Polymerization aldol group transfer

A related technique called aldol GTP makes use of the reaction of an aldehyde with a silyl vinyl ether. The cocatalysts used are again Lewis acids, and if used with t-butyl dimethyl silyl vinyl ether, the combination will polymerize aldehydes  

Hydrolysis of the trialkyl silyl groups gives poly(vinyl alcohol). Block copolymers in which one block is poly( vinyl alcohol) can be synthesized if a telechehc with an aldehyde terminal group is used to initiate the aldol GTP structures such as polyfsty-rene-h/oc -vinyl alcohol) can be prepared in this way. Alternatively, as silyl ketene acetals can react with aldehydes, block structures can be formed by a coupling process. 

During this reaction, cleavage of the silyl groups by fluoride ion in the presence of methanol forms the vinyl alcohol block. 

---

Webster, O. W. andD. Y. Sogah, Group Transfer and Aldol Group Transfer Polymerization, pp. 163—169 in Comprehensive Polymer Science, Vol. 4, G. C. Eastmond, A. Ledwith, S. Russo, and P. Sigwalt, eds., Pergamon Press, London, 1989. 

If the living ROMP of norbomene is terminated with a 9-fold excess of terephthalaldehyde, the chains formed carry an aldehyde end-group which, when activated by ZnCl2, can be used to initiate the aldol-group-transfer polymerization of tert-butyldimethylsilyl vinyl ether621. 

Aldol group transfer polymerization of ferf-butyldimethylsilyl vinyl ether [62] was initiated by pendant aldehyde functions incorporated along a poly(methyl methacrylate) (PMMA) backbone [63]. This backbone was a random copolymer prepared by group transfer polymerization of methyl methacrylate (MMA) and acetal protected 5-methacryloxy valeraldehyde. After deprotection of the aldehyde initiating group, polymerization proceeded by activation with zinc halide in THF at room temperature. The reaction led to a graft copolymer with PMMA backbone and poly(silyl vinyl ether) or, upon hydrolysis of the ferf-butyldimethylsilyl groups, poly(vinyl alcohol) branches. 

Charleux, B. and Pichot, C. (1993) Styrene-terminated poly(vinyl alcohol) macromonomers. 1. Synthesis by aldol group transfer polymerization. Polymer, 34, 195. 

More recently the workers at Du Pont have developed aldol group transfer polymerization, in which a silyl vinyl ether is polymerized using an aldehyde as the initiator to give a living silylated poly(vinyl alcohol), e.g. 

The key to the success of this synthesis of the complex spiroketal 3 was the assembly of 7 in one step as a single diastereomer from the readily-available building blocks 4,5, and 6. This process, reminiscent of group transfer polymerization, will be a useful complement to the cascade organocatalyzed aldol condensations that have recently been developed. 

A variation of GTP, referred to as aldol GTP, involves polymerization of a silyl vinyl ether initiated by an aldehyde [Sogah and Webster, 1986 Webster, 1987]. Both initiation and propagation involve nucleophilic addition of the vinyl ether to the aldehyde carbonyl group with transfer of the trialkyl silyl group from vinyl ether to the carbonyl oxygen (Eq. 5-82). The reaction has similar characteristics as GTP. The product is a silated poly(vinyl alcohol) (PVA), which can be hydrolyzed by acid to PVA (Eq. 5-83). 

In a process related to GTP, aldehydes initiate the polymerization of silyl vinyl ethers and silyl diene ethers. Here the silyl group is present in the monomer and transfers to the aldehyde ended chains regenerating aldehyde ends [17] (Scheme 8). A Lewis acid catalyst is required. terf-Butyldimethylsilyl works best as a transfer group for vinyl ether while trimethylsilyl is suitable for diene ethers [18]. Even though aldol GTP provides a route to polyvinyl alcohol segments in the subsequent block polymer synthesis, the projected cost of the monomers discouraged further research aimed at commercialization. 

---