Steric Control in Cationic Polymerization

Ionic polymerizations yield highly stereoregular polymer when control is exercised over monomer placement. Much of the steric control in cationic polymerization occurs at low temperatures. The earliest stereospecific vinyl polymerizations were observed in preparation of poly (isobutyl vinyl ether) with a BF3-ether complex catalyst at 70C. An isotactic polymer formed in this reaction [93]. The same catalyst was employed later to yield other stereospecific poly (vinyl ether) s [93–96]. The amount of steric placement increases with a decrease in the reaction temperature and, conversely, decreases with an increase in the temperature [96, 97]. Various mechanisms were proposed to explain steric placement in cationic polymerization. Most of them pertain to vinyl ethers. There is no general agreement. Some of the suggested mechanisms are discussed in this section. Most were offered for homogeneous conditions with soluble initiators like BF₃–O(C₂H₅)₂. There are, however, also some explanations of steric control with insoluble catalysts, like Al (SO₄)₂ •H₂SO₄.

Not all explanations of steric control under homogeneous conditions give equal weight to the influence of the counterion. A Bawn and Led with mechanism [99] for the polymerization of vinyl ethers is based on data that suggests that only one mesomeric form of the ethers exists, presumably trans [98]

An alkyl substituent composed of a three-carbon chain causes steric blocking of one side of the bond:

A five-carbon substituent should exhibit the highest degree of steric hindrance, which, on the other hand, should decrease with a decrease in the size of the group. There should be no blocking with an ethyl group or with an isopropyl one [98]. This was, demonstrated experimentally [99]. It was suggested, therefore, that in homogeneous polymerizations of vinyl ethers, the growing cations are stabilized by a form of neighboring group interaction. This interaction (or intramolecular solvation) would be with oxygen atoms from the penultimate monomer units [98]. These are forms of “backside” stabilization of the growing chains that force reactions to occur at the opposite sides from the locations of the counterions. The mechanisms are forms of SN2 attacks with retention of the configurations. These configuration are formed between existing and newly formed carbon cations in the transition states:

Solvations of the new cations might even occur before they are completely formed, maintaining the steric arrangement throughout, provided that the monomers enter as shown above [98]. One weakness of the above mechanism is that it fails to consider the nature of the counterions. Another mechanism, proposed by Cram and Kopecky [100], places emphasis on formation of six membered rings. The growing polymeric chains in vinyl ethers occupy equatorial arrangements with the–OR groups attached to the growing ends by virtue of their size, because they are larger. In reactions between the monomers and the six-membered ring oxonium ions the relative configurations of the two asymmetric centers that form determine total chain configurations. If the configurations are similar, the chains become isotactic, but if they are different they become syndiotactic. Molecular models suggest that isotactic placement should be more likely [100].

The CramandKopeckymechanism[43]fails to explain the influence of the various R groups upon the stereospecificity of the final product. In a mechanism proposed by Kunitake and Aso [101] two factors were given primary importance. These are: (1) steric repulsions determine the conformations of the propagating chains with a special arrangement of the counterions and those of the incoming monomers. (2) The directions of the monomer attacks are determined by the tightness of the growing ion pairs. It is assumed that the growing carbon cations are essentially sp2 hybridized and that the conformations with the least steric repulsion will, therefore, be [101] as shown in Fig. 4.1. The position of the counterion is assumed to be at the side of the carbon cation and away from the penultimate unit. The stability of such conformations should be very dependent on the temperature of the polymerization and on the size of the substituents. Experimental evidence confirms this. Thus, it is known that the stereoregular polymers, whether isotactic or syndiotactic, form only at low

Fig. 4.1 Steric arrangement. L large substituent; S small substituent

Fig. 4.2 Propagation mechanism , temperatures in homogeneous polymerizations (as stated earlier). This suggests that the fixation of the conformations of the growing chain ends is very important in enhancing polymer stereoregularity. In polar solvents the counterions interact only weakly with the growing cations. The steric effects become major factors in deciding the courses of propagation. In such situations the carbon cations [101] attack the least hindered side (frontal side attacks). These give rise to syndiotactic structures. The terminal carbon cations probably can rotate freely, so the vinyl monomers should be capable of approaching from any direction. In non-polar solvents, on the other hand, if the ion pairs are tight enough, the incoming monomers may approach the cations from the back sides only, giving rise to isotactic placements. This is illustrated in Fig. 4.2. If there is steric hindrance to backside approaches due to the large sizes of the penultimate substituents, front-side attacks take place. This occurs even in non-polar medium [101]. The incom ing monomers can, therefore, attack the cation either from the frontside or from the backside. All depends upon the tightness or the Coulombic interaction of the ion pair and on the difference in the steric hindrance between the two modes of attack. In the above reaction mechanism, the possible interactions of the counterions and the monomers are ignored. This was justified by weak interactions of electron-rich monomers, like a-methyl styrene and vinyl ethers with weak anions [101]. The nature of the counterions as such, however, is not ignored in this mechanism, because the tightness of the ion pairs is considered. Later work by Hirokawa et al. [102], suggested that the sizes of the R groups of alkenyl ethers play an important role in determining the steric structures of the resultant polymers. For instance, allyl vinyl ethers can be polymerized to highly isotactic polymers with the aid of SbCl5. 1H and 13CNMR data shows no evidence, however, of steric control, though, it does show a relationship between active chain ends and incoming monomers. In addition, the amounts of isotactic placement do not differ significantly at 10Cor at 75C [103]. This suggests that isotactic selection is generated by orienting the substituents in the monomer and in the chain away from each other. A Coulombic attraction is visualized between the counterion and the positively polarized oxygen of the monomer. Also, in studies with optically active vinyl ethers it was observed [104] that trimethyl vinyl silane, which is bulky and non-chiral forms highly syndiotactic polymers. Equally bulky, but chiral (-) menthyl vinyl ether, however, produces isotactic polymers in polar solvents. This suggests that isotactic propagation is preferred in a polar medium because of helical conformation of the polymer chain and is forced by a bulky chiral substituent. Kunitake and Takarabe [105], therefore, modified the original Kunitake and Aso mechanism. The growing chain ends are crowded by bulky substituents. This may result in steric interference between a bulky side group and the counterions. The interactions of the propagating ion pairs decrease when the sizes of the counterions increase. Afrontal attack and syndiotactic placement of the monomers results. When, however, the monomer side groups are less bulky, steric repulsion becomes insignificant. Larger counterion becomes responsible for retarding the frontside attack and gives more isotactic placement. Studies of model reactions for cationic polymerization of alkyl propenyl ethers showed that the mode of double bond opening is independent of the geometric structure of the ether. Mainly a threo opening takes place, but the mode of monomer addition is dependent on the geometric structure of the monomer or on the bulkiness of the substituent [106]. Finally, in still another investigation of model systems, UV and visible spectroscopy were used together with conductivity measurements. Results showed that charge-transfer complexes do form between the counterions and p-acceptors, which can be Lewis acids or acceptor solvents [196]. This led Heublein to suggest that interactions with monomers lead to alterations of the solvation spheres of the ion pairs in the direction of the counterions. The temporary dissymmetry of the sphere of solvation affects stereoregularities of the structures of the polymers that form. As a result, the propagation reactions are seen by Hueblein as competing interaction between the chain carriers and the monomers, the counterions, and the solvents [106].

Control in Heterogeneous Polymerizations , Several reaction mechanisms were also proposed to explain stereospecific placement with insoluble catalysts. Furukawa [46] suggested that here the mechanism for cationic polymerization of vinyl ethers depends upon multicentered coordinations. He felt that coordinations of the polymeric chains and monomers with the catalysts are possible if the complexed counter anions have electrically positive centers. This can take place in the case of aluminum alkyl and boron fluoride:

Further coordination of aluminum alkyl to the anions is possible if the coordination number of the central atom is sufficiently large [46]:

The products are complex counterions that enable multicentered coordination polymerization. Thus, the mechanism of vinyl ether polymerization proposed by Furukawa [46] is as follows. Two neighboring ether oxygens that are linked to the polymer chain close to the terminal cation become coordinated to the metal center of the complexed counterion. The molecules of the monomer can then approach the growing chain only from the opposite side and isotactic placement results:

where B represents boron and Me aluminum. A different mechanism, however, was offered by Nakano and coworkers [47]. They felt that there must be a relationship between the crystal structures of the heterogeneous catalysts and the resultant stereoregularity of the polymers. If the crystal structures of the catalysts are tetrahedral and the crystals have active edges, stereoregular polymers should form even at room temperature. In addition, shorter active edges make the catalysts more suitable for stereospecific polymerization. The following mechanism was, therefore, proposed [47]. If the terminal end of the growing chain ends:

have sp² type configurations, vacant orbitals on the terminal carbon atoms of the growing polymeric chains are in a state of resonance with the lone pair of electrons on the adjacent oxygen atoms. This means that the positive charges are distributed to the adjacent oxygens and are not localized on the carbons:

The monomer can potentially add in four different ways:

Reaction 3 yields isotactic polymers and should be the mode of addition [47] when isotactic polymer forms.

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مواضيع ذات صلة

Living Cationic Polymerizations

Pseudo-Cationic Polymerization

Propagation in Cationic Polymerization

Electro Initiation of Polymerization

Charge Transfer Complexes in Ionic Initiations

One Electron Transposition Initiation Reactions

Metal Alkyls in Initiations of Cationic Polymerizations

Lewis Acids in Cationic Initiations

Initiation by Protonic Acids

Cationic Polymerization

The Chemistry of Ionic Chain-Growth Polymerization

Polymer Preparation Techniques