Isotactic polystyrene, epimerization

A general method has been developed for the estimation of model parameters from experimental observations when the model relating the parameters and input variables to the output responses is a Monte Carlo simulation. The method provides point estimates as well as joint probability regions of the parameters. In comparison to methods based on analytical models, this approach can prove to be more flexible and gives the investigator a more quantitative insight into the effects of parameter values on the model. The parameter estimation technique has been applied to three examples in polymer science, all of which concern sequence distributions in polymer chains. The first is the estimation of binary reactivity ratios for the terminal or Mayo-Lewis copolymerization model from both composition and sequence distribution data. Next a procedure for discriminating between the penultimate and the terminal copolymerization models on the basis of sequence distribution data is described. Finally, the estimation of a parameter required to model the epimerization of isotactic polystyrene is discussed. 

Epimerization of Isotactic Polystyrene.This application considers modelling the epimerization of isotactic polystyrene by Monte... 

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. 

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... 

Our previous reports concerned 20 MHz 13C-NMR spectra of partially epimerized polystyrenes. The aromatic C-l carbon resonances were recorded at room temperature and were poorly defined when the extent of epimerization was high. A need for remeasuring these resonances at higher temperature, using higher field spectrometers was clearly evident. This paper is therefore concerned with the 75 MHz 13C-NMR specrra of partially epimerized isotactic polystyrenes. 

HARWOOD ETAL. Polystyrene and Epimerized Isotactic Polystyrenes 201... 

Aromatic C-l Resonance. Figures 1-6 show aromatic C-l carbon resonance patterns observed for various epimerized isotactic polystyrene samples. The patterns are easily divided into six resonance areas, that are designated A-F in order of increasing field. Areas A, C and E are clear in these Figures while areas B, D and F are cross-hatched. Except for the resonance pattern observed for atactic polystyrene (Figure 6), the resonances appear to be assignable as follows ... 

Carbon Resonance Pattern of an Epimerized Isotactic Polystyrene Sample Having a mm-Content of 0.70. 

Epimerized Isotactic Polystyrene Samples from Aromatic C—1 Carbon Resonances and Calculated by Monte Carlo Simulation... 

Smmmmmm can be measured from the chemical shift of isotactic polystyrene or from that of the largest signal in polymers epimerized to low extents and is 146.40 ppm for the conditions employed in the present study. Accordingly, the chemical shift of any heptad can be calculated by the following general formulas and specific examples. 

Table IV. Heptad Stereosequences Responsible for Minor Signals in Partially Epimerized Isotactic Polystyrene Samples having mm-Contents Below 0.50.

Although the aromatic C-l carbon resonance of polystyrene is very complex, relatively simple aromatic C-l carbon resonance spectra are observed for partially epimerized isotactic polystyrenes. Studies on such "model polystyrenes" provide the information needed to interpret the spectrum of polystyrene itself. Based on assign-... 

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