Stereosequence length

Several probability models have been suggested for characterizing stereosequence length and relating it to the polymerization (4, 7,9, 10, 11). However,... 

Characterizing Stereosequence Length of Propylene Oxide Polymers from Different Catalysts... 

This paper describes the theory which permits us to characterize adequately the stereosequence length in stereoregular polymers from the equilibrium percent crystallinity at room temperature and from the melting points of the polymers. Results based on this theory are given on the characterization of the isotactic stereosequence length in the crystalline fractions of polypropylene oxide polymers made from the following catalyst systems (a) ferric chloride (17, 19) (b) diethyl zinc-water (10) (c) diethyl zinc-water-isopropylamine (d) diethyl zinc-water-cyclohexylamine (14). 

The stereoregularity—i.e., distribution of the stereosequence length in these polymers—has a marked effect on the crystallization rates and the morphology of the crystalline aggregates. These differences, in turn, influence the dynamic mechanical properties and the temperature dependence of the dynamic mechanical properties. In order to interpret any differences in the dynamic mechanical properties of polymers and copolymers of propylene oxide made with different catalysts, it was interesting to study the differences in the stereosequence length in the propylene oxide polymers made with a few representative catalysts. 

On the other hand, the temperature of melting and crystallinity measurement can describe only those stereosequences that crystallize. In propylene oxide polymers which we have studied, for example, isotactic sequences are the only ones that crystallize. Therefore, in these polymers we can determine only the stereosequence length of isotactic units and the average length of the uncrystallizable units. The syndiotactic and atactic sequences in the uncrystallizable blocks are not distinguishable from the crystallinity and melting temperature measurements. 

Characterizing the Stereosequence Length of Propylene Oxide Polymers. The samples used in this study were the crystalline fractions of propylene oxide polymers made with various catalysts. Two methods were used to separate the fraction showing crystalline order (as judged from x-ray diffraction patterns) from the amorphous fraction ... 

Table II. Stereosequence Length in Propylene Oxide Polymers...

Effect of Stereosequence Length On Crystallization Kinetics and Morphology. 

Figure 4 shows photomicrographs of (a) a polypropylene oxide with long average stereosequence length (Sample A), and (b) a polypropylene oxide with a short stereosequence length (Sample C). 

The stereosequence length also has a marked effect on the isothermal crystallization kinetics of the propylene oxide polymers. These studies and analysis of results on crystallization kinetics will be described in detail in another communication. Here we summarize briefly the main conclusions of the effect of stereosequence length on the isothermal crystallization rates. 

Figure 4. Effect of stereoregularity on texture of propylene oxide polymers at same degree of supercooling A T - 30° C. Sample A with long stereo sequence length sample C with short stereosequence length.

On the other hand, stereocomplexes and solvatophobic bonds are specific to macromolecular multimerization. Certain polymers with mutually complementary stereostructures form what are called stereocomplexes whose stoichiometry is dependent on the stereosequence length. Examples of such stereocomplexing are provided by the pairs poly(7-benzyl-D-glutamate) with poly(7-benzyl-L-glutamate) and isotactic with syndiotactic poly(methyl methacrylates) (PMMA). The specific enthalpy of stereocomplex formation with it- and st-PMMA is, for example, a linear function of the syndiotactic diad mass fraction, with a maximum at Wst = 0.58 (Figure 6-11). If, however. 

A number of catalyst systems have been developed (2 fL>A which result In propylene oxide polymers of different stereosequence distribution. In the following, we review some of our work on the characterization of stereosequence length In propylene oxide polymers prepared with different catalysts, and more importantly, studies on the effect of the differences in stereose-sequence length on the crystallization behavior and mechanical properties of the polymers. 

The Effect of Stereosequence Length on Morphology and Crystallization Kinetics... 

The effect of stereosequence distribution on crystallization kinetics Is dramatic. We have previously reported our studies on the Important effects of stereosequence length on crystallization kinetics and morphology of propylene oxide polymers (22). Here we summarize the main conclusions of this study, so that results on the time-temperature dependence of mechanical response may be fully appreciated In the light of these conclusions. 

Stereosequence length of Isotactic units in poly(propylene oxide) is determined by the catalyst system used. The stereosequence length of isotactic units in stereoregular poly(pro-pylene oxide) polymers may be characterized from their melting point and degree of crystallinity. The stereosequence length of isotactic units in poly(propylene oxide) from ferric chloride catalyst is considerably longer than those from other catalyst systems such as diethyl zinc-water and diethyl zinc-water-isopropylamine. 

---