Compositions of resin-linear organosiloxane block copolymers

1. A process for preparing an organopolysiloxane block copolymer comprising:

I) reacting

a) a linear organosiloxane having the formula

R 1 2 (OH)SiO(R 1 2 SiO 2/2 ) n Si(OH)R 1 2 ,

R 1 3-q (E) q SiO(R 1 2 SiO 2/2 ) n Si(E) q R 1 3-q , or

 combinations of the two linear organosiloxanes,

wherein each R 1 , at each occurrence, is independently a C 1 to C 30 hydrocarbyl,

n is 10 to 400,

E is a hydrolyzable group containing at least one carbon atom,

each q is, independently, 1, 2 or 3, and

b) an organosiloxane resin comprising at least 80 mole % of [R 2 SiO 3/2 ] siloxy units in its formula and 1-30 mole % of siloxy units containing a Si-E bond where E is a hydrolyzable group containing at least one carbon atom, wherein each R 2 , at each occurrence, is independently an aryl or C 1 to C 20 hydrocarbyl,

in c) an organic solvent,

to form a resin-linear organosiloxane block copolymer;

wherein the amounts of a) and b) used in step I) are selected to provide the resin-linear organosiloxane block copolymer with 40 to 90 mole % of disiloxy units [R 1 2 SiO 2/2 ] and 10 to 60 mole % of trisiloxy units [R 2 SiO 3/2 ], and

wherein at least 95 weight percent of the linear organosiloxane used in step I) is incorporated into the resin-linear organosiloxane block copolymer;

II) reacting the resin-linear organosiloxane block copolymer from step I) to crosslink the trisiloxy units of the resin-linear organosiloxane block copolymer sufficiently to increase the molecular weight of the resin-linear organosiloxane block copolymer by at least 50%;

III) optionally, further processing the resin-linear organosiloxane block copolymer; and

IV) optionally, removing the organic solvent.

2. The process of claim 1 , wherein the linear organosiloxane has the formula (HO)(CH 3 ) 2 SiO[(CH 3 ) 2 SiO 2/2 )] n Si(CH 3 ) 2 OH.

3. The process of claim 1 , wherein the linear organosiloxane has the formula (HO)(CH 3 )(C 6 H 5 )SiO[(CH 3 )(C 6 H 5 )SiO 2/2 )] n Si(CH 3 )(C 6 H 5 )(OH).

4. The process of claim 1 , wherein R 2 is phenyl or methyl.

5. The process of claim 1 , wherein E is an oximo, epoxy, carboxy, amino, or amido group.

6. The process of claim 1 , wherein E is (CH 3 )(CH 3 CH 2 )C═N—O-(methylethylketoximyl).

7. The process of claim 1 , wherein E is H 3 CC(═O)O-(acetoxy).

8. The process of claim 1 , wherein step II) further comprises adding an organosilane having the formula R 5 q SiX 4-q , where R 5 is a C 1 to C 8 hydrocarbyl or a C 1 to C 8 halogen-substituted hydrocarbyl, X is a hydrolyzable group, and q is 0, 1, or 2.

9. The process of claim 8 , wherein the organosilane is an alkyltriacetoxysilane.

10. The process of claim 9 , wherein the alkyltriacetoxysilane is a mixture of two alkyltriacetoxysilanes.

11. The process of claim 10 , wherein the mixture of two alkyltriacetoxysilanes comprises methyltriacetoxysilane and/or ethyltriacetoxysilane.

12. The process of claim 8 , wherein the amount of organosilane added provides a molar stoichiometry of 2 to 15 mole % of organosilane per mole of Si in the organosiloxane resin.

13. The process of claim 8 , wherein said further processing comprises reacting the resin-linear organosiloxane from step II) with an endcapping compound selected from an alcohol, oxime, or trialkylsiloxy compound.

14. The organopolysiloxane block copolymer prepared by the method of claim 1 .

15. The organopolyxiloxane block copolymer of claim 14 , wherein the organopolysiloxane block copolymer is the reaction product of step II).

16. A solid, curable composition comprising the organopolysiloxane block copolymer of claim 14 .

17. The cured product of the composition of claim 16 .

18. A solid film composition comprising the composition of claim 16 .

19. The solid film composition of claim 18 , wherein the solid composition has an optical transmittance of at least 95%.

20. An LED encapsulant comprising the compositions of claim 16 .