Well-defined, model long chain branched polyethylene. 1. Synthesis and characterization

We describe the synthesis and characterization of a number of polymers with well-defined structures that serve as models for polyethylene with long ch ain branching. All of them have been made by using anionic polymerization t echniques and controlled chlorosilane chemistry to give nearly monodisperse polybutadienes with precise control of the number, length, and placement o f long ((M) over bar(w) > 1500 g/mol) branches on each chain. This was foll owed by hydrogenation to give saturated polymers with the same well-defined long chain branching and the local structure of a typical linear low-densi ty polyethylene. That is, both the backbones and the long branches had 17-2 5 ethyl branches per 1000 total carbons. Among the structures made were som e with no long branches ("linears"), some with a single long branch ("stars "), others with exactly two branch points (the alpha-omega type, "H's", "su per-H's", and "pom-poms"), and some with several long branches randomly dis tributed along the backbone ("combs"). Essentially all types of branching f rom a linear backbone can be made by the techniques described herein. While linear and symmetrical star models of polyethylene have been made previous ly, the other structures are the first examples of polyethylene models with multiple branches and precise control of the molecular architecture. We us e the results given here to discuss how long chain branching can be detecte d in polyethylene. We also show how the branching structure controls chain dimensions. The Zimm-Stockmayer model works well to describe the sizes of t he lightly branched molecules, but its predictions are too small for those with many long branches. This is presumably due to crowding of the branches . The rheological properties of these polymers will be described in subsequ ent publications.