N. Hadjichristidis et al., Well-defined, model long chain branched polyethylene. 1. Synthesis and characterization, MACROMOLEC, 33(7), 2000, pp. 2424-2436
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.