Alternating copolymer, defined

Figure 16 1. Schematic representation of copolymers with well-defined conjugation length A is a conjugated chromophorc, B is an interruption unit (a) alternating copolymer (b) polymer with slereo-chcmically defined non-coplanar linkages.

Fig. 13 Cooling (solid line) and heating (dashed lines) curves of crystallinity and averaged length of crystallized sequences for slightly alternating copolymers with a comonomer mole fraction 0.24. The crystallized sequences are defined as the monomer sequences more than half of whose bonds are in crystalline states [124]...

Fig. 15 Cooling curves of crystallinity (solid line) and demixing parameter of comonomers (dashed line). The latter is defined as the mean fraction of neighboring sites occupied by other comonomers around each comonomer. The cooling program is a stepwise increase of Ep/(k T) from zero with a step length of 0.002 and a step period of 300 Monte Carlo cycles, a The slightly alternating copolymer with a comonomer mole fraction 0.36 b the heterogeneous copolymer with a comonomer mole fraction of 0.36 [52]...

Figure 2.18 Four possible configurations in chain of alternating copolymer between ethylene and cw-2-butene. Sequences of (+) and (—) bonds are also indicated. Note that in Ref. 119 relative configurations racemo and meso were defined as threo and erythro, respectively.

A polymer is considered to be a copolymer when more than one type of repeat unit is present within the chain. There are a variety of copolymers, depending on the relative placement of the different types of repeat units. These are broadly classified as random, block, graft, and alternating copolymers (see Fig. 2.1 for structural details Cheremisinoff 1997 Ravve 2000 Odian 2004). Among these stmctures, block copolymers have attracted particular attention, because of their versatility to form well-defined supramolecular assemblies. When a block copolymer contains two blocks (hydrophobic and hydrophilic), it is called an amphiphilic diblock copolymer. The immiscibility of the hydrophilic and lipophilic blocks in the polymers provides the ability to form a variety of assemblies, the stmctures and morphologies of which can be controlled by tuning the overall molecular weight and molar ratios of the different blocks (Alexandridis et al. 2000). 

Highly efficient green photoluminescence has also been realized from SCPs. Copolymers 11 (Fig. 5) derived from 2,7-fluorene and 2,3,4,5-tetraphenylsilole show absolute PL quantum yields up to 84%.28 A well-defined alternating copolymer 12 with a repeating unit made up of ter-(2,7-fluorene) and 2,5-silole possesses an absolute PL quantum yield >80%.29 SCPs 13 with a main chain structure of 3,6-carbazole-2,7-fluorene-2,5-silole also show absolute PL quantum yields up to 86%.30 An energy transfer copolymer 14 of 2,7-dibenzosilole and... 

Green electroluminescence is also achieved from the well-defined alternating copolymer 12 with a repeating unit made up of ter-(2,7-fluorene) and 2, 5-silole.29 With its neat film as the emissive layer, the EL device shows a maximum //Kr of 0.47%, but the device performance can be largely improved to a maximum //Ki. of 1.99% when using a copolymer/PF8 blend film as the emissive layer. Copolymer 14 derived from 2,7-dibenzosilole and 2,1,3-benzothiadiazole is also an excellent green EL polymer.26 A maximum 7el of 3.81% can be realized in EL devices. 

Dithienosilole-thiophene alternating copolymer 21 is the important example for the electrical conduction of SCPs.42 When the copolymer is doped with iodine, a high electrical conductivity of 400 S cm-1 can be achieved. This value is the highest among SCPs and is also close to that of well-defined poly(3-alkylthiophene). 

Polymers that contain more than one type of monomeric repeat unit are called copolymers. By copolymerizing two or more monomers in varying ratios and arrangements, polymeric products with an almost limitless variety of properties can be obtained. As shown in Eq. (20), there are four basic types of copolymers as defined by the distribution of comonomers A and B, for example. Although the properties of a random copolymer are intermediate between those of the two homopolymers, block and graft copolymers exhibit the properties of both homopolymers. The properties of an alternating copolymer are usually unique. 

Recent studies in our laboratory were aimed at defining more closely the conditions governing Intramolecular excimer formation in dilute polymer solutions (15). An alternating copolymer of styrene with maleic anhydride or methylmethacrylate showed no excimer emission, confirming that interactions of other than neighboring phenyl residues made no significant contribution to... 

Copolymers may be described as alternating copolymers, block copolymers or graft copolymers. The molecular architecture of copolymers may, however, be more complicated than represented in Fig. 8.1. Homopolymers can be linear, star or branched (Fig. 8.2), giving rise to so-called star block copolymers defined by the number of arms (n). 

For PFs with HTM grafting as side chain, the alternating copolymer 18 with electron-deficient moiety (4-ferf-butylphcnyl-l,3,4-oxadiazole) functionalized fluorene and monomer of PFO was synthesized by Shu and coworkers [31]. The device with the configuration ITO/PEDOT PSS/18/Ca/Ag showed improved performance turn-on voltage of 5.3 V (defined as voltage needed for brightness of 1 cdm-2), maximum brightness 2770 cdm-2 at 10.8 V, and maximum external quantum efficiency 0.52% at 537 cdm-2 rela-... 

Furthermore, in calculations performed manually instead of using software implementing our method, the calculation of the properties of many homopolymers with large repeat units can be simplified by treating them formally as alternating copolymers of smaller repeat units of polymers whose properties have already been calculated. Simple additivity is then assumed to hold for the extensive properties of the alternating copolymer, such as its connectivity indices, cohesive energy, and molar volume. All extensive properties can thus be calculated. Intensive properties, such as the solubility parameter, are defined in terms of extensive properties. Their prediction therefore does not require any detailed calculations either. 

Optical densities at 269.5 nm for polystyrene solutions at concentrations of 0-1 X 10"2 mole/liter and for poly(styrene-co-methyl methacrylate) solutions at a total concentration of 1 X 10 2 mole/liter are presented in Figure 1 as functions of styrene content. The solvents were (from the top) dioxane, chloroform, tetrahydrofuran (THF), tetrachloroethane (TCE), and dichloro-ethane (DCE). It is evident that the linear relationship between optical density and styrene concentration that is valid for a polystyrene at all concentrations (open circles) does not hold for the statistical copolymers (solid circles). For example, copolymer (25-80 mole % styrene) solutions in chloroform deviate markedly from linearity the maximum per cent decrease in extinction coefficient (hypochromism) corresponds to a copolymer containing 50 mole % styrene. We define hypochromism as the decrease in absorption intensity at 269.5 nm per chromophore of the statistical copolymer relative to that of the atactic polystyrene. It is also evident from Figure 1 that the alternating copolymer also gives a sharp hypochromism whereas block copolymers and mechanical mixtures of polystyrene and poly (methyl methacrylate) do not deviate from the straight line. Similar results were obtained with the other solvents, but the composition range where hypochromism appears depends on the solvent used. 

The degree of randomness (B) can be defined as fire sum of tlie two probabilities (TWt + Ptn)- For random copolymers, B = I, for alternative copolymers, B =2, while for block copolymers or physical blends, B is close to zero. 

Figure 7.71 illustrates the change of the glass transition for poly(styrene-co-acrylonitrile) as a function of the run number, defined in Fig. 7.70. The branches of the Barton equation (mA > mg, and mA < mB) are symmetric to the line which extends to the alternating copolymer. This treatment of T can also be applied to triads, and one can use the other equations of Fig. 7.69 as base, but with an increase in complexity. Stereo-specific copolymers consisting of meso and racemic dyads can be treated if the samples of different tacticity have different glass transitions [32].

Alternatively, one can measure the volume fraction of the chains that participate in aggregates, using a set of simulations performed at various (p. This volume fraction of the aggregates is then extrapolated to zero, and the value of

critical micelle concentration [35]. When simulations are performed with the only nonzero energies being Eab = Sas = ffi critical micelle concentration of the ABA triblock copolymers defined by this latter approach is found to obey a power law of the form... 

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