Production of bimodal molecular weight poly(hydroxyalkanoates)

Abstract

A poly(hydroxyalkanoate) composition having a bimodal molecular weight distribution. The composition includes of a first portion and a second portion of a poly(hydroxyalkanoate). The first portion has a first weight average molecular weight and the second portion has a second weight average molecular weight which is at least 50 percent less than the first weight average molecular weight. The poly(hydroxyalkanoate) is made up of at least 10 mole percent monomer repeat units of 3-hydroxypropionate. A method for making the composition is also disclosed.

What is claimed is: 
     
         1 . A composition comprising:
 a first portion of a poly(hydroxyalkanoate) and a second portion of the poly(hydroxyalkanoate),   wherein the first portion of the poly(hydroxyalkanoate) has a first weight average molecular weight and the second portion of the poly(hydroxyalkanoate) has a second weight average molecular weight which is at least 50 percent less than the first weight average molecular weight and   wherein the poly(hydroxyalkanoate) comprises at least 10 mole percent monomer repeat units of 3-hydroxypropionate.   
     
     
         2 . The composition of  claim 1 , wherein the poly(hydroxyalkanoate) comprises poly(3-hydroxypropionate). 
     
     
         3 . The composition of  claim 1 , wherein the poly(hydroxyalkanoate) is a copolymer or a terpolymer which comprises
 a first repeat unit which is (3-hydroxypropionate),   a second repeat unit according to Formula I   
       
         
           
           
               
               
           
         
         optionally, a third repeat unit according to Formula II, 
       
       
         
           
           
               
               
           
         
         wherein R 1  and R 2  are each independently selected from the group consisting of linear or branched alkyl groups having from 1 to 22 carbon atoms. 
       
     
     
         4 . The composition of  claim 3 , wherein the poly(hydroxyalkanoate) comprises a copolymer and R 1  is methyl. 
     
     
         5 . The composition of  claim 4 , wherein the poly(hydroxyalkanoate) copolymer comprises from about 10 to about 90 mole percent of the first repeat unit and from about 90 to about 10 mole percent of the second repeat unit. 
     
     
         6 . The composition of  claim 3 , wherein the poly(hydroxyalkanoate) comprises a terpolymer, R 1  is methyl, and R 2  is propyl. 
     
     
         7 . The composition of  claim 6 , wherein the poly(hydroxyalkanoate) terpolymer comprises from about 15 to about 75 mole percent of the first repeat unit, from about 75 to about 15 mole percent of the second repeat unit, and from about 1 to about 5 mole percent of the third repeat unit. 
     
     
         8 . The composition of  claim 1 , wherein the composition comprises from about 90 to about 50 weight percent of the first portion of the poly(hydroxyalkanoate) and from about 50 to about 10 weight percent of the second portion of the poly(hydroxyalkanoate). 
     
     
         9 . The composition of  claim 1 , wherein the first weight average molecular weight is at least 100,000 daltons, as determined by ASTM D5296 and the second weight average molecular weight is less than about 25,000 daltons, as determined by ASTM D5296. 
     
     
         10 . The composition of  claim 1 , wherein the composition has a polydispersity of at least 1.9, as determined by ASTM D5296. 
     
     
         11 . The composition of  claim 1 , wherein the weight ratio of the first portion of the poly(hydroxyalkanoate) to the second portion of the poly(hydroxyalkanoate) from about 1:1 to about 9:1. 
     
     
         12 . A film comprising the composition of  claim 1 . 
     
     
         13 . A fiber comprising the composition of  claim 1 . 
     
     
         14 . A method for making a composition having a first portion of a poly(hydroxyalkanoate) and a second portion of the poly(hydroxyalkanoate), the method comprising the steps of:
 polymerizing a first charge of at least one substituted lactone to produce a first portion of a poly(hydroxyalkanoate) having a first weight average molecular weight;   polymerizing a second charge of the at least one substituted lactone to produce a second portion of the poly(hydroxyalkanoate) having a second weight average molecular weight; and   blending the first portion and the second portion to produce a final poly(hydroxyalkanoate),   wherein the at least one substituted lactone comprises beta-propiolactone and the poly(hydroxyalkanoate) comprises monomer repeat units of 3-hydroxypropionate, and   wherein the second weight average molecular weight is at least 50 percent less than the first weight average molecular weight.   
     
     
         15 . The method of  claim 14 , wherein the poly(hydroxyalkanoate) comprises poly(3-hydroxypropionate). 
     
     
         16 . The method of  claim 14 , wherein the poly(hydroxyalkanoate) is a copolymer or a terpolymer which comprises
 a first repeat unit which is (3-hydroxypropionate),   a second repeat unit according to Formula I   
       
         
           
           
               
               
           
         
         optionally, a third repeat unit according to Formula II, 
       
       
         
           
           
               
               
           
         
         wherein R 1  and R 2  are each independently selected from the group consisting of linear or branched alkyl groups having from 1 to 22 carbon atoms. 
       
     
     
         17 . The method of  claim 14 , wherein the first charge is polymerized in a first loop reactor. 
     
     
         18 . The method of  claim 14 , wherein the second charge is polymerized in a second loop reactor. 
     
     
         19 . The method of  claim 14 , wherein the step of blending the first portion and the second portion is carried out by polymerizing the second charge to form the second portion in the presence of the first portion.