one-pot synthesis of hyperbranched polyethylenes tethered with pendant acryloyl functionalities by...

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2232

One-Pot Synthesis of HyperbranchedPolyethylenes Tethered with Pendant AcryloylFunctionalities by Chain WalkingCopolymerizations

Shawn Morgan, Zhibin Ye,* Kejian Zhang, Ramesh Subramanian

Chain walking copolymerization of ethylene with 1,4-butanediol diacrylate at elevatedconcentrations for the synthesis of low-viscosity hyperbranched polyethylenes tethered witha controllable amount of acryloyl functionality is reported. At concentrations much higherthan the critical gelation concentrations, the diacrylate comonomer is copolymerized pre-dominantly following the mono-insertion pattern. A range of hyperbranched polyethylenestethered with acryloyl functionality (1.3–4.8 mol-%) and having low melt viscosity (ca. 25 Pa � sat 25 8C) and low glass transitiontemperature (Tg ¼�69 8C) has beensynthesized. This unique method isspecific to the Pd-diimine-catalyzedchain-walking copolymerizationsystem.

Introduction

Unlike linear polymers, hyperbranched polymers have

structures and topologies similar to dendrimers, and

possess some interesting and valuable material properties,

such as low solution/melt viscosity, enhanced solubility,

an abundance of reactive terminal groups, etc.[1] However,

dendrimers often require tedious synthetic procedures[2]

whereas hyperbranched polymers are more easily pro-

duced on a large scale,[1] which encourages their potential

use in various applications, such as rheological additives,[3]

toughening agents,[4] drug delivery[5] etc. In particular,

hyperbranched polymers containing a large number of

terminal polymerizable double bonds, such as methacry-

S. Morgan, Z. Ye, K. Zhang, R. SubramanianSchool of Engineering, Laurentian University, Sudbury, OntarioP3E 2C6, CanadaFax: þ1 705 675 4862; E-mail: [email protected]

Macromol. Chem. Phys. 2008, 209, 2232–2240

� 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

loyl and acryloyl groups, have great potential as new high

performance UV/radical curable crosslinkers for thermoset

applications.[6–11] The successful synthesis of several

hyperbranched polymers containing methacryloyl/acry-

loyl groups has been reported in the literature.[6–11]

However, multi-step procedures using specially designed

monomers are generally required for synthesizing these

functionalized polymers. One-step synthetic procedures

with the use of commercially abundant monomers are

thus highly desired for commercial applications.

Chain walking polymerization of ethylene with Pd-

diimine catalysts represents a newly evolved strategy for

the one step synthesis of hyperbranched polyethylenes.[12]

In this strategy, the control of polymer chain topology is

achieved uniquely through the intrinsic chain walking

mechanism of the late transition metal catalysts while

using simple and commercially abundant ethylene as the

monomer. This is distinctly different from the conven-

tional synthetic strategies for hyperbranched polymers,

DOI: 10.1002/macp.200800407

One-Pot Synthesis of Hyperbranched Polyethylenes Tethered with Pendant Acryloyl Functionalities . . .

where the hyperbranched topology is often introduced by

using specifically designed functional monomers.[1,12] Due

to the reduced oxophilicity of the Pd-diimine catalysts, this

strategy also enables the synthesis of functionalized

hyperbranched polyethylenes tethered with various polar

functional groups by copolymerization of ethylene with

polar monomers, typically functionalized acrylates and

1-alkenes that can be copolymerized by Pd-diimine

catalysts.[13–17] Recently, we reported a unique one-pot

synthesis of hyperbranched polyethylenes tethered with

methacryloyl groups by selective chain walking ethylene

copolymerization with a heterobifunctional comonomer,

which contains one copolymerizable acryloyl or 1-alkenyl

group and one non-copolymerizable methacryloyl group.[16]

In this study, we further demonstrate a one-pot

procedure for the convenient synthesis of hyperbranched

polyethylenes tethered predominantly with acryloyl

groups via chain walking copolymerization of ethylene

with a commercial symmetric bifunctional comonomer,

1,4-butanediol diacrylate (BDA). Key to this strategy, the

diacrylate comonomer is used uniquely at concentrations

that are much elevated compared to the critical gelation

concentrations and is copolymerized predominantly

following a mono-insertion pattern with only one of its

two double bonds being incorporated and the other being

intact and pendant. Specific to this unique Pd-diimine

catalyzed copolymerization system, the elevated concen-

tration of the diacrylate comonomer leads unprecedent-

edly to the kinetic suppression of both intermolecular and

intramolecular crosslinking reactions, owing to the

significant reduction in the relative concentration of the

pendant vinyl groups compared to monomeric vinyl

groups during the copolymerization. A range of low

viscosity hyperbranched polyethylenes containing acry-

loyl groups of high and tunable contents has thus been

synthesized using this polymerization procedure.

Experimental Part

Materials

All manipulations of air and/or moisture sensitive compounds

were performed in an N2-filled drybox or using Schlenk

techniques. The Pd-diimine catalyst used in this work, [(ArN––

C(Me)–(Me)C––NAr)Pd(CH3)(N CMe)]SbF6 (Ar¼ 2,6-(iPr)2C6H3) (1),

was synthesized following a procedure reported in the litera-

ture.[13] Ultra-high purity N2 and polymer-grade ethylene (both

obtained from Praxair) were purified by passing through 3 A/5 A

molecular sieves and Oxiclear columns to remove moisture and

oxygen, respectively, before use. The diacrylate comonomer, 1,4-

butanediol diacrylate (90%, Aldrich), was dried over 4 A molecular

sieves before use. Silica bound metal scavengers, triamine-

tetraacetic-acid-derivatized silica gel (SiliaBond TAAcOH) and

thiol-derivatized silica gel (SiliaBond Thiol), were obtained from

SiliCycle and were used as received. Deuterated chloroform (99.8



at.-% D, Aldrich) was dried over 4 A molecular sieves before use.

Other chemicals, including anhydrous dichloromethane (99.8%),

methanol (>99%), tetrahydrofuran (THF) (>99%), etc., were

obtained from Aldrich and were used as received.

Chain Walking Ethylene Homopolymerization and

Copolymerization with BDA

All chain walking ethylene polymerization reactions were carried

out at 1 atm (absolute) ethylene pressure in a 500 mL jacketed

glass reactor equipped with a magnetic stirrer and a temperature-

controlled circulating water bath. The general polymerization

procedure was as follows. The glass reactor was dried overnight at

about 150 8C in an oven, followed by cooling to room temperature

under a vacuum, and then sealed using a rubber septum. The

reactor further underwent a vacuum-ethylene purge procedure for

at least three cycles, and was then pressurized with ethylene at

1 atm (absolute). Subsequently, anhydrous CH2Cl2 (90 mL) and a

prescribed amount of BDA (for copolymerizations) were injected

into the reactor. The reactor temperature was then maintained by

passing water through the jacket using the circulating bath set at

the designated polymerization temperature. After thermal

equilibration for 10 min, the Pd-diimine catalyst (0.1�10�3

mol) dissolved in anhydrous CH2Cl2 (10 mL) was injected into the

reactor to start the polymerization. Ethylene pressure was

maintained at 1 atm (absolute) during the polymerization by a

continuous feed from the supply cylinder. After a prescribed

polymerization time, the polymerization was terminated by

venting the reactor. The polymer product was obtained by

precipitation in a large amount of 2% acidified methanol. In order

to remove the catalyst residue remaining in the polymer, the dark

colored oily polymer precipitate was redissolved in THF, and the

silica bound metal scavenger, triamine-tetraacetic-acid deriva-

tized silica gel or thiol-derivatized silica gel (ca. 0.2 g), was then

added into the solution. The solution was stirred overnight and

then filtered to remove the silica. Sometimes this purification

procedure was repeated until the filtrate became clear. The

purified polymer was obtained by precipitating the filtrate in a

large amount of methanol. It was dried for 3 or 4 d under a vacuum

at room temperature and then weighed.

Polymer Characterization and Analysis

1H NMR analysis of the synthesized polymers was performed on a

Varian Gemini 2000 200 MHz spectrometer at ambient tempera-

ture. Deuterated chloroform was used as the solvent for all the

samples for NMR measurement. Differential scanning calorimetry

(DSC) measurements on the polymers were performed on a TA

Instruments Q100 DSC equipped with a refrigerated cooling

system (RCS) under a N2 atmosphere. The instrument was

operated in the standard DSC mode. A N2 purging flow of

50 mL �min�1 was used. Samples (about 10 mg) were heated from

room temperature to 150 8C at 10 8C �min�1 and cooled to �90 8Cat 5 8C �min�1, and the data were then collected in the second

heating ramp from �90 8C to þ150 8C at 10 8C �min�1.

Triple detection gel permeation chromatography (GPC) mea-

surements were carried out on a Polymer Laboratories PL-GPC220

system equipped with a differential refractive index (DRI) detector

www.mcp-journal.de 2233

S. Morgan, Z. Ye, K. Zhang, R. Subramanian

2234

(from Polymer Laboratories), a three-angle laser light scattering

(LS) detector (high temperature miniDAWN from Wyatt Technol-

ogy), and a four-bridge capillary viscosity detector (from Polymer

Laboratories). The detecting angles of the light scattering detector

were 45, 90 and 1358, and the laser wavelength was 687 nm. One

guard column (PL# 1110–1120) and three 30 cm columns (PLgel

10 mm MIXED-B 300� 7.5 mm) were used. The mobile phase was

HPLC-grade THF stabilized with 0.025% BHT and the flow rate was

1.0 mL �min�1. The GPC system including the column and detector

array was operated at 33.0 8C. The mass of the polymers injected

into the columns was typically 200 mL of a 3–4 mg �mL�1 solution.

Astra software from Wyatt Technology was used to collect and

process the data from all three detectors. Two narrow polystyrene

standards from Pressure Chemicals with weight-average mole-

cular weights (Mw) of 30 000 and 200 000 g �mol�1, respectively,

were used for the normalization of light scattering signals, and

determination of inter-detector delay volume and band broad-

ening. The specific refractive index increment, (dn/dc), value of

0.078 mL � g�1, reported in the literature,[18] was used for the

hyperbranched polyethylenes, and a value of 0.185 mL � g�1 was

used for polystyrene.

Rheological characterization of the polymers was carried out

using a TA Instrument AR-G2 rheometer. A Peltier plate

measurement configuration with 20 mm parallel plate geometry

at a gap size of about 1.0 mm was used for the measurements. The

measurements were all conducted in the small amplitude

dynamic oscillation mode within the frequency range of 0.01–

100 Hz. A strain sweep was performed at 1 Hz before frequency

sweeps to establish the linear viscoelastic region for each polymer.

The measurements were performed at 15, 25 and 35 8C.

Measurement temperature was maintained within �0.1 8C by

using the Peltier plate temperature control system.

Results and Discussion

Diacrylates such as 1,4-butanediol diacrylate (BDA) have

been extensively used as bifunctional crosslinkers in

various polymerization systems involving crosslinking.

We have recently studied chain walking copolymerization

of ethylene with a small amount of BDA as the cross-

linker using a Pd-diimine catalyst, [(ArN––C(Me)–(Me)C––

NAr)Pd(CH3)(N CMe)]SbF6 (Ar¼ 2,6-(iPr)2C6H3) (1), with

the objective of synthesizing hyperbranched polyethy-

lenes containing intermolecular crosslinking structures

and consequentially having enhanced molecular weight

without gelation.[17] Three expected types of incorporated

diacrylate structures, including pendant vinyl groups (via

mono-insertion of one of the two vinyl groups of the

diacrylate monomer), intermolecular crosslinking (via

insertion of the pendant vinyl group(s) into another

kinetic chain), and intramolecular crosslinking/cyclization

(via insertion of the pendant vinyl group(s) into the same

kinetic chain), were found in the copolymers. The feed

concentration of BDA in this copolymerization system was

very low (generally in the range of 0.0048–0.036 M) and



below the corresponding critical gelation concentration to

avoid gelation. With an incremental increase of BDA feed

concentration from zero up to the critical gelation

concentration, continuous enhancements in intermolecu-

lar crosslinking density, polymer average molecular

weights and polydispersity index were observed until

the occurrence of gelation. The contents of the pendant

acryloyl groups in the resulting copolymers were very low

given the low diacrylate concentrations used.[17]

We herein aim to synthesize low viscosity hyper-

branched polyethylenes tethered predominantly with

acryloyl groups of enhanced and controllable contents

by one-step chain walking copolymerization of ethylene

with diacrylate. In designing this one pot polymerization

process, the occurrence of intermolecular crosslinking,

which is often accompanied by simultaneous intramole-

cular crosslinking, is to be suppressed/minimized due to

the resulting undesired significant enhancement in

polymer molecular weight and polymer viscosity, even

at a low intermolecular crosslinking density. Both cross-

linking structures result from the incorporation of pendant

vinyl groups, which compete kinetically with the mono-

meric vinyl groups of BDA. Generally, the reactivity of

pendant vinyl groups is significantly lower (two or three

orders of magnitude) than the monomeric vinyl groups

because of the much reduced mobility of the higher

molecular weight macromonomer.[19] It is expected that

the crosslinking reactions will be kinetically suppressed/

minimized if the monomeric vinyl concentration is over-

whelmingly greater than that of pendant vinyl groups. Our

guideline in designing this current polymerization system

is thus to reduce the concentration of pendant vinyl groups

relative to monomeric vinyl groups, which can subse-

quently lead to minimized crosslinking reactions.

After studying several works reported by others[13] and

ourselves[15] on the chain walking copolymerization of

ethylene with various acrylate comonomers, we have

found a unique characteristic of the ethylene-acrylate

copolymerization system, that is, the overall conversion of

the acrylate comonomer generally decreases drastically

with an increase in its feed concentration (with the

other polymerization conditions fixed) despite the

increase of comonomer molar fraction in the resulting

copolymers. This is due to the much lower (over two orders

of magnitude) reactivity of the acrylate comonomers

compared to ethylene, which results in a significant

reduction in polymer productivity upon acrylate incor-

poration.[13] In the current ethylene-BDA copolymerization

system, the concentration of monomeric vinyl groups is

related to the BDA conversion (x) at a certain time

following:

½monomeric vinyl� ¼ 2½BDA�0ð1 � xÞ (1)

DOI: 10.1002/macp.200800407


where [BDA]0 is the feed concentration of BDA and the

factor of 2 represents the two vinyl groups in each BDA

monomer. The concentration of the pendant vinyl groups

would be approximately proportional to BDA conversion

given their much lower reactivity relative to monomeric

vinyl groups:

Sch

Macrom

� 2008

½pendant vinyl� / ½BDA�0x (2)

The concentration ratio of the pendant vinyl groups to the

monomeric vinyl groups is thus

½pendant vinyl�½monomeric vinyl� /

½BDA�0x2½BDA�0ð1 � xÞ

¼ x

2ð1 � xÞ (3)

Following the above characteristics for the ethylene-

acrylate copolymerization by Pd-diimine catalysis, the

BDA conversion (x) should decrease drastically with the

increase of [BDA]0, which should subsequently result in a

decrease of the concentration ratio with the increase of

[BDA]0, as in Equation (3). We thus hypothesize that, at

sufficiently high BDA feed concentrations, the crosslinking

reactions can be minimized due to the low relative

concentration of the pendant vinyl groups compared to

monomeric vinyl, thus giving rise to polymers tethered

predominantly with pendant vinyl groups and with

minimal/negligible undesired intermolecular crosslinking.

From this hypothesis, the change of BDA feed concentra-

tion can thus mediate the competition between pendant

and monomeric vinyl groups, and subsequently adjust the

structure of the resulting polymers, giving hyperbranched

polymer containing predominantly pendant vinyl groups

eme 1. Effect of BDA feed concentration on the structure of the

ol. Chem. Phys. 2008, 209, 2232–2240

WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

while with minimal crosslinking at elevated BDA feed

concentrations and hyperbranched polymer containing

crosslinking structures at low BDA concentrations

(Scheme 1).

Following this hypothesis, chain walking copolymeriza-

tions of ethylene with BDA at various elevated feed

concentrations were carried out using 1 at two different

temperatures, 15 and 25 8C, respectively. Table 1 sum-

marizes the polymerization conditions. In order to

synthesize polymers of hyperbranched chain topology, a

low ethylene pressure of 1 atm was used for all the poly-

merization runs as low ethylene pressure generally leads

to polymers of more compact structure following the chain

walking mechanism of the Pd-diimine catalysts.[12,20] At

each temperature level, the BDA feed concentration was

increased incrementally, generally in the range from 0.06

to 0.80 M, to investigate the effect of BDA feed concen-

tration on its incorporation patterns and to find out the

suitable BDA feed concentrations at which crosslinking

reactions can be neglected. These feed concentrations are

much higher compared to the corresponding critical

gelation concentrations, 0.017 M at 15 8C and 0.024 M at

25 8C, estimated experimentally in our previous study

wherein the same polymerization conditions were used

except the BDA feed concentrations.[17] Corresponding

ethylene homopolymerization runs are also included here

for comparison (run 1 and 7 in Table 1).[17]

Among the set of polymerizations carried out at 25 8C,

gelation occurred in run 2 and 3 (Table 1), which had the

lowest BDA feed concentrations (0.06 and 0.16 M, res-

pectively) in the set. During the course of these two runs,

gelation occurred at a certain point when the reaction fluid

lost its mobility and the polymerization was then termi-

nated. The occurrence of gelation indicates the relative

concentration of pendant vinyl groups in these two runs

polymer synthesized in chain walking copolymerization.



Table 1. Summary of polymerization conditions, results and polymer characterization data.

Run Temp. BDA

Conc.a)Yield BDA

Conv.

Incorporated BDA Branch densityc) GPC-LS-VIS

characterization

Total BDA

contentb)Vinyl

contentb)Selectivity of

mono-insert.Mw

d) PDId) [h]we) a

-C M g % mol-% mol-% % (per 1 000 C) kD mL � gS1

1 25 0 9.7 0 0 0 0 100 142 1.4 23 0.58

2 25 0.06 gelled – – – – – – – – –

3 25 0.16 gelled – – – – – – – – –

4 25 0.27 5.8 10 1.5 1.3 85 105 132 1.8 20 0.44

5 25 0.53 1.8 3.5 3.6 3.1 85 93 57 1.4 14 0.47

6 25 0.80 1.8 3.2 5.4 4.8 89 86 37 1.4 12 0.48

7 15 0 12.5 0 0 0 0 103 141 1.5 27 0.61

8 15 0.27 5.0 5.8 0.94 0.61 64 94 165 2.3 24 0.44

9 15 0.53 2.4 3.1 2.2 1.9 87 92 49 1.4 15 0.51

10 15 0.80 1.7 2.3 3.8 3.6 95 90 31 1.3 12 0.50

a)Other polymerization conditions: 10S4 mol Pd-diimine catalyst, solvent CH2Cl2, total volume 100 mL, ethylene pressure 1 atm, reaction

time 22 h for run 1 and 7 and 24 h for all the other runs without gelation; b)Total BDA content and pendant vinyl content of the copolymers

determined using 1H NMR spectroscopy; c)Branching density of the ethylene sequences determined from 1H NMR spectroscopy; d)Weight-

average molecular weight (Mw) and polydispersity index (PDI) determined from the light scattering measurements; e)Weight-average

intrinsic viscosity ([h]w) determined from the capillary viscosity detector.

Figure 1. 1H NMR spectrum of ethylene/BDA copolymer synthes-ized in (a) run 4 and (b) run 6.

2236

was still high enough to compete with monomeric vinyl

groups to generate significant intermolecular crosslinking

structures. For run 4–6 at higher BDA feed concentrations

(0.27–0.80 M) in the set, gelation, however, did not occur.

Compared to run 2 and 3, the absence of gelation in these

three runs verifies our hypothesis, and suggests the

enhanced suppression of the intermolecular crosslinking

with the increase of BDA feed concentration. For the set of

copolymerization carried out at 15 8C (run 8–10), gelation

did not occur either within the employed BDA feed

concentration range from 0.27 to 0.80 M.1H NMR spectroscopy was used to characterize the

microstructure of the copolymers and to investigate BDA

incorporation patterns and their dependency on BDA feed

concentration. Representatively, Figure 1 shows the1H NMR spectra of the copolymers synthesized in run 4

and 6, respectively. Besides the three major resonances

assigned to methyl, methylene and methine protons of the

ethylene sequences, several characteristic resonances (a, b,

b�, d and e labeled in Figure 1) resulting from the incor-

porated BDA units can be clearly observed in the

spectra.[17] The presence of the pendant acryloyl groups

resulting from mono-inserted BDA units, CH2––CH–C(O)O–

CH2–, is confirmed from the resonances of the vinyl

protons (d, e in the region from 5.7 to 6.7 ppm) and

the methylene protons (b at 4.19 ppm) most adjacent to

the acryloyl group. Each mono-inserted BDA unit also

possesses two methylene resonances (b� at 4.10 ppm and



triplet resonance a at 2.30 ppm) of the –CH2–C(O)O–CH2–

group at the incorporated end. Small quantities of cross-

linking structures, including both intermolecular and

intramolecular crosslinks that are not distinguishable

from NMR spectroscopy, were also found in the copoly-

DOI: 10.1002/macp.200800407


Figure 2. Gel permeation chromatography elution traces fromDRIdetector for (a) polymers synthesized in the set of runs at 25 8Cand (b) polymers synthesized in the set of runs at 15 8C.

mers, contributing to the resonances of a and b� due to the

two incorporated acryloyl groups of each BDA unit.[17]

From the triplet methylene resonance a that is the only

resonance in the region of 2.2–2.5 ppm, the acrylate

functionalities are incorporated at the end of branches

(shown in Figure 1), which is characteristic of coordinative

insertion of acrylate by Pd-diimine catalysts.[13,15–17]

The above characteristic resonances resulting from the

incorporated BDA units were observed in the 1H NMR

spectra of all copolymers synthesized without gelation.

The gelled polymers in run 2 and 3 were not characterized

as they are not the target of this work. Quantifications of

the incorporated structures were performed by integrating

(a) vinyl protons (dþ e) in the region from 5.7 to 6.7 ppm,

(b) methylene protons (bþ b�) in the region from 4.0 to 4.4

ppm and (c) resonances of the ethylene sequences in the

region from 0.7 to 1.5 ppm. The BDA content, pendant

vinyl content and the selectivity for mono-inserted BDA

units were calculated for each copolymer, and the results

are listed in Table 1. With the increase of BDA feed

concentration from 0.27 to 0.80 M, both overall BDA

content and pendant vinyl content increase significantly

in each set of copolymers. Comparing the two sets of

polymers synthesized at the same BDA feed concentra-

tions, the BDA content and pendant vinyl content tend to

increase with the temperature increase from 15 to 25 8C.

The highest pendant vinyl content is 4.8 mol-% found in

run 6. From Table 1, the BDA units are incorporated

predominantly in terms of pendant vinyl groups at these

elevated BDA feed concentrations with selectivity gen-

erally above 85% except in run 8. In each set of copolymers,

the selectivity for pendant vinyl group is generally

enhanced with the increase of BDA feed concentration,

regardless of the increase in the content of pendant vinyl

groups. For example, the selectivity is 64% in run 8 at a

BDA concentration of 0.27 M, while it is enhanced to 95% in

run 10 at a BDA concentration of 0.80 M. The polymer yield

and BDA conversion, however, generally decreased with

an increase in BDA feed concentration as shown in Table 1.

These results further validate our hypothesis that the

crosslinking reactions involving the pendant vinyl groups

are indeed significantly suppressed with the increase of

BDA feed concentrations due to the resulting reduced

relative concentration of pendant vinyl groups at reduced

BDA conversion. The branching densities of the polymers,

resulting from catalyst chain walking, were calculated

using the methyl, methylene and methine resonances of

the ethylene sequences in the 1H NMR spectra. The values

are generally in the range of 86–105 per 1 000 carbon

atoms, indicating the highly branched nature of these

copolymers.

Triple-detection GPC equipped with on-line DRI, three-

angle light scattering (LS) and capillary viscosity detectors

was used to characterize polymer absolute molecular



weight and distribution, dilute solution properties, and

chain topology. Figure 2(a) and 2(b) show the GPC elution

traces recorded from the DRI detector for the two res-

pective sets of polymers. All the polymers exhibit mono-

modal GPC elution curves except the polymer synthesized

in run 8, which shows an additional weak shoulder peak at

the high molecular weight end. With the increase of BDA

feed concentration, a consistent shift of the GPC elution

traces toward longer retention time (i.e., lower molecular

weight) is found with the polymers synthesized in each

set of runs. The absolute molecular weight data of the

polymers were determined using the three-angle LS

detector coupled with the DRI detector. The weight-

average molecular weight (Mw) and polydispersity index

(PDI) data calculated from the light scattering signals are

listed in Table 1. The Mw value of the copolymers in each

set (except the copolymer in run 8) generally decreases

with the increase of BDA feed concentration, which is

often found with copolymers of ethylene and monofunc-

tional acrylate comonomers synthesized with Pd-diimine



Figure 3. Intrinsic viscosity vs. molecular weight plot for (a) poly-mers synthesized in the set of runs at 25 8C and (b) polymerssynthesized in the set of runs at 15 8C. In both (a) and (b), the curvefor the homopolymer synthesized at the other temperature level(either run 1 or run 7) is also included for comparison.

2238

catalysts.[13] The PDI values of the copolymers, except the

two synthesized in run 4 and 8 at the lower BDA

concentration in each set, are the same or even slightly

lower compared to those of corresponding homopolymers.

These results are opposite to those found with the

polymers synthesized in our prior study with BDA feed

concentrations below the critical gelation concentrations,

which show evolving bimodal GPC elution curves and

continuously enhanced Mw and PDI values with the

increase of BDA feed concentration due to the increased

intermolecular crosslinking density.[17] The different

results further confirm the predominant mono-insertion

of BDA at these elevated feed concentrations.

In the current two sets of copolymers, the presence of a

very small amount of intermolecular crosslinking is only

apparent in the two copolymers synthesized in runs 4 and

8, both of which show a longer tail at the high molecular

weight end in the GPC elution traces and possess a broader

molecular weight distribution compared to the corre-

sponding homopolyethylene. In particular, the copolymer

synthesized in run 8, which has the lowest selectivity of

mono-insertion, has an even enhanced Mw value com-

pared to the corresponding homopolymer synthesized in

run 7. Though 1H NMR data still indicate the existence of

crosslinking structure resulting from dually inserted BDA

units (5–15%), the presence of intermolecular crosslinking

seems to be negligible in the other copolymers synthesized

at higher BDA feed concentrations in each set on the basis

of their similar or even slightly lower PDI values and

reduced Mw values compared to homopolymers. Therefore,

the crosslinking structures shown in 1H NMR spectra are

probably mostly intramolecular crosslinking, whose pre-

sence does not affect polymer molecular weight and

molecular weight distribution. It is easier for intramole-

cular crosslinking to occur compared to intermolecular

crosslinking due to the higher local density of pendant

vinyl groups within the polymer coil.[19]

The above GPC results thus validate the hypothesized

suppression of intermolecular crosslinking in the resulting

copolymers at elevated BDA feed concentrations owing to

the drastic decrease of BDA conversion and subsequent

reduction of the relative concentration of pendant vinyl

groups with the increase of BDA feed concentration.

Figure 3 shows the Mark-Houwink plots (intrinsic viscosity

vs. molecular weight plots) for the two sets of polymers. In

these plots, the intrinsic viscosity data were determined

using the capillary viscometer of the triple-detection GPC

and the molecular weight data were measured using the

light scattering detector. The copolymers exhibit a similar

dependence of intrinsic viscosity on polymer molecular

weight compared to the corresponding hyperbranched

homopolymer, indicating that the copolymers possess

similar hyperbranched chain topologies compared to the

corresponding homopolymer.[15–17,20] The Mark-Houwink



a values and the weight-average intrinsic viscosity values

([h]w) in THF at 30.5 8C are also listed in Table 1. The a

values of the copolymers are generally close to 0.50, which

is typical of hyperbranched polyethylenes.[17]

Like the homopolymers, all the copolymers synthesized

in both sets of runs are completely amorphous oil-like

liquids of low viscosity at room temperature. DSC

measurements of the polymers show similar thermal

behaviors with a glass transition centered at ca. �69 8C and

a weak endotherm centered at ca. �38 8C, which is possibly

a melting endotherm attributable to the highly branched

ethylene sequences.[15–17] Figure 4(a) and 4(b) show the

complex viscosity spectra obtained from small amplitude

dynamic oscillation measurements performed at 25 8C for

the two sets of polymers synthesized at 25 and 15 8C,

respectively. With the increase of BDA feed concentration

in each set, the polymer exhibits continuously reduced

DOI: 10.1002/macp.200800407


Figure 4. Complex viscosity spectra measured from small ampli-tude dynamic oscillation measurements at 25 8C for (a) the poly-mers synthesized in the set of runs at 25 8C and (b) the polymerssynthesized in the set of runs at 15 8C.

Newtonian viscosity and the shear-thinning becomes

increasingly insignificant in the studied angular frequency

range due to their reduced molecular weights. For

example, the homopolymer synthesized in run 1 has a

Newtonian viscosity of 89 Pa � s while the Newtonian

viscosity values for the copolymers are 53 (run 4), 27 (run 5),

and 19 Pa � s (run 6), respectively. Along with the above

Mark-Houwink plots, the low Newtonian viscosity values

of these polymers also confirm their hyperbranched chain

topology.[20] The presence of intermolecular crosslinking

even at a low density often tremendously enhances

polymer viscosity and induces more significant shear

thinning.[17] The low Newtonian viscosity and reduced

shear-thinning found here further suggest that the

presence of intermolecular crosslinking in these copoly-

mers is insignificant and the crosslinking structures found

in 1H NMR measurements are mostly intramolecular ones.



Conclusion

We have demonstrated a convenient one-step synthesis of

hyperbranched polyethylenes tethered with pendant

acryloyl groups via chain walking copolymerization of

ethylene with commercial BDA at elevated concentrations

(0.27, 0.53 and 0.80 M). These concentrations are said to be

elevated here as they are much higher compared to the

critical gelation concentrations. Under these elevated BDA

concentrations, the occurrence of crosslinking, more

importantly intermolecular crosslinking, is significantly

suppressed, particularly when BDA concentration is over

0.53 M, resulting hyperbranched polymers tethered pre-

dominantly with pendant acryloyl groups. A range of

hyperbranched polymers having an acryloyl content in the

range from 1.3 to 4.8 mol-% has been successfully

synthesized. These polymers have been found to have

low melt viscosity at room temperature and possess low

glass transition temperature, and have great potential for

applications as UV/radical curable inkjet-printable macro-

crosslinker.

Acknowledgements: The financial support from the NaturalScience and Engineering Research Council of Canada (NSERC) isgreatly appreciated. The authors would also like to thank theCanadian Foundation for Innovation (CFI) for funding researchinstruments and facilities. Z.Y. thanks the Laurentian University forgranting a NSERC Research Capacity Development Faculty Award.

Received: August 6, 2008; Accepted: August 8, 2008; DOI:10.1002/macp.200800407

Keywords: crosslinking; glass transition; hyperbranched;synthesis; viscosity

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DOI: 10.1002/macp.200800407

one-pot synthesis of hyperbranched polyethylenes tethered with pendant acryloyl functionalities by...

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