Branched polymers isomers

The rate of solvent diffusion through the film depends not only on the temperature and the T of the film but also on the solvent stmcture and solvent-polymer iuteractions. The solvent molecules move through free-volume holes iu the films and the rate of movement is more rapid for small molecules than for large ones. Additionally, linear molecules may diffuse more rapidly because their cross-sectional area is smaller than that of branched-chain isomers. Eor example, although isobutyl acetate (IBAc) [105-46-4] has a higher relative evaporation rate than -butyl acetate... 

The branched aromatic polyanhydrides synthesized by Sanders et al. (1999 Mathiowitz et al., 1990b) demonstrated lower Tgs than the corresponding P(PCPP-SA) copolymers. The para-xylyl polymers synthesized by Anastasiou and Uhrich (2000a) (Pp-o-CPX and Pp-m-CPX) had systematically higher Tgs than the ortho-isomers (Po-o-CPX, Pm-o-CPX, P/uo-CPX). 

In order to prolong the pot life of the system, a reduction in polydispersity is quite important. Shape polydispersity of the polymer, which is determined by the distribution of topological isomers, is expressed through a quantity Frechet and coworkers originally termed degree of branching [78], which reaches unity for... 

Macromolecules having identical constitutional repeating units can nevertheless differ as a result of isomerism. For example, linear, branched, and crosslinked polymers of the same monomer are considered as structural isomers. Another type of structural isomerism occurs in the chain polymerization of vinyl or vinylidene monomers. Here, there are two possible orientations of the monomers when they add to the growing chain end. Therefore, two possible arrangements of the constitutional repeating units may occur ... 

Another type of geometric arrangement arises with polymers that have a double bond between carbon atoms. Double bonds restrict the rotation of the carbon atoms about the backbone axis. These polymers are sometimes referred to as geometric isomers. The X-groups may be on the same side (cis-) or on opposite sides (trans-) of the chain as schematically shown for polybutadiene in Fig. 1.12. The arrangement in a cis-1,4-polybutadiene results in a very elastic rubbery material, whereas the structure of the trans-1,4-polybutadiene results in a leathery and tough material. Branching of the polymer chains also influences the final structure, crystallinity and properties of the polymeric material. 

Spectroscopic analysis revealed that the thermally initiated [3 + 2] polycycloaddition produced 1,4- and 1,5-substituted triazole isomers in an approximately 1 1 ratio. This ratio appears to be statistic and dependant on the bulkiness of the organic moieties. For example, hfr-r-P[30(4)-20] with butyl spacers contained slightly more 1,4-triazole isomers than did hb-r-P[30(6)-20] with hexyl spacers. This becomes clearer if we look at the proposed transition states a and b of the [3 + 2]-dipolar cycloaddition (Scheme 16). Because of their molecular orbital symmetry, the acetylene and azide functional groups arrange in two parallel planes, a so-called two-plane orientation complex [48], which facilitates a concerted ring formation. If the monomer fragment or the polymer branch ( ) attached to the functional groups are bulky, steric repulsion will come into play and transition state a will be... 

Isobutane (4-1) and n-butane (4-2) are familiar examples of constitutional isomers. Each has the molecular formula C4H10 but the C and H atoms are joined differently in these two molecules. In polymers the major types of constitutional differences involve positional isomerism and branching. 

The primary structure of macromolecules is defined as the sequential order of monomers connected via covalent chemical bonds. This structural level includes features such as chain length, order of monomer attachment in homopolymers (head-to-head, head-to-tail placement), order of monomer attachment in various copolymers (block copolymers, statistical and graft copolymers, chemical composition of co-monomers), stereoregularity, isomers, and molecular topology in different branched macromolecules and molecular networks. Structure at this primary level can be manipulated by polymer synthesis [4]. With AFM it is possible to visualize, under certain conditions, single macromolecules (Fig. 3.2) and it is even possible to manipulate these (i.e. push with AFM tips). Characteristics of chain-internal... 

It is ideal to measure reference Raman spectra of all ingredients in their pure forms, using the same conditions as the sample. Because Raman spectroscopy is sensitive not only to molecular structure but also to the local environment of molecules (degree of hydration and crystallinity, different isomers and polymorphic forms, long-chain polymers of different level of branches and saturations. 

An example of this behavior is shown in Table 11. In the reported experiments, a typical Cr/silica catalyst was tested for ethylene polymerization with small amounts of butene added to the reactor. Three different butene isomers were used in three series of experiments 1-butene, 2-butenes (cis and trans), and isobutylene. In the first series, as 1-butene was added to the reactor, the density of the polymer declined significantly, indicating the presence of ethyl branches on the chains from the incorporation of the comonomer (branching disrupts crystallinity and creates more amorphous polymer, which lowers the average density). The MI values of the polymers in this series went up as 1-butene was added, as would be expected from the greater ease with which a (3-hydride can be abstracted from the tertiary carbon resulting from 1-butene incorporation. This is the behavior typical of all a-olefin comonomers. 

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