Oxymethylene polymers and process for the preparation thereof

1. A method for the preparation of oxymethylene polymers comprising the following step:

polymerization of a monomer, which forms —CH 2 —O— units in the presence of

i) an acetal of formaldehyde or a polyhydric alcohol and

ii) an initiator for cationic polymerization

wherein said initiator is a heteropoly acid or an acid salt thereof which is dissolved in an alkyl ester of a polybasic carboxylic acid.

2. The method according to claim 1 , wherein the heteropoly acid is a compound represented by the general formula (1):

H x [M m M′ n O z ].y H 2 O  (1)

wherein

M represents an element selected from the group consisting of P, Si, Ge, Sn, As, Sb, U, Mn, Re, Cu, Ni, Ti, Co, Fe, Cr, Th and Ce,

M′ represents an element selected from the group consisting of W, Mo, V and Nb,

m is 1 to 10,

n is 6 to 40,

z is 10 to 100,

x is an integer of 1 or above, and

y is 0 to 50.

3. The method according to claim 1 , wherein the heteropoly acid is a compound represented by the following formula:

H x [M m .M′ n O z ].y H 2 O

wherein

M represents an element selected from the group consisting of P and Si;

M′ represents a coordinating element selected from the group consisting of W, Mo and V;

z is 10 to 100;

m is 1 to 10;

n is 6 to 40;

x is an integer of at least 1; and

y is 0 to 50.

4. The method according to claim 1 , wherein the heteropoly acid is selected from the group consisting of phosphomolybdic acid, phosphotungstic acid, phosphomolybdotungstic acid, phosphomolybdovanadic acid, phosphomolybdotungstovanadic acid, phosphotungstovanadic acid, silicotungstic acid, silicomolybdic acid, silicomolybdotungstic acid, silicomolybdotungstovanadic acid and acid salts thereof.

5. The method according to claim 1 , wherein the heteropoly acid is selected from 12-molybdophosphoric acid (H 3 PMo 12 O 40 ) and 12-tungstophosphoric acid (H 3 PW 12 O 40 ) and mixtures thereof.

6. The method according to claim 1 , wherein the alkyl ester of a polybasic carboxylic acid is an alkyl ester of an aliphatic dicarboxylic acid of formula (2):

(ROOC)—(CH 2 ) n —(COOR′)  (2)

wherein

n is an integer from 2 to 12, and

R and R′ represent independently from each other an alkyl group having 1 to 4 carbon atoms.

7. The method according to claim 1 , wherein the alkyl ester of a polybasic carboxylic acid is a compound of formula (3):

(ROOC) 2 —CH—(CH 2 ) m —CH—(COOR′) 2   (3)

wherein

m is an integer from 0 to 10, and

R and R′ are independently from each other alkyl groups having 1 to 4 carbon atoms.

8. The method according to claim 1 , wherein the alkyl ester of a polybasic carboxylic acid is a compound of formula (4):

(ROOC)—C 6 H 4 —(COOR′)  (4)

whereby R and R′ represent independently from each other alkyl groups having 1 to 4 carbon atoms.

9. The method according to claim 1 , wherein the alkyl ester of a polybasic carboxylic acid is selected from the group consisting of dimethyl glutaric acid, dimethyl adipic acid, dimethyl pimelic acid, dimethyl suberic acid, diethyl glutaric acid, diethyl adipic acid, diethyl pimelic acid, diethyl suberic acid, diemethyl phthalic acid, dimethyl isophthalic acid, dimethyl terephthalic acid, diethyl phthalic acid, diethyl isophthalic acid, diethyl terephthalic acid, butantetracarboxylic acid tetramethylester and butantetracarboxylic acid tetraethylester as well as mixtures thereof.

10. The method according to claim 1 , wherein the melting point of the alkyl ester of the polybasic carboxylic acid is lower than 100° C.

11. The method according to claim 1 , wherein the boiling point of the alkylester of the polybasic carboxylic acid at 1 bar is higher than 100° C.

12. The method according to claim 1 , wherein the alkyl ester of the polybasic carboxylic acid does not contain a nitrogen atom.

13. The method according to claim 1 , wherein the heteropoly acid is dissolved in the alkyl ester of the polybasic carboxylic acid in an amount ranging from 0.01 to 5 wt.%, wherein the weight is based on the entire solution.

14. The method according to claim 1 , wherein formaldehyde or trioxane is used as monomer which forms —CH 2 —O— units.

15. The method according to claim 1 , wherein, as acetal of formaldehyde, a compound of the formula (5) is used

R 1 —(O—CH 2 ) q —O—R 2   (5)

in which

R 1 and R 2 represent independently of each other alkyl groups, and

q is an integer from 1 to 100.

16. The method according to claim 15 , wherein, as acetal of formaldehyde, a compound of the formula (5) is used in which q=1.

17. The method according to claim 16 , wherein as acetal of formaldehyde, methylal is used.

18. An oxymethylene polymer formed according to the method of claim 1 .

19. The oxymethylene polymer according to claim 17 , wherein the range of oxymethylene polymers having a number average molecular weight Mn of less than 8,000 Dalton is less than 3% by weight, wherein the weight is based on the weight of the entire polymer.