Tetraphenylporphyrin Aluminum and Zinc Initiators

One of the most widely studied systems for the polymerization of cyclic ethers is the tetra-phenylporphyrinato aluminum system, (TPP)AIX. Most investigations have focused on the chloride complex, (251), which initiates the living ROP of EO, PO, and Et-EO.936 For example, 400 equivalents of EO require 3 hours in CH2C12 at 25 °C to reach 80% conversion. Mn values increase linearly with monomer conversion, with polydispersities typically 1.10, and chain lengths controlled by the initial monomer initiator ratio. 

The initiation mechanism comprises nucleophilic attack of the metal chloride at the least-hindered carbon. Thus hydrolysis of the 1 1 reaction product of (251) with PO yields l-chloropropan-2-ol.937 

The rate of polymerization is heavily dependent upon the size of the epoxide substituents. Consequently, Et-EO polymerizes slower than either PO or EO,936 and (TPP)AICI inserts only one molecule of Bu-EO over a period of 5 days. Reactivity follows the order EO PO -Et-EO cis-BO ECH Bu-EO. Further, in a 1 1 mixture of cis- and trans-BO, the cis monomer is opened preferentially, possibly because coordination of the monomer to the metal center occurs prior to ring cleavage. It has also been shown that the 1° alkoxide derived from the ringopening of EO is much more reactive than the 2° alkoxide resulting from PO. 

The living nature of the system has been utilized to prepare a range of block copolymers, including PEO-b-PPO,938 939 P(Et-EO)-b-PEO, P(Et-EO)-b-PPO and the triblock P(Et-EO)-b-PEO-b-PPO.936 Random copolymerization of EO and PO also affords narrow molecular weight distributions.940 However, for the copolymerization of PO with slower monomers, such as ECH, the more active PO incorporates preferentially to afford block-like materials. 

Analogous alkoxides, phenoxides, and carboxylates will also initiate the ROP of epoxides, all forming propagating alkoxide species.779 Block copolymers of epoxides with /3-butyrolactone have been prepared via the addition of EO or PO to living poly(ester) chains.782 The oxygen-bound enolate of living PMMA will also react with epoxides to yield diblocks such as PEO-b-PMMA and PPO-b-PMMA (Mn= 12,800, Mw/Mn = 1.16) 787 

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