Direct Alternating Copolymerization of PO and CO

In 1963, Heck reported the ring opening of propylene oxide by the carbonylating reagent tetracarbonylhydridocobalt(I) in the presence of carbon monoxide, which results in a stable acyl cobalttetracarbonyl compound (Fig. 15). However, no polymeric products were reported, which would result from multiple ring opening and CO insertion processes [58, 59]. 

The opened epoxide shown in Fig. 15 is also an intermediate in the catalytic ring expansion of epoxides to p-lactones (see Sect. 8.4). In this context, Drent and coworkers patented a catalytic system of dicobaltoctacarbonyl Co2(CO)g and 

The molecular weights of the produced polymers are quite small, ranging from 4,000 g/mol to 7,000 g/mol. The products feature an atactic microstructure, which is not desired for industrial purposes. Many other catalytic systems were investigated, such as Zn[Co(CO)4]2 and Hg[Co(CO)4]2 [62], Small improvements were achieved by using Ph3SiCo(CO)4 as catalyst-precursor since this compound is able to 

Alper and coworkers investigated the influence of more bulky bases (2,2 -bipyridine, 1,10-phenanthroline, and 6,7-dihydro-5,8-dimethyldibenzo[b,j]-l,10-phenantroline) and para-toluene sulfonic acid (p-TsOH) as proton source (proton functions as a Lewis acid). In the presence of p-TsOH a molecular weight up to 10,000 g/mol was reported. This could be slightly increased by the use of benzylbromide instead of p-TsOH, forming PhCH2COCo(CO)4 as catalyst in situ. In this case, a molecular weight comparable to that found in Rieger s work was achieved, but the polymers remain atactic [69]. 

To date, no catalytic system has been found that converts racemic PO to tactic PHBs. Since the synthesis of enantiopure PO is very expensive, only racemic PO can be used on an industrial scale. Therefore, other pathways to yield feasible materials are area of investigation. 

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