Distribution stereosequence

Many radical polymerizations have been examined from the point of view of establishing the stereosequence distribution. For most systems it is claimed that the tacticity is predictable within experimental error by Bemoullian statistics [i.e. by the single parameter P(m) - see 4.2.1],... 

The physical properties (7-10) of our E-V copolymers are sensitive to their microstructures. Both solution (Kerr effect or electrical birefringence) and solid-state (crystallinity, glass-transitions, blend compatibility, etc.) properties depend on the detailed microstructures of E-V copolymers, such as comonomer and stereosequence distribution. I3C NMR analysis (2) of E-V copolymers yields microstructural information up to and including the comonomer triad level. However, properties such as crystallinity depend on E-V microstructure on a scale larger than comonomer triads. 

Because of the close connection between stereosequence distribution and... 

The polymer stereosequence distributions obtained by NMR analysis are often analyzed by statistical propagation models to gain insight into the propagation mechanism [Bovey, 1972, 1982 Doi, 1979a,b, 1982 Ewen, 1984 Farina, 1987 Inoue et al., 1984 Le Borgne et al., 1988 Randall, 1977 Resconi et al., 2000 Shelden et al., 1965, 1969]. Propagation models exist for both catalyst (initiator) site control (also referred to as enantiomorphic site control) and polymer chain end control. The Bemoullian and Markov models describe polymerizations where stereochemistry is determined by polymer chain end control. The catalyst site control model describes polymerizations where stereochemistry is determined by the initiator. 

Although several techniques have been used to characterize stereosequence distribution, we suggest that the percent crystallinity and temperature of melting measurements are more generally applicable than any other technique presently available. Bovey and co-workers (7) showed how NMR measurements can be used to determine the triad distribution in polymers such as polymethyl methacrylate in which there is sufficient difference between the NMR spectra corresponding to syndiotactic, isotactic, and heterotactic triads to allow quantitative measurements to be made. This type of measurement unfortunately is restricted to few systems and would lead to a unique description of the stereostructure of the chain only when a model involving one or two probability parameters is applicable (See Appendix I). 

The bar graph in Figure 7 compares the triad stereosequence probabilities for the PVF samples examined here with values calculated for an equilibrium stereochemical distribution which would result from epimerization at 50 °C (18). All PVF samples, except for the urea polymer, rather fortuitously have nearly an equilibrium stereosequence distribution. Furthermore, both the precursor PVCF and final PVF derived therefrom have nearly identical p(m) values and stereosequence distributions, so that the effect of racemization during reductive dechlorination (19) is not apparent. 

Whereas the stereosequence distribution in isoregic and aregic PVF is nearly ideal random (Bernoullian with p(m) — 0.5), the latter has a first-order Markov regiosequence distribution. Accordingly the monomer sequence isomerism in PVF cannot be described by a single parameter such as the % defect, and requires two reactivity ratios for complete specification. 

Isotactic polystyrene was epimerized to various extents by reaction with KOtBu in hexamethylphosphor-amide solution at 100°. The 13C-NMR spectra of the epimerized samples and of polystyrene in 9 1 trichlorobenzene nitrobenzene-d6 solution at 150° were recorded and analyzed using stereosequence distributions that were calculated for the samples by Monte Carlo simulation of the epimerization process. 

Resonance area measurements were made by cutting and weighing spectra tracings. Spectra simulations were done using a Calcomp plotter with the aid of a program written by Dr. B.L. Bruner of the University of Kentucky, using stereosequence distributions calculated for the polymers by Monte Carlo simulations of the epimeri-... 

Table III. Correlation of A-F Aromatic C-l Carbon Resonance Areas Measured for Polystyrene and Epimerlzed Isotactic Polystyrene Samples with Stereosequence Distributions Calculated by Monte Carlo Simulation (V=0.65)...

In some cases, a site isomerization (i.e. a chain migration from a site to the other without monomer insertion) can occur. When this mechanism influences the stereosequence distribution (as is the case in syndiotactic polymerizations), the statistical model should contain a probability parameter to describe this event. 

The quantitative stereosequence distribution was then reproduced by making use of matrix multiplication methods in the framework of appropriate stochastic models (7). 

Analyses of polymers to determine stereosequence distributions and understand the propagation mechanism can be carried out with NMR spectroscopy aided by statistical propagation models. 75 detailed discussion of the subject is beyond this book. The following is a brief explanation of the concepts. The Bernoulli, Markov, and Colman-Fox, models describe propagation reactions with chain end control over monomer placement. The Bernoulli model assumes that the last monomer unit in the propagating chain end determines the stereochemistry of the polymer. No consideration is given to the penultimate unit or other units further back. In such an event, two modes... 

Difluorocarbene, generated under mild neutral conditions, adds to 1,4-cw- and 1,4-rra/w-poly-butadienes to give materials containing cyclopropane groups. The addition takes place randomly, to give atactic stereosequence distributions ... 

In the case of inherently more complicated stereoblock polypropylenes containing (close-to-)atactic blocks, with all possible stereosequences present in comparable amounts, the requirements for a meaningful microstructural analysis are even more stringent. Routine NMR characterization of propylene polymers gives access to the stereosequence distribution at the pentad... 

MHz C NMR Stereosequence Distribution of the Polypropylene Sample Shown in Figure 8.11 and Best-Fit Calculated Stereosequence Distributions in Terms of the Chain-End (CE) or Enantiomorphic-Sites (ES) Stochastic Model of Chain Propagation ... 

In the NMR spectra of PLA, the observed resonances can be assigned to stereosequence distribution in the polymer and reflect its history including the stereochemistry of the feed composition, polymerization kinetics, and extent of transesterification and racemization [57-60]. The assignments are designated as various combinations of z isotactic pairwise relationship (RR and SS) and s syndiotactic pairwise relationship (RS and SR). In the NMR spectra, the combinations RR and SS are indistinguishable and have identical chemical shifts, as would RS and SR [57, 61]. 

Table III lists calculated MA-centered triad fractions, F, Fg and Fq values used in this determination. Figure 6 shows plots of the left-hand parts (L.H.P.) of Eqs. 1-3 vs. (F ms+ SHM)/ SMS The plots are linear and their slopes and intercepts define a O g value of 0.47. This suggests that MS and SM placements in the copolymers have nearly equal probabilities of being coisotactic or cosyndio-tactic. Since poly(methyl acrylate)and poly(a-fluorostyrene) have been shown to have random stereosequence distributions when prepared by free radical initiated polymerizations, the copolymers may be considered to have atactic stereosequence structures.

The quaternary aromatic carbon resonance of the copolymers (6q=140.8 ppm) is observed as a group of four resonances, the relative intensities of which are influenced by C-F couplings, by sequence distribution effects and probably also by stereosequence distribution effects. In the spectrum of poly(a-fluorostyrene), this resonance region contains two strong signals separated by 28 Hz (1.4 ppm), which corresponds to for this polymer. The... 

Several studies have been conducted using NMR spectroscopy to determine the stereosequence distribution in PLA. Kricheldorf and Kreiser-Saunders (1990) has pioneered the use of methine resonance in and NMR spectra on various synthesis methods of PLA as well as the initiators/catalysts... 

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