Synthetic polymer exists as a distribution of chain lengths and a distribution of molecular weight. These distributions are typically characterized as the number average molecular weight (Mn) and the weight average molecular weight (Mw). The Z average molecular weight (Mz) and the Z+1 average molecular weight (Mz+1) are indicators of the high molecular weight (high-MW) portion present in a polymer.
It is known that the high-MW portion can affect the downstream fabrication processes for a given polymer. For example, the high-MW portion in polypropylene is problematic during fiber spinning Fiber spinning requires the polypropylene molecules to disentangle quickly and over a very short distance (from die to draw-down). However, the presence of long molecule chains (i.e., the high-MW portion) in the polypropylene increases the risk of fiber break during spinning Fiber break during spinning is detrimental as it halts fiber production, requires manpower to troubleshoot and remedy, and reduces production efficiency.
The art recognizes the need for propylene-based polymer with a reduced amount of high-MW portion, and particularly for improved fiber and improved fiber production. The reduced high-MW portion is also important for the production of visbroken propylene-based polymer for high speed fiber spinning Even though the peroxide chain scission reduces the M, and narrows the molecular weight distribution, it has been found that that the fiber spinning performance of the visbroken product depends on the absence of high-MW species in the polymer prior to visbreaking.