Stereochemically asymmetric polymerizations

Definitions relating to stereochemically asymmetric polymerizations (lUPAC Recommendations 2001), PureAppl. Chem. 74, 915-922 (2002). Reprinted as Chapter 5, this edition. 

Definitions relating to stereochemically asymmetric polymerizations Pure Appl. Chem. 74 (2002) 915. 

These experiments revealed the important stereochemical features of polyisocyanide. First, racemic polyisocyanides consisting of achiral repeating units exist as a 1 1 mixture of right- and left-handed helices. Second, the right- or left-handed helical conformation is stable enough to exist as either enantiomer at ambient temperature in solution, although the stability may depend upon the AT-substituent as well as the degree of polymerization. Based on these ideas, efficient systems for asymmetric polymerization of isocyanides have been further pursued. 

Stereochemically asymmetric ROP polymerizations of cyclic esters (hereafter stereocontrolled processes) involve chiral monomers. There are two major cyclic esters that bear centers of chirality p-BL and LAs (see structures 4). In this section, a notion of the absolute configuration will be used thus, the relative configurations d and l correspond to the absolute configurations R and S, respectively. 

The polymerization of enantiomerically pure monomers presents no relevant stereochemical problems when the asymmetric carbon atom is not involved in the reaction and no new centers of stereoisomerism are formed. This is the case, for example, in polycondensation of chiral diacids with diamines (274) and in ring-opening polymerization of substituted lactams (275) and A -carboxyanhy-drides of a-amino acids (276). Interest here lies mainly in the properties of the polymer. Accidental racemization may sometimes occur but is not necessarily related to the mechanism of polymerization. 

The hypothesis of stereochemical control linked to catalyst chirality was recently confirmed by Ewen (410) who used a soluble chiral catalyst of known configuration. Ethylenebis(l-indenyl)titanium dichloride exists in two diaste-reoisomeric forms with (meso, 103) and C2 (104) symmetry, both active as catalysts in the presence of methylalumoxanes and trimethylaluminum. Polymerization was carried out with a mixture of the two isomers in a 44/56 ratio. The polymer consists of two fractions, their formation being ascribed to the two catalysts a pentane-soluble fraction, which is atactic and derives from the meso catalyst, and an insoluble crystalline fraction, obtained from the racemic catalyst, which is isotactic and contains a defect distribution analogous to that observed in conventional polypropylenes obtained with heterogeneous catalysts. The failure of the meso catalyst in controlling the polymer stereochemistry was attributed to its mirror symmetry in its turn, the racemic compound is able to exert an asymmetric induction on the growing chains due to its intrinsic chirality. 

Basie definitions of terms relating to polymerization reactions [1,2] and stereochemical definitions and notations relating to polymers [3] have been published, but no reference was made explieitly to reaetions involving the asymmetric synthesis of polymers. It is the aim of the present doeument to recommend classification and definitions relating to asymmetrie polymerizations that may produce optically active polymers. 

Full understanding of the stereospecific polymerization, especially of the asymmetric one, is one of the most fascinating problems in synthetic organic chemistry as well as in synthetic polymer chemistry. Requirement posed on the stereospedfic polymerization may be severer than that on the stereospedfic reaction of low molecular wdght compound, because any side reaction, if occurred, puts the structurally and stereochemically irregular units, which cannot be separated from the regular one, in the polymer molecule. 

Despite its inherent difficulties, carbometallation has, in fact, played important roles in catalytic asymmetric carbon-carbonal bond formation. Isotactic and syndiotactic alkene polymerization involving both heterogeneous and homogeneous Ti and Zr catalysts must involve a series of face-selective carbometallation processes, although the main stereochemical concern in poly(alkene) formation is diastereoselectivity rather than enantioselectivity. This fascinating topic, however, is outside the scope of this chapter, and the readers are referred to Chapter 11 and other previous reviews [6]. 

PMPS, PMHS, PMDS, and PDHS. Individual resonances are observed for each carbon type, except for the dodecyl polymer in which C-4 to C-9 in the side chain are not completely resolved. The carbon resonances can also be assigned without the use of model compounds by using standard two-dimensional NMR techniques (2). C-1 and, in some cases, C-1, C-2, or both in the three asymmetrically substituted polymers show shift dispersion, which results from the many different stereochemical sequences along the polymer chain. The methyl carbon in PMPS is resolved to at least the pentad level of stereosequences. This chemical shift information can be analyzed to provide a description of the chain statistics resulting from a particular polymerization. 

Since complexes 92 and 93 are achiral, it is likely that they form tactic polymers via a chain-end control process (vide infra). To increase the isospecificity of complex 93 for polymerization, Schrock replaced the achiral alkoxide groups with a range of Q-symmetric diols to create asymmetric complexes 94—98. These complexes, especially rac-95, exert an exceptionally high degree of stereochemical control in the polymerization of 90 and 91, producing polymers that are >99% cis and have >99% isotactic (mm) triads. Even complex 98 that employs a conformationally flexible ligand produces highly isotactic, cis polymers. 

In addition to this structural problem, there is also a stereochemical problem. Unlike vinyl monomers or a-olefins, propylene oxide has an asymmetric carbon atom before polymerization. Thus, it is possible to obtain optically active polymers if the polymerization proceeds with either complete retention or complete inversion at the asymmetric center. Four different dimer units in the main chain are therefore possible [eq.(l)]. 

The asymmetric induction polymerizations mentioned up to now have been performed in the presence of optically active catalysts which, even without the presence of chiral components, are capable of controlling the stereochemical path of the reaction and give stereoregular polymers following an ionic polymerization mechanism. Asymmetric in-... 

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