Free Radical Polymerization of Ethylene
2.2 Free Radical Polymerization of Ethylene
Origins of free radical polymerization of ethylene to produce LDPE were dis-cussed in Chapter 1, stemming from the seminal work of chemists at ICI in the early 1930s. Agents that foster free radical polymerization of ethylene are called
"initiators" and sometimes "catalysts." (The latter is not technically correct, since the agents are consumed in the process.) Organic peroxides are the most commonly used initiators for free radical polymerization of ethylene.
Because of the extremely high pressures (15,000 to 45,000 psig), ethylene exists in the liquid phase and polymerization occurs in solution. Owing to high tem-peratures (typically >200 °C), polyethylene is also dissolved in monomer and the reaction system is homogeneous. LDPE precipitates only after the reaction mass is cooled in post-reactor separation vessels. Relative to other processes, reactor residence times are very short (<30 seconds for the autoclave process and <3 min for the tubular process) (7).
In the polymerization reactor, organic peroxides dissociate homolytically to generate free radicals. Polymerization of ethylene proceeds by a chain reaction.
Initiation is achieved by addition of a free radical to ethylene. Propagation proceeds by repeated additions of monomer.
Termination may occur by combination (coupling) of radicals or disproportion-ation reactions. Chain transfer takes place primarily by abstraction of a proton from monomer or solvent by a macroradical. A low molecular weight hydro-carbon, such as butane, may be used as chain transfer agent to lower molecular weight. Initiation, propagation, termination and chain transfer reactions are shown in Figure 2.1. Termination reactions illustrated in Figure 2.1 show that the end groups in LDPE are most commonly a vinyl group or an ethyl group.
Mechanisms for formation (called "backbiting") of n-butyl and 2-ethylhexyl branches are shown in Figures 2.2 and 2.3.
In addition to the use of chain transfer agents, molecular weight may also be varied by adjusting pressure and temperature. Higher pressures lead to higher molecular weight. Branching tends to increase at higher temperatures.
FREE RADICAL POLYMERIZATION OF ETHYLENE 25
Initiation:
ROOR ► 2 RO'
(See Figure 2.4 for structures of organic peroxides)
2RO' + CH2=CH2 ► ROCH2CH2'
Propagation:
ROCH2CH2- + (x+í;CH2=CH2 ► ROCH2CH2(CH2CH2)vCH2CH2· = Rp·
Termination:
Coupling: Rp· + Rp· ► Rp- Rp
Disproportionation: ~CH2CH2' + CH2CH2~ ». ~CH=CH2 + CH3CH2~
Chain Transfer:
~CH2Cri2 + CH2=CH2 ^ ~CH=CH2 + Crl3CH2
~CH2CH2 + CH2=CH2 ^ ~CH2CH3 + CH2=CH
~CH2CH2' + R'H* ► ~CH2CH3 + R"
* R'H = solvent, CTA, etc.
Figure 2.1 Free radical polymerization of ethylene.
Intramolecular transfer of a radical from a terminal to an internal carbon atom is called "backbiting" and results in short chain branching. This usually occurs on the fifth carbon atom from the macroradical terminus (δ to the radical), as illus-trated in Figures 2.2 and 2.3. Ethyl, butyl groups and 2-ethylhexyl groups comprise most of the short chain branching in LDPE (8,9). Intermolecular transfer between a radical and an internal carbon of another chain results in long chain branching, a characteristic feature of LDPE.
Chain propagation during copolymerization of ethylene with polar comonomers can proceed in several ways depending on the nature of the macroradical end group and the monomer being added, illustrated with vinyl acetate in eq 2.1-2.4:
Self-propagation:
~CH2CH2' + CH2=CH2 kn
> ~CH2CH2CH2CH2· (2.1)
Cross-propagation:
~CH2CH2· + CH2=Cs O OCCH3
H
k12
OCCH
~CH2CH2CH2C
1 1
H
(2.2)
Cross-propagation:
O
II
OCCH,
o II
0CCH3
~CH7CH· + CH,=CH 2- ^ ± i2 -> ~CH,CHCH,CH, (2.3)
Self-propagation:
O O
OCCH3 OCCH3 I / -CH2CH- + CH2=C
\ H
O O
^22
OCCH3 OCCH3 I / -CH2CHCH2
C-I H
(2.4)
The ratio of the reaction rate (kn) of an ethylene terminus with ethylene mono-mer to the reaction rate (k]2) of an ethylene terminus with vinyl acetate is defined as the reactivity ratio (r,):
r , = kn/ k . 12 (2.5)
FREE RADICAL POLYMERIZATION OF ETHYLENE 27 Intermediate that can result in formation of 2-ethylhexyl branching in LDPE (See Figure 2.3)
Figure 2.2 Mechanism of "backbiting" in formation of short chain branching initiated by attack of radical on a δ carbon-hydrogen bond. In the reaction above, homolytic bond scission occurs resulting in a free radical on the 5th carbon atom and an «-butyl branch.
R is a polymeric alkyl group.
Similarly, the ratio of the reaction rate (k21) of a vinyl acetate terminus with eth-ylene monomer to the reaction rate (k22) of a vinyl acetate terminus with vinyl acetate provides a different reactivity ratio (r ):
r2 = k21/k22 (2.6)
Reactivity ratios are indicative of each monomer's tendency to self-propagate or cross-propagate and determine the composition distribution of the polymer.
C H2
' \ / CH
\ 2 CH2
\ CH2
RP 7C H | H homolytic scission
CH CH2
CH2
/ CH2
\ CH3
Intermediate from Figure 2.2 backbiting mechanism
C H2 C H2 C H2 C H2
\ / \ / \ /
RD C * CH CH? CH3
CH2 / CH3
Figure 2.3 Mechanism of formation of 2-ethylhexyl branch in LDPE. As in
backbiting mechanism for /¡-butyl group formation, homolytic scission of a CH bond occurs down the chain. R is a polymeric alkyl group.
If Tj > 1, ethylene tends to self-propagate. If ri < 1, copolymerization is favored.
If r1 ~ r2 ~ 1, the monomers have nearly identical reactivities and comonomer incorporation is highly random. This means that the composition of the copoly-mer will closely reflect the proportions of ethylene and comonocopoly-mer charged to the reactor. For EVA, the ethylene reactivity ratio and reactivity ratio for vinyl acetate are very close (rj = 0.97 and r2 = 1.02), which translates into uniform dis-tribution of VA in the copolymer (10).
Reactivity ratios are important in determining reactor "feed" composition of ethylene and comonomer required to produce a copolymer with the target comonomer content. Because the relative proportion of comonomer changes as polymerization proceeds, adjustment of comonomer feed with time may be necessary. A detailed discussion of the derivation of reactivity ratios for copoly-merizations has been provided by Stevens (11).
In addition to potential hazards of handling peroxides (see section 2.3), ethylene itself can decompose violently under the extreme conditions used in high
FREE RADICAL POLYMERIZATION OF ETHYLENE 29
pressure processes for free radical polymerization of ethylene. Anyone living or working within earshot of an LDPE manufacturing facility is familiar with the occasional boom that accompanies the bursting of rupture discs from what is innocuously called a "decomp." This is a result of spontaneous decomposition of ethylene to carbon, hydrogen and methane as depicted in simplified eq 2.7.
2CH2=CH2 -> 3C + CH4 + 2H2 (2.7)
This reaction occurs at >300 °C and is highly exothermic and releases a pressure pulse that bursts the discs. Even if the standard operating temperature is well below 300 °C, localized "hot spots" from the high heat of polymerization (-24 kcal/mole) can initiate decomposition. Fortunately, decomposition of ethylene is relatively rare.
Nevertheless, engineering design must accommodate "decomps." Instrumentation must be capable of detecting excessive pressure or exotherms within microseconds of the event. Mitigating measures (12-14) include:
• rapid depressurization of reactor systems
• injection of nitrogen and water into gases vented upon disc-rupture