Isotactic stereosequences

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). 

Sodium montmorilonite can also be used to polymerize polar monomers between the lamellae. Here, too, the organization of monomer molecules within the monolayers influences the structure of the resultant polymers. Poly (methyl methacrylate) formed in sodium montmorilonite is composed of short, predominantly isotactic stereosequences. The percent of isotactic component... 

Fig. 4.17 illustrates the potential of carbon-13 NMR to detect the presence of isotactic (a), syndiotactic (b), and atatactic (c) vinyl polymers with polypropylene as sample [521], The spectrum of atactic polypropylene (Fig. 4.17(c)) displays the signals of all possible stereosequences including iso- and syndiotactic ones. Using the empirical increment systems for alkane carbon shift prediction [85, 201, 202] and including y effects of Zy = — 5 ppm specifically obtained by analysis of stereoisomeric polypropylene partial sequences between 3,5-dimethylheptane and 3,5,7,9,11,13,15-heptamethylheptadecane as a heptad model, the methyl carbon-13 shifts of all 36 possible heptads can be calculated... 

Spin-lattice relaxation times of carbon-13 in different polypropylene stereosequences differ slightly while nuclear Overhauser enhancements are almost identical (1.8-2.0) [533] isotactic sequences display larger Tx values than the syndiotactic stereoisomers. Other vinyl polymers behave correspondingly [534]. Carbon-13 spin-lattice relaxation times further indicate that dynamic properties in solution depend on configurational sequences longer than pentads. The ratio 7J(CH) 7J(CH2) varies between 1.6 to 1.9 thus, relaxation can be influenced by anisotropic motions of chain segments or by unusual distributions of correlation times [181],... 

Isoblock polypropylene is characterised by the appearance of stereosequences of m diads bridged by pairs of r diads [20,21], Such polypropylene, containing a small number of long blocks, is usually referred to as isotactic polypropylene. Note that the polymer chains in this case contain helix sequences of the same handedness (right or left) in the chain, which is due to the same relative configuration of tertiary carbon atoms in these sequences. Stereoblock polypropylene, which is also called block-isotactic polypropylene,... 

When using the aluminium isopropoxide catalyst to promote the polymerisation of lactide, all three isopropoxide groups have been found to initiate the polymerisation [137,139]. Ring cleavage in the polymerisation with aluminium isopropoxide and diethylaluminium isopropoxide occurred at the C(0)-0 bond, leading to aluminium alcoholate propagating species [139], Lactide polymerisation with other catalysts containing multinuclear species proceeded similarly [138-140]. Analysis of stereosequences of lactide polymers obtained with catalysts such as aluminium isopropoxide, diethylaluminium alcoholate and methylaluminoxane indicates that none of these catalysts favours the formation of isotactic blocks [139]. 

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). 

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. 

The dependence of CD-IC formation from solution for polymers with different stereosequences was investigated for isotactic (i) and atactic (a) poly(3-hydroxy-butyrates) (i-PHB and a-PHB) [26], i-PHB was found to form an IC with a-CD, but not with 0- or y-CDs, while only y-CD formed an IC with a-PHB. From these observations it was concluded that extended conformations available to i-PHB were too narrow, thin for a tight-packing fit with 0- or y-CDs, while the much wider, thicker extended conformations available to a-PHB chains precluded their inclusion in the narrower a- and 3-CDs, but fit tightly in the more spacious channels of its IC formed with y-CD. 

A monosubstituted vinyl monomer yields polymers having three regioregular arrangements of configuration (Fig. 5), described by triad stereosequences. The isotactic structure has all R groups on the same side of the backbone (mm) the syndiotactic structure has R groups on alternate sides of the polymer s backbone (rr) and the heterotactic or atactic structure has R groups randomly oriented on either side of the polymer s backbone. 

H and 13C NMR relaxation studies in concentrated chloroform solution307 308 and 13C relaxation studies of cross-linked gels in benzene- 309 were carried out on PMMA. The dependence of the segmental mobility on stereosequence was not affected by the presence of covalent cross-links in the chain. The isotactic sequences are more mobile than the syndiotactic sequences.309... 

The major splitting of the isoregic resonance into the three peaks is due to triad stereosequences, as noted above. Assuming that polymerization at low temperature in urea favors syndiotactic propagation, we assign these peaks to isotactic, heterotactic, and syndiotactic sequences in order of increasing field as... 

The isotactic (mm), heterotactic (mr), and syndiotactic (rr) triad stereosequence probabilities for (1) commercial PVF, (2) PVF derived from PVCF, (3) PVF derived from PCFE, (4) urea PVF and (5) PVF epimerized at 50°C 08). Note that the observed heterotactic probability is mr + rm. 

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. 

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)...

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

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. 

The 220-MHz H NMR spectra of three polypropylene (PP) samples are presented in Fig. 20.8 [5-7]. Note the apparent greater resolution of the spectra in (a) and (c) recorded for the stereoregular samples (isotactic and syndiotactic) than for atactic PP in (b). The impression of degraded resolution in the spectrum for atactic PP is a consequence of the overlapping of many slightly different resonance frequencies or chemical shifts corresponding to the various triad and tetrad stereosequences present in the atactic sample. Only the rr (rrr) and mm (mmm) triads (tetrads) are present in the stereoregular syndiotactic and isotactic samples, respectively. 

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