) Butadiene 2,3-dimethyl butadiene 

AI(i-Bu)3/Til4/Bu2S (Al/Ti/BujS = 8/1/16) Butadiene 2,3-dimethyl butadiene 

AlHCl2Et20/TiCl4/All3 (5.5/1/1) Butadiene 2-phenyl butadiene 

It is highly unhkely that the reactivities of the various monomers would be such as to yield either block or alternating copolymes. The quantitative dependence of copolymer composition on monomer reactivities has been described [Korshak et al., 1976 Mackey et al., 1978 Russell et al., 1981]. The treatment is the same as that described in Chap. 6 for chain copolymerization (Secs. 6-2 and 6-5). The overall composition of the copolymer obtained in a step pol3fmerization will almost always be the same as the composition of the monomer mixture since these reactions are carried out to essentially 100% conversion (a necessity for obtaining high-molecular-weight polymer). FiuTher, for step copolymerizations of monomer mixtures such as in Eq. 2-192 one often observes the formation of random copolymers. This occurs either because there are no differences in the reactivities of the various monomers or the polymerization proceeds under reaction conditions where there is extensive interchange (Sec. 2-7c). The use of only one diacid or one diamine would produce a variation on the copolymer structure with either R = R or R = R [Jackson and Morris, 1988]. 

Statistical copolymers containing repeating units each with a different functional group can be obtained using appropriate mixture of monomers. For example, a polyestermide can be synthesized from a ternary mixture of a diol, diamine, and diacid or a binary mixture of a diacd and amine-alcohol [East et al., 1989]. 

The alternating copolymer of composition XXXV cannot be synthesized. However, it is jxts-sible to synthesize an alternating copolymer in which R = R by using a two-stage process [Ueda, 1999]. In the first stage, a diamine is reacted with an excess of diacid to form a trimer 

The trimer is then reacted with an equimolar amount of a second diamine in the second stage  

Alternating copolymers with two different functional groups are similarly synthesized by using preformed reactants [Adduci and Amone, 1989 Gopal and Srinivasan, 1986 Liou and 

Wu et al. [378] have recently reported a new spiro-linked PF (289). Unlike the spiro-co-PF discussed in the previous section, the conjugation in 289 is completely interrupted by the spiro-bifluorene units. As a result, the copolymer showed significant blue-shifted absorption 

SCHEME 2.45 Synthesis of triblock copolymers 288. (From Kong, X. and Jenekhe, S.A., Macromolecules, 37, 8180, 2004.) 

SCH EME 2.46 Suzuki-coupling polymerization route to alternating fluorene-arylene copolymers 290. (From Liu, B., Yu, W.-L., Lai, Y.-H., and Huang, W., Chem. Mater., 13,1984, 2001.) 

An increase in the PL QE of the fluorene-thiophene copolymers can be achieved by introduction of -oxidized thiophene units (although no efficient EL from such materials was reported). This aspect and the chemical structures of thiophene-iS,5 -dioxide-fluorene copolymers are discussed in more detail in Section 2.4. 

A very efficient green-emitting fluorene copolymer 304 was synthesized by Shim and coworkers [390] via Suzuki coupling of dibromothieno[3,2-b]thiophene with dialkylfluorene-diboronic acid [390]. The authors compared the EL properties of this copolymer with PFO homopolymer 196 and PFO-bithiophene copolymer 295. Both the absorption and emission spectra of 304 are red-shifted compared with PFO 196 but slightly blue-shifted compared to bithiophene-based copolymer 295. PLEDs fabricated in the configuration ITO/ PEDOT/304/LiF/Al showed a pure green emission (CIE . v 0.29, r 0.63) close to the 

When there is a strong tendency for the comonomeric units to altemate, i.e. when p -c Xa, a large depression of the melting temperature is predicted by Eq. (5.26). This expectation is based on the assumption that only the A units crystallize and the crystal structure corresponding to that of the homopolymer forms over the complete composition range. This condition is usually difficult to fulfill. 

An example of the melting temperature-composition relation for an alternating copolymer is that of ethylene and chlorotrifluoroethylene, shown in Fig. 5.22.(155) The observed melting temperatures are plotted against the mole fraction of the ethylene units. A maximum in the melting temperature is observed at equal molar ratios of the two components. This temperature, 264 °C, corresponds to the 

A plot of the observed melting temperature against the mole percent of CO is given in Fig. 5.23 for the alternating copolymer, polymerized by Y-radiation.(158) From about 39% to 50% CO there is a linear increase in melting temperature until the equimolar composition is reached. The maximum melting temperature of this copolymer is 244 °C and is substantially higher than that of linear polyethylene and any of its radom copolymers. 

To analyze the melting temperature-composition relation in more detail we assume, following Starkweather, that the new AB type crystal structure can be treated as the crystallizing unit in a random copolymer.(163) Thus, if y represents the fraction of CO, the concentrations of crystallizable CH2CH2 CO and noncrystallizable CH2CH2 units are proportional to y and 1 — 2y respectively. The fraction of crystallizable units X is then given by 

The Flory relation, for random sequence distribution, then becomes 

High Performance Polymers, New York, WUliam Andrew, 2008. 

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Ethylene-Chlorotrifluoroethylene Copolymer. Ethylene-chlorotrilluoroethylene copolymer consists of linear chains in which the predominant 1 1 alternating copolymer is... 

The successive repeat units in strucutres [VI]-[VIII] are of two different kinds. If they were labeled Mj and M2, we would find that, as far as microstructure is concerned, isotactic polymers are formally the same as homopolymers, syndiotactic polymers are formally the same as alternating copolymers, and atactic polymers are formally the same as random copolymers. The analog of block copolymers, stereoblock polymers, also exist. Instead of using Mj and M2 to differentiate between the two kinds of repeat units, we shall use the letters D and L as we did in Chap. I. 

Alternating copolymers Alternative fuel Althesin Alto... 

The radical-catalyzed polymerization of furan and maleic anhydride has been reported to yield a 1 1 furan-maleic anhydride copolymer (89,91). The stmcture of the equimolar product, as shown by nmr analyses, is that of an unsaturated alternating copolymer (18) arising through homopolymerization of the intermediate excited donor—acceptor complex (91,92). 

Acrylate polymerizations are markedly inhibited by oxygen therefore, considerable care is taken to exclude air during the polymerization stages of manufacturing. This inhibitory effect has been shown to be caused by copolymerization of oxygen with monomer, forming an alternating copolymer (81,82). 

Two of the perfluoropolyether fluid stmctures yet to be commercialized are interesting. The first stmcture is a strictly alternating copolymer of ethylene oxide and methylene oxide, which has the longest Hquid range of any molecule containing carbon (40). The second stmcture is the perfluoromethylene oxide polyether which has low temperature Hquid properties down to —120° C ... 

It has been discovered that styrene forms a linear alternating copolymer with carbon monoxide using palladium II—phenanthroline complexes. The polymers are syndiotactic and have a crystalline melting point - 280° C (59). Shell Oil Company is commercializing carbon monoxide a-olefin plastics based on this technology (60). 

Styrene—maleic anhydride (SMA) copolymers are used where improved resistance to heat is required. Processes similar to those used for SAN copolymers are used. Because of the tendency of maleic anhydride to form alternating copolymers with styrene, composition drift is extremely severe unless the polymerization is carried out in CSTR reactors having high degrees of back-mixing. 

Alternating equimolar copolymers of vinyl acetate and ethylene and alternating copolymers of vinyl acetate and acrylonitrile have been reported (127,128). Vinyl acetate and certain copolymers can be produced directly as films on certain metallic substrates by electroinitiation processes in which the substrate functions as one electrode (129). 

New teipolymers of vinyl acetate with ethylene and carbon monoxide have been prepared and their uses as additives to improve the curing and flexibihty of coating resins, eg, nitrocellulose, asphalt, phenoHcs, and polystyrene, have been described (130—132). Vinyl acetate and vinyUdene cyanide form highly alternating copolymers. 

VEs can also copolymerize by free-radical initiation with a variety of comonomers. According to the and rvalues of 0.023 and —1.77 (isobutyl vinyl ether), VEs are expected to form ideal copolymers with monomers of similar and e values or alternating copolymers with monomers such as maleic anhydride (MAN) that have high values of opposite sign (Q = 0.23 e = 2.25). 

PMVEMA, supphed as a white, fluffy powder, is soluble in ketones, esters, pyridine, lactams, and aldehydes, and insoluble in aUphatic, aromatic, or halogenated hydrocarbons, as well as in ethyl ether and nitroparaffins. When the copolymer dissolves in water or alcohols, the anhydride group is cleaved, forming the polymers in free acid form or the half-esters of the corresponding alcohol, respectively. Table 7 illustrates the commercially available alternating copolymers and derivatives. 

Vinyl ethers can also be formulated with acryHc and unsaturated polyesters containing maleate or fumarate functionaHty. Because of their abiHty to form alternating copolymers by a free-radical polymeri2ation mechanism, such formulations can be cured using free-radical photoinitiators. With acryHc monomers and oligomers, a hybrid approach has been taken using both simultaneous cationic and free-radical initiation. A summary of these approaches can be found in Table 9. 

The Q and e values of VP are 0.088 and —1.62, respectively (125). This indicates resonance interaction of the double bond of the vinyl group with the electrons of the lactam nitrogen, whence the electronegative nature. With high e+ monomers such as maleic anhydride, VP forms alternating copolymers, much as expected (126). With other monomers between these Q and e extremes a wide variety of possibiHties exist. Table 14 Hsts reactivity ratios for important comonomers. 

A variety of trichloroethylene copolymers have been reported, none with apparent commercial significance. The alternating copolymer with vinyl acetate has been patented as an adhesive (11) and as a flame retardant (12,13). Copolymerization with 1,3-butadiene and its homologues has been reported (14—16). Other comonomers include acrylonitrile (17), isobutyl vinyl ether (18), maleic anhydride (19), and styrene (20). 

In the Type II case, the copolymerization tends toward an alternating arrangement of monomer units. Curve II of Figure 1 shows an example of an alternating copolymer that has an azeotropic copolymer composition, ie, a copolymer composition equal to the monomer feed at a single monomer feed composition. This case is analogous to a constant Foiling mixture ia vapor—Hquid equihbria.T) III... 

Alternating copolymers of chloroprene have been prepared from a number of donor acceptor complexes in the presence of metal haUdes. 

Other copolymer forms are alternating copolymers, block copolymers and graft polymers. 

Figure 2.16. (a) Random copolymer, (b) alternating copolymer, (c) block copolymer, (d) graft... 

The regular structure of the alternating copolymer with its absence of side chains enables the polymer to crystallise with close molecular packing and with interchain attraction augmented by the carbonyl groups. As a result these polymers exhibit the following characteristics ... 

Low molecular weight liquid nitrile rubbers with vinyl, carboxyl or mercaptan reactive end groups have been used with acrylic adhesives, epoxide resins and polyesters. Japanese workers have produced interesting butadiene-acrylonitrile alternating copolymers using Ziegler-Natta-type catalysts that are capable of some degree of ciystallisation. 

Copolymers of chlorotrifluoroethylene and ethylene were introduced by Allied Chemicals under the trade name Halar in the early 1970s. This is essentially a 1 1 alternating copolymer compounded with stabilising additives. The polymer has mechanical properties more like those of nylon than of typical fluoroplastic, with low creep and very good impact strength. Furthermore the polymers have very good chemical resistance and electrical insulation properties and are resistant to burning. They may be injection moulded or formed into fibres. 

Using the above method, Bamford and Han [59] have succeeded in synthesizing and characterizing copolymers in which each block is an alternating copolymer. The block copolymers prepared are of general structure ... 

It was recently found that j3-PCPY can also be used as a radical initiator to obtain an alternate copolymer of MMA with styrene [35], which was only possible in the presence of Lewis acids [36,37] in the past. The kinetics of the system has been formulated as Rp a[/3-PCPY] a[MMA] (l/a[Styrene] The values of kp /k, and AE were evaluated as 1.43 x 10 L mol -s and 87 kJ/ mol, respectively, for the system. NMR spectroscopy was used to determine the structure composition and stereochemistry of copolymers. Radical copolymerization of AN with styrene [38] by using /3-PCPY as the initiator at 55-65°C also resulted in an alternate copolymer. Rp is a direct function of /3-PCPY and AN, and is inversely related to styrene. 

Alternating copolymer can be synthesized using most of the ylides as radical initiators even in the absence of Lewis acids otherwise essential for other conventional radical initiators. 

Alternating copolymer—In these copolymers, the monomers are arranged in an alternate sequence. Alternating copolymers are produced by special copolymerization processes. 

An alternating copolymer of a-methyl styrene and oxygen as an active polymer was recently reported [20]. When a-methyl styrene and AIBN are pressurized with O2, poly-a-methylstyreneperoxide is obtained. Polymerization kinetic studies have shown that the oligoperoxides mentioned above were as reactive as benzoyl peroxide, which is a commercial peroxidic initiator. Table 1 compares the overall rate constants of some oligoperoxides with that of benzoyl peroxide. 

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