Multi-layer core golf ball having thermoset rubber cover

1. A golf ball comprising at least a core layer and an outer cover layer, each comprising a cross-linked thermoset rubber composition having a primary rubber component of polybutadiene in an amount of 80% by weight or greater, wherein the polybutadiene of said core layer has a first structure and the polybutadiene of the outer cover layer has a second structure, the first structure being different than the second structure; the thermoset composition of each of the core layer and the outer cover layer further comprising balata in an amount of about 20% by weight or less; and wherein said outer cover layer is a moisture vapor barrier layer that has a moisture vapor transmission rate of less than about 0.95 grams·mm/(m 2 ·day) and is comprised of metal flakes, wherein said outer cover layer has a specific gravity of about 1.2 g/cm 3 or greater.

2. A golf ball comprising at least a core layer and an outer cover layer, each comprising a cross-linked thermoset rubber composition having a primary rubber component of polybutadiene in an amount of 80% by weight or greater, wherein the polybutadiene of said core layer has a first structure and the polybutadiene of the outer cover layer has a second structure, the first structure being different than the second structure; the thermoset composition of each of the core layer and the outer cover layer further comprising balata in an amount of about 20% by weight or less; and wherein said outer cover layer is a moisture vapor barrier layer that has a moisture vapor transmission rate of less than about 0.95 grams·mm/(m 2 ·day), and is comprised of metal flakes, wherein said outer cover layer has a specific gravity of about 1.5 g/cm 3 or greater.

3. A golf ball comprising at least a core layer and an outer cover layer, each comprising a cross-linked thermoset rubber composition having a primary rubber component of polybutadiene in an amount of 80% by weight or greater, wherein the polybutadiene of said core layer has a first structure and the polybutadiene of the outer cover layer has a second structure, the first structure being different than the second structure; the thermoset composition of each of the core layer and the outer cover layer further comprising balata in an amount of about 20% by weight or less; and wherein said outer cover layer is a moisture vapor barrier layer that has a moisture vapor transmission rate of less than about 0.95 grams·mm/(m 2 ·day), wherein said outer cover layer has a specific gravity of about 2.0 g/cm 3 or greater.

4. A golf ball comprising at least a core layer and an outer cover layer, each comprising a cross linked thermoset rubber composition having a primary rubber component of polybutadiene in an amount of about 80% by weight or greater, the polybutadiene of the core layer having a first structure and the polybutadiene of the outer cover layer having a second structure, the first structure being different than the second structure wherein the polybutadiene of the core layer comprises a cis-1,4-polybutadiene content (δ cr ) and a trans-1,4-polybutadiene content (ε cr ) such that (δ cr )>(ε cr ) and wherein the polybutadiene of the cover layer comprises a cis-1,4-polybutadiene content (δ cv ) and a trans-1,4-polybutadiene content (ε cv ) such that (δ cv )<(ε cv ); said cross linked thermoset rubber composition of each of the core layer and the outer cover layer further comprising balata in an amount of 20% by weight or less; and wherein the cover layer is a moisture vapor barrier layer comprising hydrophobic microparticles having a size of about 335 microns or lower, wherein the moisture barrier layer has a moisture vapor transmission rate of less than about 0.95 grams·mm/(m 2 ·day), and wherein said outer cover layer has a specific gravity of about 1.2 g/cm 3 or greater.