Head random copolymers

The products have been described as pseudo-random copolymers. The reason for this is that successive head-to-tail styrene-styrene insertions do not occur. The structure may thus be formally represented as... 

We have designed PBUILD, a new CHEMLAB module, for easy construction of random copolymers. A library of monomers has been developed from which the chemists can select a particular sequence to generate a polymeric model. PBUILD takes care of all the atom numbering, three dimensional coordinates, and knows about stereochemistry (tacticity) as well as positional isomerism (head to tail versus head to head attachment). The result is a model of the selected polymer (or more likely a polymer fragment) in an all trans conformation, inserted into the CHEMLAB molecular workspace in literally a few minutes. 

Ribbed helices (costal helices) are important in organic chemistry because linear polymers contain side chains as well as backbones. We may, then, discern not only the catenal helix of the backbone, but the intercostal helix formed by all of the ribs and the infracostal helicesof the individual side chains. The intercostal helix may be iterative (as in an isotactic head-to-tail vinyl polymer or homogeneous poly-a-amino acid) or non-iterative (as in a random copolymer, an atactic polymer or typical protein). The intracostal helices can best be analyzed as short-chain crooked lines, as in Section III. Important as costal helicity is, it is secondary to catenal helicity and we therefore limit our attention to the primary helicity, that of long chains. Indeed, we limit our attention to catenal helices having chain motifs of two atoms and two bonds as found in head-to-tail vinyl homopolymers ... 

Amino acid-based norbomene random and block copolymers have been synthesized by Sanda, Masuda et al. [178]. The blocks were constructed with monomers containing either the ester or carboxyl amino acid forms, and C4 was used. While the random copolymers were partially soluble in acetone, the block copolymers were soluble through formation of reverse micelles (Scheme 24). Moreover, the diameter of these aggregates was around 100 nm as measured by DLS and AFM. Amino acid-based ROMP monomers with a different cyclic core, i.e., cyclobutenecarbonyl glycine methyl esters, were polymerized by Sampson et al., leading to head-to-tail-ordered polymers without stereocenters [179]. C6 was used and polydispersities between 1.2 and 1.6 were obtained. 

The work of Natta et aL was so misunderstood that some authors actually wrote (4) that it is concluded from that work that random copolymers do not contain regularly head-to-tail enchained Cj units . 

The number of possible polymers is increased further by the many isomers. Isomers have the same number of atoms in each of their repeating units, but arranged in different structures. A simple type of isomer is the positional isomer, observed in most vinyl polymers as indicated at the top of Fig. 1.20. Fortunately the reaction mechanism prefers often one of the isomers (usually the head-to-tail isomer) to such a degree that the concentration of the other one is small. Uncontrolled isomerization causes a loss of regularity, similar to that in random copolymers. The irregular... 

A series of regioregular (head-to-tail), random copolymers (R=dodecyl/R =methyl) has been prepared by McCullough and Jayaraman.(5 Small red shifts in the optical spectra (450 458 470 466 nm) occur upon... 

HEAD-TO-TAIL COUPLED, RANDOM COPOLYMERS OF ALKYL THIOPHENES SUBSTITUENT EFFECT ON ORDER... 

Head-to-tail cou ded, random copolymers of alkyl thiophenes... 

The earliest studied main-chain polymers consisted of chromophores with their dipole moments oriented head-to-tail along the polymer chain [127-129]. It was expected that this head-to-tail arrangement of the molecular dipole moments might result in a coherent enhancement of the second-order nonlinear properties [130]. Williams et al. [131,132] reported a series of head-to-tail materials based on 2-cyano-3-(4-hydroxyphenyl)-2-propenoate ester random copolymers [Structure (15)] prepared by transesterification. The homopolymers are insoluble, intractable materials, whereas copolymers are soluble and can be spun to form thin films. In those polymers, in which the dipoles of the individual chromophores point in one direction, large dipole enhancement will be observed. By electric field-induced second-harmonic generation (EFISH) measurements in solution, enhancement was observed but when extended to the polymer films, the expected enhancement in dipole moments and stability have been not realized. 

All polymer molecules have unique features of one sort or another at the level of individual repeat units. Occasional head-to-head or tail-to-tail orientations, random branching, and the distinctiveness of chain ends are all examples of such details. In this chapter we shall focus attention on two other situations which introduce variation in structure into polymers at the level of the repeat unit the presence of two different monomers or the regulation of configuration of successive repeat units. In the former case copolymers are produced, and in the latter polymers with differences in tacticity. Although the products are quite different materials, their microstructure can be discussed in very similar terms. Hence it is convenient to discuss the two topics in the same chapter. 

For instance, the Dow experimental membrane and the recently introduced Hyflon Ion E83 membrane by Solvay-Solexis are "short side chain" (SSC) fluoropolymers, which exhibit increased water uptake, significantly enhanced proton conductivity, and better stability at T > 100°C due to higher glass transition temperatures in comparison to Nafion. The membrane morphology and the basic mechanisms of proton transport are, however, similar for all PFSA ionomers mentioned. The base polymer of Nation, depicted schematically in Figure 6.3, consists of a copolymer of tetrafluoro-ethylene, forming the backbone, and randomly attached pendant side chains of perfluorinated vinyl ethers, terminated by sulfonic acid head groups. °... 

Fig. 1 NMR spectra in the carbonate region in CDCI3 of poly(propylene carbonate) A regioselective 94% head-to-tail selective copolymer prepared using a (salan)CrCl catalyst B regioirregular random polymer commercially available from Aldrich Chemicals...

As a result of the catalyst and process conditions used in their manufacture, the particular copolymers of current major interest are atactic, and contain typically up to about 50 mol% ( 80 wt%) styrene. These materials have been described as pseudo-random , since successive head-to-tail styrene chain insertions have been shown to be absent, even at high levels of styrene incorporation [1,2]. The term ethylene-styrene interpolymer (ESI) is used here to describe the specific ethylene-styrene copolymers produced via INSITE Technology. For convenience, all subsequent comonomer contents are expressed in weight percentages, unless otherwise stated. For example, the code ES70 refers to an interpolymer having 70 wt.% comonomer styrene incorporation. 

When polymerization of unsymmetrical monomers such as propylene occurs, addition can occur in a head-to-tail or a head-to-head shion to provide a variety of different structures. If two or more monomers are copolymerized, a variety of monomer sequences ((fyads, triads, tetrads, pentads, etc.) are possible as illustrated by the possible pentad monomer sequences for poly(l-chloro-l-fluoroethylene-co-isobulylene), (PCFEI), shown in Table 1. In general, it is possible to form copolymers having blodcs of monomers, a statistically random distribution of two monomers, and all possible compositional variations between these two extremes. In the example of PCFEI, it is possible to form pentads with five sequential E s, five sequential I s, and all permutations of E s and Fs between those extremes. The relative number of different monomer sequences provides information about the blockiness of the polymer, and is useful for determining the polymer s monomer composition. 

By contrast electron-rich heteroaromatic units such as 2,5-pyridines or 2,5-thiophenes provide a way to redshift the emission of PAVs (Figure 4.5). The lower symmetry of the pyridine than the phenylene ring means that poly(pyridine vinylene) can be produced as a random polymer or in two regioregular forms—head-to-tail (35) and head-to-head (36) [74]. The EL emission maxima of these appear at 575, 584, and 605 nm, respectively. The thiophene-containing copolymer 37 has even more redshifted emission (Amax = 620 nm) [107,108]. The most redshifted emission yet to be reported from PAV is near-infrared emission (Amax = 800 nm) from the polymers 38 (Amax = 740 nm) [109] and 39 (Amax=800 ntn) [110,111]. A wide range of other heteroaromatic units have been incorporated into PAVs with emission colors ranging from green to red. 

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