Copolymerization tapered

GopolymeriZation Initiators. The copolymerization of styrene and dienes in hydrocarbon solution with alkyUithium initiators produces a tapered block copolymer stmcture because of the large differences in monomer reactivity ratios for styrene (r < 0.1) and dienes (r > 10) (1,33,34). In order to obtain random copolymers of styrene and dienes, it is necessary to either add small amounts of a Lewis base such as tetrahydrofuran or an alkaU metal alkoxide (MtOR, where Mt = Na, K, Rb, or Cs). In contrast to Lewis bases which promote formation of undesirable vinyl microstmcture in diene polymerizations (57), the addition of small amounts of an alkaU metal alkoxide such as potassium amyloxide ([ROK]/[Li] = 0.08) is sufficient to promote random copolymerization of styrene and diene without producing significant increases in the amount of vinyl microstmcture (58,59). 

Block copolymer synthesis from living polymerization is typically carried out in batch or semi-batch processes. In the simplest case, one monomer is added, and polymerization is carried out to complete conversion, then the process is repeated with a second monomer. In batch copolymerizations, simultaneous polymerization of two or more monomers is often complicated by the different reactivities of the two monomers. This preferential monomer consumption can create a composition drift during chain growth and therefore a tapered copolymer composition. 

On the other hand, when sCL is copolymerized with dilactones such as GA [38] and (D- andD,L-)LA [39], tapered or pseudoblock copolymers are obtained with a reactivity ratio much in favor of the dilactone. As an example, the reactivity ratios in the copolymerization of eCL and D,L-LA in toluene at 70 °C are r = 0.92 (e-CL) and r2=26.5 (D,L-LA). Very similar reactivity ratios were calculated for copolymerization between eCL and L-LA, other experimental conditions being kept unchanged. However the control over the polymerization is lost due to transesterification side reactions perturbing the propagation step. Such a behav-... 

Because of the enhanced effectiveness of the cobalt(III) complex with piperidinium end-capping arms (Scheme 6) compared to standard (salen)CoX catalysts for the copolymerization of propylene oxide and CO2, Nozaki and coworkers were able to prepare in a stepwise manner a tapered block terpolymer by first copolymerizing propylene oxide/C02 followed by 1-hexene oxide/C02 [31]. 

A new system for grafting copolymerization was described by Kojima and collaborators (126), in 1971, involving tri-n-butyl borane and water, at 37° C, for 2 hrs, in a taper joint glass tube. This system was inefficient when organic solvents like cyclohexanone, n-hexane, tetrahydrofurane, and toluene were used instead of water. 

As in the case of olefin or diene homopolymerization by RLi, copolymerization is particularly sensitive to solvent effects. Initial-charge (all monomers added together) copolymerization of butadiene and styrene tends to result in a tapered block copolymer (a block of butadiene with increasing levels of styrene, followed by a block of styrene) in hydrocarbon solvents and a random copolymer (a uniform distribution of butadiene and styrene) in polar media. 

Initial-charge RLi copolymerization of butadiene and styrene in hydrocarbons tends toward a tapered-block placement of monomer units. However, it is possible to generate a random copolymer in such solvents if the B S monomer ratio is kept constant by programmed monomer addition or continuous copolymerization (77-78a). 

Triblock polymers are generally made in sequential addition processes with the aid of sodium naphthalene or alkyl lithium. They can also be produced in a "coupling" process wherein a dlblock is prepared first and is coupled with the aid of phosgene or alkyl dlhalides to form the ABA triblock. Tapered blocks can be made by starting with a polystyrene block and then copolymerizing a mixture of styrene and diene. 

The possibilities inherent in the anionic copolymerization of butadiene and styrene by means of organolithium initiators, as might have been expected, have led to many new developments. The first of these would naturally be the synthesis of a butadiene-styrene copolymer to match (or improve upon) emulsion-prepared SBR, in view of the superior molecular weight control possible in anionic polymerization. The copolymerization behavior of butadiene (or isoprene) and styrene is shown in Table 2.15 (Ohlinger and Bandermann, 1980 Morton and Huang, 1979 Ells, 1963 Hill et al., 1983 Spirin et al., 1962). As indicated earlier, unlike the free radical type of polymerization, these anionic systems show a marked sensitivity of the reactivity ratios to solvent type (a similar effect is noted for different alkali metal counterions). Thus, in nonpolar solvents, butadiene (or isoprene) is preferentially polymerized initially, to the virtual exclusion of the styrene, while the reverse is true in polar solvents. This has been ascribed (Morton, 1983) to the profound effect of solvation on the structure of the carbon-lithium bond, which becomes much more ionic in such media, as discussed previously. The resulting polymer formed by copolymerization in hydrocarbon media is described as a tapered block copolymer it consists of a block of polybutadiene with little incorporated styrene comonomer followed by a segment with both butadiene and styrene and then a block of polystyrene. The structure is schematically represented below ... 

This method is used to form a block copolymer, which consists of two segments of essentially homopolymeric stracture separated by a block of a tapered segment of random copolymer composition. These are usually prepared by taking advantage of the differences in reaction rates of the component monomers. When polymerized individually in hexane, butadiene reacts six times more slowly than styrene however, when styrene and butadiene are copolymerized in a hydrocarbon solvent such as hexane, the reaction rates reverse, and the butadiene becomes six times faster than the styrene. This leads to a tapering of the styrene in a copolymerization reaction. For more details on the synthesis techniques, refer to Chapters 2 and 13. 

The NIR in situ process also allowed for the determination of intermediate sequence distribution in styrene/isoprene copolymers, poly(diene) stereochemistry quantification, and identification of complete monomer conversion. The classic one-step, anionic, tapered block copolymerization of isoprene and styrene in hydrocarbon solvents is shown in Figure 4. The ultimate sequence distribution is defined using four rate constants involving the two monomers. NIR was successfully utilized to monitor monomer conversion during conventional, anionic solution polymerization. The conversion of the vinyl protons in the monomer to methylene protons in the polymer was easily monitored under conventional (10-20% solids) solution polymerization conditions. Despite the presence of the NIR probe, the living nature of the polymerizations was maintained in... 

As it was later discovered, it has been more convenient to carry out tapered block copolymer formation with the acid monomer reacting first followed by the hydrophobic monomer in a single-stage FRRPP copolymerization method (see Section 4.1). The result was an anionic polymer surfactant after neutralization of the acid segments with a base (Caneba and Dar, 2005). 

Tapered Block Copolymers. The alkyllithium-initiated copolymerizations of styrene with dienes, especially isoprene and butadiene, have been extensively investigated and illustrate the important aspects of anionic copolymerization. As shown in Table 15, monomer reactivity ratios for dienes copolymerizing with styrene in hydrocarbon solution range from approximately 8 to 17, while the corresponding monomer reactivity ratios for styrene vary from 0.04 to 0.25. Thus, butadiene and isoprene are preferentially incorporated into the copolymer initially. This type of copolymer composition is described as either a tapered block copolymer or a graded block copolymer. The monomer sequence distribution can be described by the structures below ... 

Block Copolymerization. A polymerization with long chain lives can be used to make block copolsrmers (qv). An important commercial example is styrene/butadiene blocks produced by anionic polymerization (qv). A solution polymerization is done in a batch reactor, starting with one of the two monomers. That monomer is reacted to completion and the second monomer is added while the catalytic sites on the chains remain active. This produces a block copolymer of the AB form. Early addition of the second monomer produces a tapered block. If the second monomer is reacted to completion and replaced by the first monomer, an ABA triblock is obtained. This process is not easily converted to continuous operation because polsrmerizations inside tubes rarely approach the piston-flow environment that is needed to react one monomer to completion before adding the second monomer. Designs using static mixers (also known as motionless mixers) are a possibility. 

An interesting variation on the pure block copolymer is the tapered block copolymer, which is formed by copolymerizing two monomers which enter the polymer chain at very different rates. If A enters the chain faster than B, the first part of the chain will be almost polyA. As A is consumed, and the B/A ratio rises, B will enter the chain occasionally, and with increasing frequency. Toward the end of the reaction, with very little A left, the last part of the chain will be almost polyB. While this does not fit the theoretical model nearly as well, experimental results suggest that such tapered block copolymers may actually be... 

The u-butyllithium-initiated polymerizations of myrcene proceed in a living manner in benzene (5-30°C) as well as in tetrahydrofuran (THF —30-15°C). Quantitative conversions can be obtained within 2 h (benzene, 30°C) or less than 1 h (THF, 15°C). The polymers have MWs in the range of 5-30 kg/mol, and PDl values are 1.4-1.6 (benzene) and 1.1-1.5 (THF). The polymyrcenes prepared in benzene consist of 85-89% 1,4 units and 11-15% 3,4 units, similar to those obtained by radical polymerization (see above). Increasing either polymerization temperature or initiator concentration causes an increase of the fraction of 3,4 units. On the other hand, polymyrcenes prepared in THF exhibit 40-50% 1,4 units, 39 14% 3,4 units, and 10-18% 1,2 units. Also here, the amount of 1,4 units is found to decrease with increasing polymerization temperature or initiator concentration. The copolymerization of myrcene and styrene results in the formation of block-like or tapered copolymers. The initial copolymers formed in benzene are rich in myrcene, and styrene is preferably incorporated at later stages of the reaction the situation is reversed when the copolymerization is performed in THF [38]. 

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