Block-statistical copolymers

As typically observed in the case of non-ionic block and graft copolymers, the immiscibility of the constituent blocks within the copolymers can induce microphase separation beyond even that which normally occurs due to hydrophobic and hydrophilic sites within statistical copolymer PEMs such as Nation. A relatively recent area of PEM research, ionic block and graft copolymers are interesting from the point of view of providing fundamental understanding about the influence of morphology upon proton conduction... 

Hotnopolytners Block Copolymers Statistical Copolymers Gradient Copolymers Graft Copolymers... 

While the percentage composition of copolymers (i.e., the ratio of comonomers) is not given, copolymers with architecture other than random or statistical are identified as alternating, block, graft, etc. Random or statistical copolymers are not so identified in the CA index. Oligomers with definite structure are noted as dimer, trimer, tetramer, etc. 

Fig. 10 Illustrations of the microchannel confined surface-initiated polymerization (p-SIP) route for producing gradient polymer brush libraries a route for making polymer molecular weight and block copolymer libraries b route for making statistical copolymer libraries. Red arrows show the flow of monomer solution from a syringe pump used to gradually fill the microchannel. See text for details...

The copolymer described by Eq. 6-1, referred to as a statistical copolymer, has a distribution of the two monomer units along the copolymer chain that follows some statistical law, for example, Bemoullian (zero-order Markov) or first- or second-order Markov. Copolymers formed via Bemoullian processes have the two monomer units distributed randomly and are referred to as random copolymers. The reader is cautioned that the distinction between the terms statistical and random, recommended by IUPAC [IUPAC, 1991, in press], has often not been followed in the literature. Most references use the term random copolymer independent of the type of statistical process involved in synthesizing the copolymer. There are three other types of copolymer structures—alternating, block, and graft. The alternating copolymer contains the two monomer units in equimolar amounts in a regular alternating distribution ... 

The characterization of block or graft copolymers is generally much more difficult than that of random copolymers (see Sect. 23.2.7). Especieilly, DSC measurements are useful for the characterization of the different segments (determination of Tg). Also dynamic-mechanical measurements are used to distinguish statistical copolymers from those with block or graft structure. 

The copolymers tested preliminarily were of the statistical, azeotropic, and block type and the separation experiment was carried out by a concentration-gradient development, for which chloroform and ethyl acetate were used as the initial and second solvents, respectively. It was found that the block copolymers could hardly be developed under the condition which allowed the statistical copolymers to migrate, despite the fact that the block copolymers had higher styrene contents, hence, lower affinities to adsorbent, than the statistical copolymers. 

Apparently the above results indicate that the chromatographic behavior of copolymers is different for different chain architecture. When the Rf values are compared at equimolar composition, the sample migration occurs first for the statistical copolymer and belatedly for the alternating, whereas the block copolymer remains immobile on the starting point. Such observations allow us to conclude that the mechanism of chromatographic separation of copolymers is related not only to the chemical composition but also to the chain architecture. 

Fjg. 5. Patterns (1), (2), and (3) of the second solvent addition in concentratiorHpadient developments for ST-MMA block and statistical copolymers (reproduced from Ref.by permission of the Hiithig Wepf Verlag, Basel)... 

In the foregoing examples the synthesis of block copolymers was based on the solubility differences between two monomers, of which one is water soluble while the other is emulsified. Another polymerization technique is based on the kinetics of the emulsion polymerization. When a water emulsion of a monomer, such as styrene, is irradiated during a short time, the reaction, continues at a nearly steady rate until practically all the monomer is used up. If a second monomer is then added, it will polymerize, being initiated by the radicals occluded in the polymer particles. Although in this case also the yields of block copolymers are low, nevertheless the physical properties of the final product are markedly different from those of statistical copolymers (4, 5, 151, 176). 

It turned out that many statistical properties of protein-like and random copolymers with the same HP composition are very different. In order to be able to distinguish whether this difference is due to the special sequence design described above, or just due to the different degree of blockiness, one can introduce for comparison also the random-block primary sequence. The random-block HP copolymers have the same chemical composition and the same average length L of uninterrupted H or P sequence as protein-like copolymers, but in other respects the HP sequence is random. In [18], the distribution of block length X was taken in the Poisson form /(A) = e LLl/ l. 

Thus a statistical copolymer of ethylene and propyiene is named poly(ethylene-stef-propylene), and an ABA tri-block copolymer of styrene (A) and isoprene (B) is named polystyrene-block-polyisoprene-block-polystyrene. In certain cases, additional square brackets are required. For example, an alternating, copolymer of styrene and maleic anhydride is named poly[styrene-d/f-(maleic anhydride)]. 

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