Polymerization of propylene oxide

Fig. 2. Reaction scheme for the anionic polymerization of propylene oxide.

Polymerization to Polyether Polyols. The addition polymerization of propylene oxide to form polyether polyols is very important commercially. Polyols are made by addition of epoxides to initiators, ie, compounds that contain an active hydrogen, such as alcohols or amines. The polymerization occurs with either anionic (base) or cationic (acidic) catalysis. The base catalysis is preferred commercially (25,27). 

Co/Zn double metal cyanide catalyzed ring-opening polymerization of propylene oxide effect of cocataiysts on polymerization behavior... 

Polymerizations of propylene oxide were rarried out by using 1 L autoclave (Parr) at... 

Cationic complexes, such as 55 and 56, catalyze the polymerization of propylene oxide, cyclohexene oxide, and of e-caprolactone with substantially higher activities than neutral zinc complexes. 

We have reported a gas phase and solution phase study comparing mono-and dinuclear CF" complexes as catalysts for the ring opening polymerization of propylene oxide. (Adapted from Schon et al., 2004)... 

We will now consider segments of a polymer derived from the polymerization of propylene oxide. Here the simplest approach is to simply consider this an extension of the case immediately above except where the chloride atoms are substituted by methyl radicals and the next methylene is now an oxygen atom. Thus, we can make the same assignments based on the meso, racemic considerations. 

Excluding polymerizations with anionic coordination initiators, the polymer molecular weights are low for anionic polymerizations of propylene oxide (<6000) [Clinton and Matlock, 1986 Boileau, 1989 Gagnon, 1986 Ishii and Sakai, 1969 Sepulchre et al., 1979]. Polymerization is severely limited by chain transfer to monomer. This involves proton abstraction from the methyl group attached to the epoxide ring followed by rapid ring cleavage to form the allyl alkoxide anion VII, which isomerizes partially to the enolate anion VIII. Species VII and VIII reinitiate polymerization of propylene oxide as evidenced... 

Anionic polymerization of propylene oxide is usually limited to producing a relatively low-molecular-weight polymer. Discuss the reasons for this occurrence. 

Epoxidation of olefins over Mo containing Y zeolites was studied by Lunsford et al. [86-90]. Molybdenum introduced in ultrastable Y zeolite through reaction with Mo(C0)g or M0CI5, shows a high initial activity for epoxidation of propylene with t-butyl hydroperoxide as oxidant and 1,2-dichloroethane as solvent [88]. The reaction is proposed to proceed via the formation of a Mo +-t-butyl hydroperoxide complex and subsequent oxygen transfer from the complex to propylene. The catalyst suffers however from fast deactivation caused by intrazeolitic polymerization of propylene oxide and resulting blocking of the active sites. 

Fig. 36. Polymerization of propylene oxide (PO) initiated with the (TPP)AlCl (1, X=Cl)-2-propanol (2-PrOH) system in the presence of methylaluminum bis(2,6-di-tert-butyl-4-methylphenolate) (3e) ([2-PrOH]o/[PO]o/[l]o=9/200/l) in CH2CI2 at rt. Effect of the concentration of 3e on the rate of polymerization...

In 1955, Pruitt and Baggett (4,5) reported the polymerization of propylene oxide catalyzed by the reaction product of ferric chloride and propylene oxide, i.e., Pruitt-Baggett catalyst . Polypropylene oxide obtained from DL-monomer could be fractionated into two parts, one rubbery and another resinous. The latter fraction gave a discrete crystalline X-ray diffraction pattern. 

Stereospecific Polymerization of Propylene Oxide by the R2A10A1R2 Catalyst... 

Addition of two moles of a donor substance, 5,6-benzoquinoline, to one mole of R2A OAlR2 deactivated completely the activity of the latter. This fact provides positive experimental evidence for the currently accepted assumption that the coordination of monomer to catalyst is an essential factor for occurrence of stereospecific polymerization of propylene oxide. 

The diethylzinc-alcohol (1 2) system was also extensively studied by Tsuruta and his co workers (85,86). Amorphous zinc dialkoxide was concluded to be an active species, because crystalline zinc alkoxide prepared from zinc chloride and lithium alkoxide proved to have only a very small catalytic activity. Based on kinetic studies of the polymerization of propylene oxide with the ZnEt2-CH3OH (1 2) catalyst system, the catalytically active species was concluded to be the complex formed by coordination of one molecule of monomer to the catalyst. In the polymerization of propylene oxide with the catalyst system, it was concluded that the monomer was polymerized by ring opening brought about by cleaving the CH2-0 bond (87). 

In order to find a highly stereospedfic, homogeneous catalyst for the polymerization of propylene oxide, we selected the organometallic compound-primary amine catalyst system which exhibited excellent stereospecificity in the polymerization of acetaldehyde. 

Results of the polymerization of propylene oxide by these four kinds of organometallic compounds showed that the zinc compounds are superior to the aluminum compounds with respect to their stereoregulating power. It is noteworthy that the zinc catalyst is generally superior to the aluminum catalyst as the stereospedfic polymerization catalyst. Among the two organozinc compounds, EtZnNBu ZnEt was thought to be more suitable than EtZnNHBu for detailed studies, because the former was monomeric and had no reactive hydrogen atom. 

Catalytic activities of these complexes for the polymerization of propylene oxide were tested. The pyridine complex had no catalytic activity, whereas the diethyl ether or dioxane complex had activity comparable to that of the mother organozinc compound. This fact indicates that a strong electron donor which cannot be replaced by propylene oxide inhibits the polymerization effectively. Thus, the co-... 

The propylene oxide complex not only dissociated into its components but also transformed to either an oligomer or a polymer of propylene oxide when it was allowed to stand in solution. This transformation could be followed by H-NMR techniques with the use of a-deuterated propylene oxide instead of the non-deuterated one. Its rate depended on the nature of solvent and on the temperature. This experimental result implies that the monomer liberated by dessociation of the complex is polymerized by the catalyst, that only a minute fraction of the organozinc component of the complex actually acts as a catalyst for polymerization, and that the rate of propagation is far faster than that of initiation. These implications together with the evidence that coordination of the monomer to the catalyst is a prerequisite for the stereospecific polymerization led us to the detailed studies of the bulk polymerization, that is, the polymerization of propylene oxide in propylene oxide solution. 

Fig. 15. Temperature dependence of polymer composition in the stereospecific polymerization of propylene oxide by EtZnNBu ZnEt [Oguni el. al. (90)]...

Table 11. BulK polymerization of propylene oxide by EtZnNBu ZnEt at 80° C...

Polymerization of propylene oxide-a-d was carried out by the EtZnNBu ZnEt catalyst in benzene solution in the presence of varying amounts of added water at 70° C, and was terminated after 7 days. The microstructure of the crude polymer was determined by the 1H-NMR method and the yields of amorphous and crystalline polymers were determined by a fractionation method (Fig. 16). When the amount of added water was increased up to 0.3 mole per mole of catalyst, the yield of crystalline polymer increased remarkably, whereas that of amorphous one remained nearly constant, and the isotactic dyad content (I) increased remarkably while syndiotactic one (S) remained almost constant. Thus, the striking parallel was observed between the yield of crystalline polymer and the isotactic dyad content, and between the yield of amorphous polymer and the syndiotactic dyad content. It is therefore concluded that water contributes more remarkably to the formation... 

Price, C.C, Spector.R. Partial head-to-head polymerization of propylene oxide by stereospecific catalysts. J. Am. Chem Soc. 87, 2069 (1965). 

---