The Synthesis of Hydrophobe-Modified Hydroxyethyl Cellulose Polymers Using Phase Transfer Catalysis

Union Carbide Corporation Bound Brook Technical Center Bound Brook, NJ 08805 

The rheological properties of all HMHEC polymers are profoundly affected by the hydrophobe molar substitution (MS) and the hydrophobe chain length. For any given hydrophobic moiety, there is a threshold hydrophobe MS below which there are no significant associative interactions. The most common phenomenological evidence for associative behavior is a dramatic increase in the solution viscosity of HMHEC polymers as a function of hydrophobe MS. The solution viscosity of HMHEC polymers continues to increase as a function of hydrophobe MS until the maximum limit of solubility is reached, as which point the HMHEC polymer becomes insoluble in water.33 

Because the cellulose ether alkoxide is present entirely in the aqueous phase, the rate-limiting step may be the partitioning (phase transport) of the hydrophobic electrophile across the interface from the organic to aqueous phase. If the reaction rate is controlled by diffusion of the electrophile across the interface, then one would expect a correlation between water solubility of the hydrophobe and its alkylation efficiency. The fact that the actual alkylation reaction is probably occurring in the aqueous phase (or at the interface) yet the electrophile itself is principally soluble in the organic phase has important mechanistic ramifications. This type of synthetic problem, in which one reactant is water soluble and the other organic soluble, should be amenable to the techniques of phase transfer catalysis (PTC) to yield significant improvements in the alkylation efficiency. 

Theory would predict that PTC should be useful in increasing the alkylation efficiency of hydrophobic electrophiles with cellulose ether alkoxides. However, there is very little previous work reported in using PTC in the preparation of cellulose ethers. Daly and coworkers10 reported that quaternary ammonium salts were useful in catalyzing the heterogeneous benzylation of cellulose, but when we applied this technique to the DPGE alkylation of nascent HEC in aqueous /-butyl alcohol, the presence of catalytic amounts of tetramethylammonium chloride or tetrabutylammonium bromide actually afforded lower alkylation efficiencies. 

This paper describes our efforts to apply phase transfer catalysis (PTC) to the synthesis of HMHEC polymers. The potential use of PTC in the manufacture of HMHEC polymers is important because a large portion of the manufacturing cost of HMHEC polymers is the cost of the hydrophobe reactant. Higher alkylation efficiencies would increase the overall reaction efficiency and thus reduce the overall cost of the final HMHEC product. Increased hydrophobe alkylation efficiencies would also reduce the volume of unreacted hydrophobe in the waste stream and reduce disposal costs. 

---

THE SYNTHESIS OF HYDROPHOBE-MODIFIED HYDROXYETHYL CELLULOSE POLYMERS USING PHASE TRANSFER CATALYSIS... 

---