Cycloaliphatic Polymers via Late Transition Metal Catalysis - Late Transition Metal Polymerization Catalysis - page 103

4.2
The Addition Polymerization of Cyclic Olefins

This extensive section deals with the late transition metal-catalyzed homopolymer-
ization of cyclic olefins to afford cycloaliphatic polymers, or saturated polymers in
which the cyclic structure of the monomer remains intact in the polymer back-
bone.

4.2.1
Cyclopropenes and Cyclobutenes

Unsubstituted cyclopropene is thermally unstable and polymerizes spontaneously
and uncontrollably at temperatures exceeding -78

C [14]. 3,3-Dialkylcyclopropenes
have been shown to undergo controlled addition polymerization using cationic
palladium catalysts [15]. The catalysts used were ionic (

3 -allyl)palladium com-
plexes with chelating N,N-ligands and a hexafluoroantimonate counterion. The li-
gands reported were 2,2 -bipyridyl, sparteine and C 2 -symmetric bisoxazoline. The

polymers had molecular weights ( M n ) up to in excess of 50000, comprised the ex-
pected triangular repeat units, and are partially crystalline with moderate thermal
stability.
Surprisingly there have been no reports of late transition metal-catalyzed addi-
tion polymerization of cyclobutenes. Since the homopolymerization of cyclobutene
using metallocene catalysts is exemplified in the literature [16] it is only a matter
of time before a report of a cationic palladium or nickel catalyst for the polymer-
ization appears in the literature.

4.2.2
Cyclopentene

Cyclopentene can be polymerized using a variety of metallocene catalysts to afford
poly(cyclopentene)s with 1,3-enchainment of the cyclopentane units [17]. These
polymers are invariably of low molecular weight ( M n <2000) and often highly iso-
tactic, and are generally not melt-processable [18].
Polymerization of cyclopentene by both cationic nickel and palladium catalysts
with hindered di-imine ligands (“Brookhart catalysts”) results in the formation of
new poly(cyclopentene)s with a different crystalline form [19]. The rates of poly-
merization are very slow; for example, the nickel catalyst illustrated in Fig. 4.3 in
combination with an ethylaluminum dichloride co-catalyst converted 4600 moles
of cyclopentene but took a week to do so [20]. The resulting polymer was all- cis ,
exhibited a T g of 97 C and a molecular weight

( M n ) of 73000 with a polydispersity of around 4. Average molecular weights can
be as high as 250000 using these catalysts and the broad melting points are in
the range of 240-330

C, a melting range of 160-285

C.