Isotactic polymers crystallization

In spite of the similarity of the structure of the monomer units the two corresponding isotactic polymers crystallize in two different chain conformations tiie helix of poly-3-methyl-l-butene contains four monomer units per turn (4/1) with a chain repeat of 6.85 A the helix of poly-4-methyl-l-pentene contains 3.5 units per turn (7/2) and has a repeat of 13.85 A. The copolymers tend to crystallize. Their chain conformation and cross sectional area in the crystal lattice are analogous to those of the homopolymer corresponding to the predominant comonomer. For 4-methyl-l-pentene contents higher than 50% some evidence exists that the system simultaneously contains both chain conformations. 

Natta and co-workers (35) have shown by careful x-ray investigation that crystalline polyaldehydes are isotactic polymers crystallizing in a four-fold helix with an identity period of 4.8 A. (Figure 4). 

The anomalous low density of poly(4-methyl pent-l-ene) for a crystalline hydrocarbon polymer has been resolved in a new crystallographic analysis. The isotactic polymer crystallizes as a 2 helix with symmetry PAb and unit cell with dimensions a= 18.70 and c= 13.68 A. There are four chains with seven... 

Snetivy D and Vancso G J 1994 Atomic force microscopy of polymer crystals 7. Chain packing, disorder and imaging of methyl groups in oriented isotactic polypropylene Po/yme/ 35 461... 

Term N J, Fairclough P A, Towns-Andrews E, Komanshek B U, Young R J and Ryan A J 1998 Density fluctuations the nucleation event in isotactic polypropylene crystallization Polymer 29 2381- 5... 

Stereodefects reduce the overall regularity of an isotactic polymer chain and hinder its ability to crystallize. As the concentration of defects increases, the degree of crystallinity falls, resulting in reduced density, reduced melting temperatures, lower heat distortion temperatures, reduced modulus, and reduced yield stress. 

We will then examine other flexible polymer crystallization instances which may be interpreted, at least qualitatively, in terms of the bundle model. We will concentrate on crystallization occurring through metastable mesophases which develop by quenching polymers like isotactic polypropylene, syndiotactic polypropylene etc. In principle also hexagonal crystallization of highly defective polymers, and order developing in some microphase-separated copolymer systems could be discussed in a similar perspective but these two areas will be treated in future work. A comparison between the bundle approach and pertinent results of selected molecular simulation approaches follows. 

The conformation of the chains of isotactic polymers in the crystalline state is generally helical and corresponds to a succession of nearly trans and gauche torsion angles, the exact values depending on the bulkiness of the side groups. Molecular mechanics calculations have been extensively used for the prediction of the chain conformation of polymers in the crystal.29... 

Data concerning the chain conformations of isotactic polymers are reported in Table 2.1. In all the observed cases the torsion angles do not deviate more than 20° from the staggered (60° and 180°) values and the number of monomeric units per turn MIN ranges between 3 and 4. Chains of 3-substituted polyolefins, like poly(3-methyl-l-butene), assume a 4/1 helical conformation (T G )4,45,46 while 4-substituted polyolefins, like poly(4-methyl-1-pentene), have less distorted helices with 7/2 symmetry (T G )3.5-39 When the substituent on the side group is far from the chain atoms, as in poly(5-methyl-1-hexene), the polymer crystallizes again with a threefold helical conformation (Table 2.1). Models of the chain conformations found for the polymorphic forms of various isotactic polymers are reported in Figure 2.11. 

For the isotactic polymer, highly extended chains with chain axis of 7.5-7.6 A can be obtained with a helix repeating after two structural units [s(2/l) line repetition group] when 01 02, and 03 are in the range 180° 8, with 8 being 25-30° and 8i + 82 + 83 0.125 The experimental values of the torsion angles found in the crystal structure of i-STCO123 are indeed 0i = —161.5°, 02 = 155.3°, and 03 = -171.4°. Similar values have been found by conformational... 

In the crystal structures of many other isotactic polymers, with chains in threefold or fourfold helical conformations, disorder in the up/down positioning of the chains is present. Typical examples are isotactic polystyrene,34,179 isotactic poly(l-butene),35 and isotactic poly(4-methyl-l-pentene).39,40,153,247... 

While the properties and applications of isotactic polymers have been extensively studied, those of syndiotactic polymers received less attention until relatively recently. The reason is the relative ease of forming isotactic polymers. Syndioselective polymerizations were less frequently encountered or proceeded with less efficiency compared to isoselective polymerizations. But the situation is changing fast as initiators and reaction conditions have been developed for syndioselective polymerizations. In the case of polypropene, the properties of the syndiotactic polymer have been examined [Youngman and Boor, 1967]. Syndiotactic polypropene, like its isotactic counterpart, is easily crystallized, but it has a lower Tm by about 20°C and is more soluble in ether and hydrocarbon solvents. 

Linear polymers prepared by step reaction polymerization, such as nylon 66, and linear, ordered polymers prepared by the chain polymerization of symmetrical vinylidene monomers, such as polyvinylidene chloride (PVDC), can usually be crystallized because of symmetry and secondary-bonding. Isotactic polymers, such as isotactic polypropylene (PP), usually crystallize as helices. 

Certain chiral organic compounds create crystalline environments and act as enantio-controlling media (7) even though they do not function as true catalysts. Natta s asymmetric reaction of prochiral trans-1,3-pentadiene, which was included in the crystal lattice of chiral perhydro-triphenylene as a host compound, to form an optically active, isotactic polymer on 7-ray irradiation, is a classic example of such a chiral molecular lattice (Scheme 1) (2). Weak van der Waals forces cause a geometric arrangement of the diene monomer that favors one of the possible enantiomeric sequences. 

Some information is available on other acrylates. N,N-disubstituted acrylamides form isotactic polymers with lithium alkyls in hydrocarbons (12). t-Butylacrylate forms crystallizable polymers with lithium-based catalysts in non-polar solvents (65) whereas the methyl, n-butyl, sec-butyl and isobutyl esters do not. Isopropylacrylate also gives isotactic polymer with lithium compounds in non-polar solvents (34). The inability of n-alkylacrylates to form crystallizable polymers may result from a requirement for a branched alkyl group for stereospecific polymerization. On the other hand lack of crystallizability cannot be taken as definite evidence of a lack of stereoregulating influence, as sometimes quite highly regular polymer fails to crystallize. The butyllithium-initiated polymers of methylmethacrylate for instance cannot be crystallized. The presence of a small amount of more random structure appears to inhibit the crystallization process1. 

Poly a-methyl styrene is also reported to have a predominantly syndiotactic structure as prepared with butyllithium in cyclohexane (11). Sakurada and co-workers (90) have suggested that there is the possibility of error in the N. M. R. band assignments and that the polymer may be predominantly isotactic. It is difficult to assess the validity of this claim without details of the crystallization of the supposedly isotactic polymer. 

I, 3-diene polymerization. Monomer molecules are included in chiral channels in the matrix crystals, and the polymerization takes place in chiral environment. The y-ray irradiation polymerization of trans- 1,3-pentadiene included in 13 gives an optically active isotactic polymer with a trans-structure. The polymerization of (Z)-2-methyl-1,3-butadiene using 15 as a matrix leads to a polymer having an optical purity of the main-chain chiral centers of 36% [47]. 

After having studied in our laboratory, polymer blends of amorphous polymers poly-c-caprolactone and poly (vinyl chloride) (1,2) (PCL/ PVC), blends with a crystalline component PCL/PVC (3,4), poly(2,6-dimethyl phenylene oxide) (PPO) with isotactic polystyrene (i-PS) (5) and atactic polystyrene (a-PS) with i-PS (6), we have now become involved in the study of a blend in which both polymers crystallize. The system chosen is the poly(1,4-butylene terephthalate)/poly(ethylene terephthalate) (PBT/PET) blend. The crystallization behavior of PBT has been studied extensively in our laboratory (7,8) this polymer has a... 

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