Stereorandom polymers

The m ority of the stereochemical studies described in the above cited papers surest an S]s 2 mechanism of propagation. However, Hail observed in the polymerization of some bicyclic acetals (e.g. 2,6-dioxabicyclo[2.2.2)octane) the formation of a stereorandom polymer, indicating an Si>(l proems. The randomization proceeds at higher temperatures whereas at low temperatures, a stereoregular polymer is exclusively formed by an Si 2 process l... 

Polymerization of l-LA, which is the most readily available enantiomerically pure form of lactide, leads to the formation of poly(L-LA) providing that no epimerization of the monomer stereocenter occurs. Epimerization can be avoided by use of the majority of metal coordination catalysts (Scheme 25.2) and by using the Lewis base/alcohol process (Scheme 25.3). However, in the polymerization of rac-LA or meso-LA, stereorandom polymers are generally produced. Figure 25.3 shows polylactides with various tacticities. Epimerization occurs only with the most highly active of catalysts, such as organolithium and organomagnesium initiators. 

One equivalent of PEO was weighed into a dried round-bottom flask. The macroinitiator was then melted at 1S0°C and flushed with nitrogen for approximately thirty minutes (until no more bubbles are seen) to remove any remaining water. Half of an equivalent of Stannous (II) 2-ethyl hexanoate catalyst was added to the PEO, followed by immediate addition of either L-iactide monomer to create the stereoreguiar polymer or the meso-DL-lactide monomer to create the stereorandom polymer. The flask was then capped and the polymerization was carried out in the bulk at ISO C for 24 hours with stirring (Figure 1). When done, the mixture was quenched with methanol, dissolved in tetrahydrofuran, and precipitated in hexanes. Dissolution and precipitation was repeated three more times, and the polymer was dried under vacuum at room temperature for approximately 2 days. 

Apparently, with Li+ as the counterion, very little, if any, control over the pathway of ring opening can be achieved. The result is then a random distribution of inversions and retentions leading to stereorandom polymer. Therefore, to achieve some degree of control over whether the polymerization occurs with retentions or inversion of configuration at the attacked silicon atom and the newly formed reactive center, a modified initiator must be employed. 

Epichlorohydrin Elastomers without AGE. ECH homopolymer, polyepichlorohydrin [24969-06-0] (1), and ECH—EO copolymer, poly(epichlorohydrin- (9-ethylene oxide) [24969-10-6] (2), are linear and amorphous. Because it is unsymmetrical, ECH monomer can polymerize in the head-to-head, tail-to-tail, or head-to-tail fashion. The commercial polymer is 97—99% head-to-tail, and has been shown to be stereorandom and atactic (15—17). Only low degrees of crystallinity are present in commercial ECH homopolymers the amorphous product is preferred. 

Neither PVC nor polystyrene is very crystalline and polystyrene often has poor mechanical strength. Both of these maybe results of the stereorandom nature of the polymerization process. The substituents (Cl or Ph) are randomly to one side or other of the polymer chain and so the polymer is a mixture of many diastereoisomers as well as having a range of chain lengths. Such polymers are called atactic. In some polymerizations, it is possible to control stereochemistry, giving (instead of atactic polymers) isotactic (where all substituents are on the same side of the zig-zag chain) or syn-diotactic (where they alternate) polymers. 

ECH elastomers have more than 97% of head-to-tail sequences, they are amorphous (no cristallinity detected by DSC or x-ray analysis) and with a stereorandom distribution of enantiomeric units. These polymers are thus atactic. In ECH/EO copolymer the mole ratio of both monomers was kept to approximately I. In the terpolymer the amount of AGE is small (around 2.5%). 

Two catalyst systems, RhCl and allyl nickel iodide which give high trans polymer were examined and found to give identical, stereorandom results. Examination of recovered monomer shows no isomerization the stereorandomness of the bond forming event must be inherent to the mechanism. 

While several explanations may be given for these findings, we focus here on our contention that this result supports the importance of allyl isomerization in diene polymerization. We postulate initially that the loss of bond forming stereospecificity in the diene polymerization is the result of allyl isomerization. This is particularly well supported by the completely stereorandom bond formation found for trans polymer. This is satisfactorily accounted for by rapid isomerization of the label, via primary allyls, to a random(l l) mixture of syn and anti deuterium. Any other mechanism for randomization, for example... 

A unique mechanism is shown in Figure 1. This scheme starts with an allyl/diene complex with both the growing polymer chain and the deuterium label in an anti configuration initially. Rapid allyl isomerization (left hand branch) leads to trans backbone and stereorandom deuterium rapid bond formation (right hand... 

An atactic polymer is stereorandom, but an isotactic or syndiotactic polymer is stereoregular. These three classes of polymers, even if derived from the same monomer, will have different physical properties. 

Polymerization of l-LA (or d-LA) leads to an isotactic polymer that is assigned as Hi at the tetrad level. The stereorandom polymerization of rac-LA leads to five possible tetrads Hi, Us, sH, isi, and sis. Because each monomer unit in rac-LA contains an isotactic junction, there can be no ss junctions. Similarly, in the polymerization of meso-LA (which contains an s junction) there can be only five tetrads, namely, sss, ssi, iss, sis, and isi. No ii junctions are possible for po y(meso-LA). However, if a mixture of L-, d-, and meso-LA is polymerized, or if pure L-, D-, or meso-LA is polymerized with concomitant epimerization (for l-LA or d-LA) or transesterification (for rac-LA or... 

In this chapter, various synthesis routes for trifluorovinyl aromatic ethers are discussed. PFCB polymers are prepared via thermally activated [2+2] cycloaddition of aryl trifluorovinyl ether monomers. The cyclodimerizaton proceeds in a stereorandom fashion giving a roughly equal distribution of cis- and trans- stereoisomers. The PFCB technology can serve as a versatile materials platform for many industrial... 

He concluded that this remarkable difference in physical properties resulted from identical configuration of all the asymmetric centers in the crystalline polymer. The liquid polymer from racemic monomer, on the other hand, was evidently a stereorandom atactic polymer (4). Both polymers were later shown to be formed almost exclusively by head-to-tail polymerization. Vandenberg, Price, and others later showed that addition of each epoxide unit to the polymer chain occurs with inversion of configuration at the carbon atom where ring opening occurs. The asymmetry in this case is not disturbed, however, because the bond between the oxygen and the asymmetric carbon is never broken. 

A DSC thermogram of a stereorandom copolymer is shown in Figure 5. Similar to the L copolymers discussed above, the R polymers exhibit an endotherm between 50 and 53 C corresponding to melting of the crystalline PEO mid-block. However, there are no further phase changes over the sampled temperature range. In this case, the copolymers do not show melting of PLA because the stereocenter dictates amorphous PLA end blocks. 

Usually free-radical polymerization of acrylic systems produces mainly stereorandom (atactic or heterotactic) polymers, but Duran and Gramain have demonstrated a correlation between tacticity triads and the spacer length in acrylate monomers subjected to free-radical solution... 

---