Polymer resonances

Figure 9.2 500 MHz H NMR spectra of a commercial stabilised PE product solvent and temperature as in Figure 9.1. See Figure 9.1 for structural designations (S and P refer to residual solvent and polymer resonances, respectively). After Schilling et al. [48]. Reprinted from Polymer Degradation and Stability, 31, F.C. Schilling et al., 141-152, Copyright (1991), with permission from Elsevier...

The step 1 product (200 mg) was treated with phenol (100 mg) and heated to 80°C and then further treated with borontrifluoride diethyletherate (0.09 mmol). The mixture immediately turned red and increased in viscosity. The vial was then heated to 105°C for 1 hour and then treated with 10 ml of distilled water and stood overnight at ambient temperature. The mixture was dissolved in 3 ml of toluene and sonicated. The soluble fraction did not show any polymer resonances by 1H-NMR spectroscopy indicating that the polymer has become crosslinked. 

The main problem for NMR analysis of on-resin products, with the exception of TentaGel, is that the H NMR spectra are generally broad with featureless line widths around 10 Hz or more (28). The additional complication of large polymer resonances can be attenuated by using a spin-echo sequence but the residual peaks can still be problematic. Peak assignment can be difficult due to the loss of coupling information relegating the interpretation of the... 

In the J-resolved projection, the aromatic rings of the SCAL are clearly present where as the polymer resonances have dropped out of the spectrum. In addition, the proton coupling constants arising from alloc group are readily measured as seen in the expanded spectrum (Fig. 13). If a more detailed measure of the couplings is desired, then the full 2D J-resolved spectrum can be evaluated in the normal manner as exemplified in Fig. 14. A similar method to obtain accurate proton-proton coupling constants based on E.COSY spectra has also appeared recently (43). 

Recently, multidimensional NMR has become a standard technique for the assignment of polymer resonances and for characterization of polymer microstructure. Two-dimensional and three-dimensional NMR have been useful for identification of resonances from trace structures such as block junctions, chain-ends, or chain branches that are usually obscured in ID-NMR spectra because of overlap with polymer backbone resonances. Complex spectral features are simplified... 

In the same NMR spectrum the pure polymer resonates at a separate chemical shift value. Host-guest cross peaks are diagnostic of the nanoscale topological relationship, giving an insight into the incommensurate crystal structure. This kind of spectroscopy was extended from polymethylene chains to a number of polymer nanocomposites, including rubbery polymers. The most interesting examples, those are formed with elastomers, where the crystalline adducts act as reinforcement for the elastomeric material [55]. 

The molecular weights of these polymers have been determined by various methods. For example, the molecular weights (M ) of [PhPHBH2]n and [iBuPHBH2]n as determined by static light scattering methods are 20,000 and 13,100, respectively. The phosphorus chemical shifts of the phosphine-borane adducts and the linear polymers are nearly similar. Thus, while the P-NMR signal of PhPHz.BHj is at -47 ppm, the linear polymer resonates at -48.9 ppm. 

Pramanik P and Chakrovorty R (2004) The unique story of a high-tech polymer, Resonance 9 39-50. 

The UV and NMR spectra of the polymers changed with solvent. As shown in Figure 2 for the case of polymer le, in a methanol-rfVchloroform-fl mixture with a methanol content of 20 vol %, the amide proton of the polymer resonates at 5--8.1. This resonance peak progressively downfield-shifts to S 8.4, when the methanol content is increased to 80%. The shift in the resonance peak of the... 

The polymer concentration profile has been measured by small-angle neutron scattering from polymers adsorbed onto colloidal particles [70,71] or porous media [72] and from flat surfaces with neutron reflectivity [73] and optical reflectometry [74]. The fraction of segments bound to the solid surface is nicely revealed in NMR studies [75], infrared spectroscopy [76], and electron spin resonance [77]. An example of the concentration profile obtained by inverting neutron scattering measurements appears in Fig. XI-7, showing a typical surface volume fraction of 0.25 and layer thickness of 10-15 nm. The profile decays rapidly and monotonically but does not exhibit power-law scaling [70]. 

Monomer (Section 6 21) The simplest stable molecule from which a particular polymer may be prepared Monosaccharide (Section 25 1) A carbohydrate that cannot be hydrolyzed further to yield a simpler carbohydrate Monosubstituted alkene (Section 5 6) An alkene of the type RCH=CH2 in which there is only one carbon directly bonded to the carbons of the double bond Multiplicity (Section 13 7) The number of peaks into which a signal IS split in nuclear magnetic resonance spectroscopy Signals are described as singlets doublets triplets and so on according to the number of peaks into which they are split... 

Inhibitors slow or stop polymerization by reacting with the initiator or the growing polymer chain. The free radical formed from an inhibitor must be sufficiently unreactive that it does not function as a chain-transfer agent and begin another growing chain. Benzoquinone is a typical free-radical chain inhibitor. The resonance-stabilized free radical usually dimerizes or disproportionates to produce inert products and end the chain process. 

For most vinyl polymers, head-to-tail addition is the dominant mode of addition. Variations from this generalization become more common for polymerizations which are carried out at higher temperatures. Head-to-head addition is also somewhat more abundant in the case of halogenated monomers such as vinyl chloride. The preponderance of head-to-tail additions is understood to arise from a combination of resonance and steric effects. In many cases the ionic or free-radical reaction center occurs at the substituted carbon due to the possibility of resonance stabilization or electron delocalization through the substituent group. Head-to-tail attachment is also sterically favored, since the substituent groups on successive repeat units are separated by a methylene... 

The various mechanical properties of polyamides may be traced in many instances to the possibility of intermolecular hydrogen bonding between the polymer molecules and to the relatively stiff chains these substances possess. The latter, in turn, may be understood by considering still another equilibrium, this one among resonance structures along the chain backbone ... 

Once the radicals diffuse out of the solvent cage, reaction with monomer is the most probable reaction in bulk polymerizations, since monomers are the species most likely to be encountered. Reaction with polymer radicals or initiator molecules cannot be ruled out, but these are less important because of the lower concentration of the latter species. In the presence of solvent, reactions between the initiator radical and the solvent may effectively compete with polymer initiation. This depends very much on the specific chemicals involved. For example, carbon tetrachloride is quite reactive toward radicals because of the resonance stabilization of the solvent radical produced [1] ... 

For both copolymers and stereoregular polymers, experimental methods for characterizing the products often involve spectroscopy. We shall see that nuclear magnetic resonance (NMR) spectra are particularly well suited for the study of tacticity. This method is also used for the analysis of copolymers. 

Figure 7.6 Chemical shift (from hexamethyldisiloxane) for acrylonitrile-methyl methacrylate copolymers of the indicated methyl methacylate (Mj) content. Methoxyl resonances are labeled as to the triad source. [From R. Chujo, H. Ubara, and A. Nishioka, Polym. J. 3 670 (1972).]...

Figure 7.10 Nuclear magnetic resonance spectra of three poly(methyl methacrylate samples. Curves are labeled according to the preominant tacticity of samples. [From D. W. McCall and W. P. Slichter, in Newer Methods of Polymer Characterization, B. Ke (Ed.), Interscience, New York, 1964, used with permission.]...

R. E. Florin and L. A. Wad, "Electron Spin Resonance Studies on Fluorination of Polymers," abstract no. 8, Fluorine Chemistry Div., 165th American ChemicalSodef Meeting, Dadas, Tex., Apr. 10,1973. 

The dissipation factor (the ratio of the energy dissipated to the energy stored per cycle) is affected by the frequency, temperature, crystallinity, and void content of the fabricated stmcture. At certain temperatures and frequencies, the crystalline and amorphous regions become resonant. Because of the molecular vibrations, appHed electrical energy is lost by internal friction within the polymer which results in an increase in the dissipation factor. The dissipation factor peaks for these resins correspond to well-defined transitions, but the magnitude of the variation is minor as compared to other polymers. The low temperature transition at —97° C causes the only meaningful dissipation factor peak. The dissipation factor has a maximum of 10 —10 Hz at RT at high crystallinity (93%) the peak at 10 —10 Hz is absent. 

Nuclear Magnetic Resonance Spectroscopy. Bmker s database, designed for use with its spectrophotometers, contains 20,000 C-nmr and H-nmr, as weU as a combined nmr-ms database (66). Sadder Laboratories markets a PC-based system that can search its coUection of 30,000 C-nmr spectra by substmcture as weU as by peak assignments and by fiiU spectmm (64). Other databases include one by Varian and a CD-ROM system containing polymer spectra produced by Tsukuba University, Japan. CSEARCH, a system developed at the University of Vieima by Robien, searches a database of almost 16,000 C-nmr. Molecular Design Limited (MDL) has adapted the Robien database to be searched in the MACCS and ISIS graphical display and search environment (63). Projects are under way to link the MDL system with the Sadder Hbrary and its unique search capabiHties. 

---