Sequences in Polystyrene

Static secondary ion mass spectrometry (SIMS) [31] has been applied to structural characterisation [32-35] and surface analysis of polymers [36, 37]. Polymers can be identified from fingerprint spectra or from fragments characteristic of the backbone and pendant groups. Liquid matrix SIMS-FAB-MS has provided only approximate oligomer distribution. 

Reprinted with permission from LV. Bletsos, D.M. Hercules, D. Van Leyen and  

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These equations show how a mmmm sequence in polystyrene can be converted into a mrrm sequence by a simple epimerization event. Should the configuration of the fourth carbon from the left in the last structure also be altered, a rmrm pentad would result. Thus, by a series of epimerization steps it is possible to change isotactic polystyrene gradually into a polymer that exhibits the same NMR spectra as the polystyrene that was prepared by free radical polymerization(17-19). A Monte Carlo program has... 

As an independent approach Fourier transform infrared studies of isotactic polystyrene gels favours isotactic sequences in an ordered conformation different from the Natta type three-fold helix (38). 

Traceless linkers enable the solid-phase synthesis of products which were formerly only accessible by tedious, multistep solution-phase chemistry. Some of these linkers tolerate a broad range of reaction conditions, giving the chemist plenty of freedom in the design of new solid-phase synthetic sequences. Interestingly, polystyrene-bound selenium reagents can also mediate useful chemical transformations of substrates during their attachment to the support, and thereby function both as reagents and as linkers. 

The physicochemical incompatibility of the polystyrene supports with the peptides has been observed to be one of the problems associated with the original Merrifield method. This has been approached successfully by Sheppard and coworkers by the introduction of the polar polyacrylamide supports. Poly(JV-acrylylpyrrolidine) resin is another polar polymeric support developed on similar grounds. The use of these polar resin supports facilitated the solid phase synthesis of a number of peptides and protein sequences in higher yield and purity than with the polystyrene resins. However, the inadequate availability of these polar supports appears to be a limiting factor for a wider utilization at present. 

Schaefer and coworkers, in another chapter in this text, used 1h - 13(j dipole-dipole "line shapes" obtained in a very clever way to investigate rotational motion of the aromatic rings in polystyrene. The method used a WAHUHA pulse sequence to decouple proton-proton dipolar interactions, cross polarization to enhance signal acquisition and an overall sampling technique synchronous with the sample rotation. The dipole-dipole interaction was mapped in rotational sideband spectra obtained from 16 "normal" CP/MAS spectra. The method, though somewhat involved, provided a measure of dipole-dipole line-shapes which can be interpreted in terms of side-chain rotation in the polymer. 

Product III has not been identified. It has a weak, unstructured fluorescence maximum ca. 390 nm, with an excitation maximum at 332 nm. This product was fully extractable from a photolyzed film with methanol, suggesting that it may be a secondary photolysis product arising from Product II. If nonaltemating sequences exist within the copolymer, Product III might be thought to correspond to the diene observed in the photolysis of polystyrene. However, Product III formed more rapidly in vacuum than in air, the reverse of the manner in which diene appeared to form in polystyrene. 

Excimer formation has been studied in polystyrene and poly(a-alkylstyrenes)189 (PS), poly(vinylcarbazole),139>140 poly-(2-vinylnaphthalene), and poly-(4-vinyl-biphenyl).141 For polystyrene films, David et a/.189 showed that the fluorescence yield increased with increasing crystallinity, at both ambient temperature and 77 K. The contribution of excimer fluorescence yield increased in the sequence atactic (0.7) < atactic oriented (0.60) < isotactic amorphous (0.28) < isotactic crystallized (0.01), with normal yields relative to excimer given in parentheses. Similar results were obtained for poly(vinylcarbazole), PVCZ, although the contribution of excimer fluorescence at 77 was independent of crystallinity. The results can be interpreted in terms of electronic energy migration to low-energy defect sites from which excimer emission can occur. In PVCZ copolymers with fumaronitrile (10), diethyl fumarate (11), and diethyl maleate (12) (Scheme 6),... 

We focus in this article on the simplest configuration made of four blocks (SlSl). In this case, the free polyisoprene end sequence plays the same role as the free polyisoprene part of the diblock in the [SIS-SI] blends, and the isoprene part trapped between the two sequences of polystyrene plays the same role as the isoprene sequence of the triblock copolymer. The characteristics of this tetrablock must be submited to the same constraints as one which are described for the [SIS-SI] blends ... 

Most probably, future concentration cartridges will incorporate both Tenax and hypercrosslinked polystyrene SPE material in sequence, in order to combine the positive characteristics of both sorbents and collect the whole range of VOCs, from the heavy ones on Tenax to the very fight on MN-200. 

A similar analysis of the surface area available and the area occupied by the polymer bound species was used to design the synthesis of bis(di-n-butylchlorotin)-tetracarbonylosmium shown in Scheme 7 (S2) The analogous reaction sequence in solution gives bisOi-dibutyltin-tetracailxrttylosniium), a cyclic dimer. A calculation based on a Poisson distribution of sites on a 100 m /g, 20% cross-linked macro-porous polystyrene led to the choice of a substitution level of 0.01-0.02 mmol/g, practical oiily for small scale syntiieses. 

A well studied example is given by the poly(oxyethylene-Z locfc-styrene). In case of atactic sequences of polystyrene, only the poly(oxyethylene), POE, can crystallize. A typical morphology of the POE is shown in Fig. 5.55. Single crystals of the copolymer can be grown from a common solvent which keeps both components mobile up to the time of crystallization of the POE-component. Figure 7.53 illustrates a growth spiral out of poly(oxyethylene-fclocfe-styrene), grown at 293 K from a solution of ethylbenzene (AB diblock, 28 wt-% oxyethylene block with a molar mass of about 10,000 Da). The crystal is comparable to the lamellar crystals of Fig. 5.55, i.e., the poly(oxyethylene) crystals are chain-folded with about 2.5 nm amorphous polystyrene layers at the interfaces. 

Organic glasses are polymers with an irregular sequence of monomers (as in polystyrene) or polymers with such complex molecules that they caimot form a compact regular network rmder cooling. Organic glasses are also known as optical polymers. 

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