Multi-layer core golf ball

1. A golf ball comprising:

an inner core layer having a diameter of from 0.500 inches to 1.580 inches and formed from a first thermoset rubber composition;

at least one outer core layer having a thickness of from 0.005 inches to 0.300 inches and a surface hardness of from 50 Shore C to 90 Shore C and formed from a highly neutralized polymer composition, wherein the highly neutralized polymer composition comprises:

an acid copolymer of ethylene and an α,β-unsaturated carboxylic acid;

a non-acid polymer selected from ethylene-alkyl acrylates and ethylene-alkyl methacrylates and present in an amount of greater than 50 wt %, based on the combined weight of the acid copolymer and the non-acid polymer;

an organic acid or salt thereof; and

a cation source present in an amount sufficient to neutralize greater than 80% of all acid groups present in the composition; and

a cover.

2. The golf ball of claim 1 , wherein the acid copolymer of ethylene and an α,β-unsaturated carboxylic acid does not include a softening monomer, and wherein the acid is selected from acrylic acid and methacrylic acid and is present in the acid copolymer in an amount of from 15 mol % to 30 mol %.

3. The golf ball of claim 1 , wherein the highly neutralized polymer composition has a solid sphere compression of 40 or less and a coefficient of restitution of 0.820 or greater.

4. The golf ball of claim 1 , wherein the highly neutralized polymer composition has a solid sphere compression of 100 or greater and a coefficient of restitution of 0.860 or greater.

5. The golf ball of claim 1 , wherein the organic acid salt is magnesium oleate, and wherein the magnesium oleate is present in an amount of 30 parts or greater, per 100 parts of acid copolymer and non-acid copolymer combined.

6. The golf ball of claim 1 , wherein the cation source is present in an amount sufficient to neutralize 100% of all acid groups present in the composition.

7. The golf ball of claim 1 , wherein the thickness of the outer core layer is from 0.010 inches to 0.150 inches.

8. The golf ball of claim 1 , wherein the thickness of the outer core layer is from 0.070 inches to 0.150 inches.

9. The golf ball of claim 1 , wherein the golf ball additionally comprises at least one outer core layer formed from a second thermoset rubber composition and having a thickness of from 0.010 inches to 0.100 inches and a surface hardness of 50 Shore C or greater.