Über Vernetzte Makromolekulare Stoffe mit Inhomogener Netzwerkdichte

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Abstract
Visual swelling characteristics of crosslinked polyester resins in good swelling agents lead one to suppose that in these resins the crosslinks are not equally distributed. When polyester resins are cured by heat without added peroxide, the maximum conversion and therefore also the maximum crosslink density have been shown to depend on the initial radical concentration in the reaction medium. After the gelling point the conversion could not be enhanced by increasing the polymerization temperature. Obviously the primary network, which arises at the gelling point, predetermines the structure of the fully cured resin. Because of the relatively high functionality of the polyester molecules, the large degree of polymerization of the arising copolymer chains and their predominantly alternating structure, growing chain radicals are rapidly attached to the network. It must be assumed that after the gelling point the polymerization proceeds mainly at certain centers, the final number and extent of which depend on the special conditions of the polymerization reaction. The change of the reactivity ratios of the monomers, normally to be observed at high conversions, is explained by the restricted mobility of the still‐unsaturated polyester chains. The conversion at which this restriction appears depends on the structure of the primary network. Under special conditions popcorn formation takes place during polymerization of polyester resins. These popcorn polymers may be considered as an extreme case of a network with an unequal distribution of crosslinks. The composition of these popcorn‐containing polyester resins indicates that their existence is due to the reduction of the mobility and reactivity of the polyester chains at a very low conversion.