A Simple and Efficient Protocol for the Catalytic Insertion Polymerization of Functional Norbornenes

Summary

We describe the catalytic insertion polymerization of 5-norbornene-2-carboxylic acid and 5-vinyl-2-norbornene to form functional polymers with a very high glass transition temperature.

Abstract

Norbornene can be polymerized by a variety of mechanisms, including insertion polymerization whereby the double bond is polymerized and the bicyclic nature of the monomer is conserved. The resulting polymer, polynorbornene, has a very high glass transition temperature, T_g, and interesting optical and electrical properties. However, the polymerization of functional norbornenes by this mechanism is complicated by the fact that the endo substituted norbornene monomer has, in general, a very low reactivity. Furthermore, the separation of the endo substituted monomer from the exo monomer is a tedious task. Here, we present a simple protocol for the polymerization of substituted norbornenes (endo:exo ca. 80:20) bearing either a carboxylic acid or a pendant double bond. The process does not require that both isomers be separated, and proceeds with low catalyst loadings (0.01 to 0.02 mol%). The polymer bearing pendant double bonds can be further transformed in high yield, to afford a polymer bearing pendant epoxy groups. These simple procedures can be applied to prepare polynorbornenes with a variety of functional groups, such as esters, alcohols, imides, double bonds, carboxylic acids, bromo-alkyls, aldehydes and anhydrides.

Introduction

Norbornene, NBE, the Diels-Alder adduct of ethylene and cyclopentadiene (obtained by "cracking" of dicyclopentadiene (DCPD)), is readily polymerized using either free-radical polymerization,¹ cationic polymerization,² ring-opening metathesis polymerization³ and catalytic insertion polymerization.^4,5,6,7 Unlike the other mechanisms, the catalytic insertion polymerization leads to the formation of a very high glass-transition temperature (T_g) polymer whereby the....

Protocol

1. Preparation of Poly(5-norbornene-2-carboxylic acid), PNBE(CO₂H)

1. Preparation of the monomer NBE(CO₂H)
   1. Weigh out acrylic acid (AA) (327 g, 4.5 mol, 2 eq.) and hydroquinone (4.9 g, 4.5 x 10^-2 mol, 0.02 eq.) and add them to a 2 L round-bottom flask equipped with a condenser and a magnetic stir bar. Heat the flask at 150 °C using a silicone oil bath.
   2. Once reflux is settled, add DCPD (300 g, 2.3 mol, 1 eq.) in a single portion, and then increase the temperature to 170 °C.
   3. Leave the reaction at this temperature for 16 h. Observe the color change from clear to yellow-brown.
   ....

Results

The NBE monomers are prepared by simple Diels-Alder reaction of DCPD and a suitable dienophile, for example acrylic acid (AA). Normally, DCPD is cracked to yield cyclopentadiene (CPD) before reaction.¹⁷ Freshly cracked CPD is then engaged in the Diels-Alder reaction. However, in this protocol, both cracking and Diels-Alder steps are performed concomitantly, in a one-pot reaction. Thus, as soon as CPD is formed, it reacts with AA to yield 5-norbornene-2-carboxylic a.......

Discussion

The method proposed here is simple, and readily amenable to scale-up. All chemicals could be used as received without any purification. Note that performing the reaction at a lower scale (e.g. scales ≤1 g) usually yields lower yields due to an unavoidable loss of material during the handling and the collection.

The catalysts are formed in situ upon the reaction of commercial Pd compounds with cationizing agents. In our hands, the yield of the reaction as well as the cha.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
acrylic acid	Sigma-Aldrich	147230
hydroquinone	Sigma-Aldrich	H9003
dicyclopendadiene	Sigma-Aldrich	454338
palladium allyl dichloride dimer	Sigma-Aldrich	222380
silver hexfluoro antimonate	Sigma-Aldrich	227730
liquid nitrogen	Local Facility	NA
ethyl acetate	Fischer Scientific	E14520
5-vinyl-2-norbornene	Sigma-Aldrich	148679
toluene	Fischer Scientific	T290-4
palladium dba	Sigma-Aldrich	227994
triphenyl phosphine	Sigma-Aldrich	93090
silica gel 40-63 microns	Silicycle	Siliaflash
methanol	Fischer Scientific	BPA412-20
dichloromethane	EMD Millipore	DX08311
formic acid	Sigma-Aldrich	F0507
acetic acid	Sigma-Aldrich	320099
hydrogen peroxide solution	Sigma-Aldrich	216763
acetone	Fischer Scientific	A18-200