Isotactic 392 Subject

As shown in the previous section the mechanical and thermal properties of polypropylene are dependent on the isotacticity, the molecular weight and on other structure features. The properties of five commercial materials (all made by the same manufacturer and subjected to the same test methods) which are of approximately the same isotactic content but which differ in molecular weight and in being either homopolymers or block copolymers are compared in Table 11.1. 

The polymerization of MMA has been shown to be subject to enantiomorphic site control when the Ci-symmetric a .va-lanthanocene complexes (196) and (197) are employed as initiators.463 When the (T)-neomenthyl catalyst (196) is used, highly isotactic PMMA is produced (94% mm at — 35 °C), whereas the (-)menthyl derived (197) affords syndiorich PMMA (73% rr at 25 °C). NMR statistical analysis suggests that conjugate addition of monomer competes with enolate isomerization processes, and the relative rate of the two pathways determines the tacticity. 

When D, L-alanine A-carboxylic acid anhydride was subjected to polymerisation with triethylaluminium as the catalyst, the polymer formed, polyalanine, was found [168,174] to consist of two fractions a water-insoluble fraction which was rich in isotactic sequences and a water-soluble atactic fraction. This indicated that some stereoselective polymerisation had occurred [75]. 

Polymers containing cyclophane-type moieties were also prepared for their unique functionalities and potential applications. Thus, isotactic poly(para-phenylene) ladder polymer (56) was prepared from the corresponding bromide and borate (55) in enantiomerically pure forms and subjected to the CD spectral measurement [62],... 

Thin films of isotactic or elastomeric [51] polypropylene can be prepared from pellets by melt pressing. For this purpose several iPP or ePP pellets are placed between two sheets of thick aluminum foil and are subjected to pressure in in a hot press (T - 175°C pressure ca. 1-10 tons). Careful Wear appropriate safety shields and gloves After cooling the sample down to ambient temperature, the foil is unwrapped. If the polymer sticks to the foil, the film can be removed after cooling it down below the glass transition temperature e.g. in liquid nitrogen. Careful For... 

Fig. 1.9 AFM tapping mode images of a spherulitic texture in isotact polypropylene. The sample was crystallized to completion at 145°C and subjected to permanganic etching prior to examination. Image (a) shows topography while (b) contains phase information. Scale bars 5 iim.

Note Characterization of stereoregular PMS is a subject of considerable controversy. Many disagree as to whether a specific sample is syndiotactic or isotactic. As a result, the method of polymerization is usually referred to in the literature instead of the stereoregularity. 

Polypropylene Blends Blends of isotactic polypropylene, PP, with a polyethylene are immiscible and, owing to their commercial importance, have been the subject of intensive studies. In these blends, PP crystallization mostly takes place in the presence of molten PE droplets. 

Mechanical Behavior Variation of an Isotactic Polypropylene Copolymer Subjected to Artificial Aging... 

The chapter shows the analysis of the mechanical properties variation of an isotactic polypropylene copolymer, when this material is subjected to artificial aging according to the standard UNE 4892. 

Once the samples of isotactic polypropylene copolymer were subjected to the artificial aging process, the next phase in the development process was to obtain and comp>are the mechanical proporties of the aged material and the mechanical properties of the original material. 

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