Polymer isomeric

Isomeric polymers can also be obtained from a single monomer if there is more than one polymerization route. The head-to-head placement that can occur in the polymerization of a vinyl monomer is isomeric with the normal head-to-tail placement (see structures III and IV in Sec. 3-2a). Isomerization during carbocation polymerization is another instance whereby isomeric structures can be formed (Sec. 5-2b). Monomers with two polymerizable groups can yield isomeric polymers if one or the other of the two alternate polymerization routes is favored. Examples of this type of isomerism are the 1,2- and 1,4-polymers from 1,3-dienes (Secs. 3-14f and 8-10), the separate polymerizations of the alkene and carbonyl double bonds in ketene and acrolein (Sec. 5-7a), and the synthesis of linear or cyclized polymers from non-conjugated dienes (Sec. 6-6b). The different examples of constitutional isomerism are important to note from the practical viewpoint, since the isomeric polymers usually differ considerably in their properties. 

The trans-poly-1,4-butadiene isomer is a harder and less soluble rigid crystalline polymer than the cis isomer. As shown by the skeletal structures for the trans isomer (Figure 1.11), chain extensions on opposite sides of the double bonds allow good fitting of adjacent polymer chains, and this, results in a rigid structure. In contrast, the os-poly-1,4-butadiene isomeric polymer units do not permit such interlocking of alternate units. Even so, chain... 

Dihydrofurans are known to polymerize readily by cationic means but not by free radical initiation (77MI11102). 2,3-Dihydro-5-methylfuran (40), for example, has been polymerized to a stereoregular, optically active polymer (Scheme 9) by the appropriate choice of a catalyst system. Interestingly, the double bond isomer 2-methylenetetrahydrofuran (41) can be cationically polymerized (74CL499) to the isomeric polymer (42 Scheme 10). Under the polymerization conditions, no isomerization of (41) to (40) occurred, and the resultant polymer was shown to be isotactic and crystalline by NMR and X-ray analysis, respectively. 

Imidazoline-functional polymers are most often prepared by modification of acrylonitrile-containing polymers (see Section 1.11.4.2.2). However, isomeric polymers (72) are obtainable via polymerization of 1-vinylimidazolinium salts (71 Scheme 18) (B-74MI11100). 

By comparing the stability constants relative to different polymers, and models, it may be observed that they increase with the number of basic nitrogens present in the repeating unit. Furthermore, the stability constants of the Cu2+ complexes of the polymers of the first class are higher than those of the isomeric polymers of the second class. This fact, and the lower d-d energy band of the complexes of the second class, has been explained with the lack of partecipation in the latter of the C = O groups to the metal coordination l02). Viscosity measurements show that for the complexes with polymers of the first class, the viscosity monotonously decreases upon increasing the pH until the formation of the complex CuL is complete, and then remain nearly constant. On the contrary, in the case of the polymers of the second class, risp/c is... 

There are relatively few data available for the synthetic polyamides. From the studies of Schaefgen and Flory (224) on 6-Nylon in sulfuric acid and those of Howard (127) on 66-Nylon in formic acid-sodium formate we derive identical values of A and of isomeric polymers is also shown in the work of Batzer and Moschle (32 f), who found that both of these Nylons and their copolymers obeyed the same viscosity-molecular weight relationship. 

In practice, the polymerization takes place mainly head-to-tail, and the proportion of other isomers is minor. Structural isomeric polymers may be obtained using different preparation reactions although starting with the same monomers. 

Problem 2.22 Account for the differences in glass transition temperatures for the following pairs of isomeric polymers having similar chemical structures ... 

The initial four chapters of the book concern several important aspects of polymer science which are relevant to a course in polymer chemistry. Following Chapter 1, which is a general introduction aimed at giving the reader an appreciation for the language, applications, and versatility of synthetic polymers. Chapter 2 is devoted to polymer characterization dealing with the size and shape of a polymer chain, polymer isomerism, polymer conformation, and thermal transitions in polymers. 

Conjugated dienes upon polymerization would result in either cis or trans isomeric polymer. One of the most important examples of this class is isoprene. Example of these isomeric polymers is shown in Scheme 3.9. 

Poly(lactic acid) has three typical optical isomeric forms (i) optically active and crystalliz-able poly(L-lactide) (i.e. poly(L-lactic acid) (PLLA)), (ii) optically active and crystallizable poly(D-lactide) (i.e. poly(D-lactic acid) (PDLA)), and (iii) optically inactive and noncrystal-lizable poly(DL-lactide) [i.e. poly(DL-lactic acid) (PDLLA)]. Of these isomeric polymers, PLLA is most frequently used because its production cost is lower due to its joint mass production of 1.4 X 10 metric tons per year by NatureWorks LLC, which is owned by Cargil... 

The reactions and properties of macromolecules are determined by chemical structure and molecular size. Consequently, it is convenient to use these parameters instead of, for example, mechanisms to classify macro-molecular reactions. Distinction is made among catalyses, isomerizations, polymer analog conversions, chain extension, and degradation reactions according to whether the chemical structure, the molar mass and/or the degree of polymerization are retained or changed. 

Isomeric polymers n. Polymers which have essentially the same percentage composition, but differ with regard to their molecular architecture. Odian GC (2004) Principles of polymerization. John Wiley and Sons Inc., New York. Elias HG (1977) Macromolecules, vols 1-2. Plenum Press, New York. 

In structural isomerism, polymer scientists refer to the olefinic carbon with the methyl group on it as the head (h) and the other olefinic carbon as the tail (t) of the monomer. The most common method of polymerization uses catalysts that link the monomers together in the head-to-tail fashion, although occasionally there is a mistake made and the monomers form a head-to-head or a tail-to-tail linkage, but these tend to be rare. 

Dworak, A. Jedlinski, Z. Investigation of the structure of poly(p-chlorophenyl glycidyl ether) by the carbon-13 NMR technique Tacticity and addition isomerism. Polymer 1980, 21, 93-96. 

As well as stereoisomerism and geometrical isomerism, polymers and copolymers can exhibit a third form of isomerism regioisomerism. Head-to-head, head-to-tail and tail-to-head isomerism is well known for simple organic compounds. Thus, a dimer of styrene monomer can exist in the following three different regioisometric forms ... 

In his pioneering study of nonlinear polycondensation [7], Stockmayer has already checked his statistical solution [Eq. (5)] by solving the mass balance equations in a batch reactor for the concentrations of functional groups A and the set of isomeric polymer molecules P with x repeating units X [Eqs. (122)]. 

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