CHEM 335 Lecture 10-14-2026. Covers radical polymerization, conjugation and molecular orbitals.

Overview

Brant Kedrowski finishes radical chemistry by tracing alkene monomers through free-radical chain growth into polymers such as polyethylene, PVC, polypropylene, and polystyrene, then shifts to conjugation and molecular orbitals. The lecture explains resonance as electron delocalization across overlapping p orbitals, using allylic radicals, 1,3-butadiene, isolated and cumulated dienes, and benzene to distinguish conjugated systems from nonconjugated ones.

Key takeaways

An allylic radical must place the unpaired electron adjacent to a C=C bond; resonance can move the radical and π bond, but cannot relocate the radical across an intervening saturated carbon.
Free-radical addition polymerization proceeds by initiator attack on an alkene, repeated addition of monomers at the radical chain end, and eventual termination.
Resin codes distinguish common plastics: HDPE is 2, PVC is 3, LDPE is 4, polypropylene is 5, polystyrene is 6, and PET is 1; the code alone does not guarantee local recycling acceptance.
Conjugation requires a continuous set of at least three aligned p orbitals, so the sp³ carbon in an isolated diene breaks electron delocalization.
The two resonance drawings of an allylic carbocation are contributors to one delocalized structure, with partial charge at both terminal carbons and partial double-bond character across the system.
Benzene’s six sp² carbons form a continuous ring of overlapping p orbitals, producing delocalized electron density above and below the ring.

Chapters

0:00 Allylic Bromination: Identify the Valid Radical Intermediates

N-bromosuccinimide allylic bromination can abstract hydrogen from three allylic positions in the example alkene.
Resonance forms A/B and C/E are valid; D is not because its radical is one carbon removed from the double bond.
The missing intermediate comes from abstraction at the third allylic position; its two resonance drawings are equivalent by molecular symmetry.

6:00 Alkene Monomers, Radical Initiators, and Polymer Notation

Polymers are large molecules assembled from repeating monomer units; natural examples include proteins, starch, cellulose, RNA, and DNA.
Benzoyl peroxide can cleave its weak O–O bond homolytically under heat, generating radicals that initiate polymerization.
Polymer notation places the smallest repeating unit in brackets with bonds extending beyond them and an n to indicate repetition.

11:00 Reading the Repeat Unit in an Addition Polymer

Alkene polymerization converts the monomer’s C=C bond into C–C bonds that connect successive monomer-derived units.
Brackets and n summarize a chain that may contain hundreds or thousands of repeat units without drawing every atom.

13:00 Everyday Polymers: Polyethylene, PVC, Polypropylene, and Polystyrene

Polyethylene has hydrogen as its R group: HDPE is resin code 2, while LDPE is code 4; branching and structure affect rigidity and flexibility.
PVC is code 3 and is used in plumbing pipes and some packaging; polypropylene is code 5 and is common in food containers and heat-resistant meal trays.
Polystyrene is code 6 and appears in insulation, foam cups, disposable cutlery, and clear packaging such as CD jewel cases.

22:00 Plastic Resin Codes, Recycling, and PET Bottles

Resin codes identify plastic types; the lecture notes that local recycling acceptance and economic value vary, with polystyrene often excluded.
Code 1 PET, used in many clear single-use water bottles, is a polyester made by a different polymer-forming process than the radical addition polymers discussed here.
HDPE and LDPE grocery bags illustrate how polymer structure can produce different properties and uses despite sharing polyethylene chemistry.

24:00 How Free-Radical Addition Polymerization Grows a Chain

An initiator radical adds to an alkene’s π bond, forming a new C–C bond and placing the radical on the other alkene carbon.
The chain-end radical adds to further monomers by the same pattern, extending the polymer one repeat unit at a time.
Termination ends growth when radical chains combine or a chain reacts with another radical; initiator-derived groups can remain at chain ends.

29:00 Polymer Growth Direction, Branching, and Other Monomers

Addition direction can favor a more substituted radical; a phenyl substituent can further stabilize a chain-end radical by resonance.
Polymer samples contain chains of varied lengths and may include differently oriented units rather than one perfectly uniform structure.
Radical transfer can create branches in polyethylene, contributing to the flexibility associated with LDPE; methyl methacrylate also polymerizes to acrylic material such as Plexiglas.

33:00 Resonance Hybrids and Delocalized Charge

Two resonance structures represent one molecule, not structures that alternate in time.
In the allylic carbocation example, shifting the π bond moves the positive charge between terminal carbons.
The resonance hybrid has partial positive charge on both ends and partial double-bond character across both C–C bonds.

37:00 Conjugation Explained by Overlapping p Orbitals

An empty p orbital adjacent to a π bond can overlap with the bond’s p orbitals, allowing electron density to spread across three carbon atoms.
Conjugation requires three or more suitably aligned p orbitals that can overlap continuously.
Delocalization stabilizes the structure and gives bonds partial character beyond the localized single- and double-bond drawings.

44:18 Conjugated, Isolated, and Cumulated Dienes

In 1,3-butadiene, the sequence C=C–C=C provides four adjacent sp² carbons whose p orbitals overlap across the molecule.
An isolated diene has an sp³-hybridized carbon between its double bonds, interrupting the p-orbital pathway and preventing conjugation.
A cumulated diene has adjacent double bonds but is not conjugated because the relevant p orbitals are oriented approximately perpendicular to one another.

53:27 Benzene’s Continuous Ring of p Orbitals

All six benzene carbons are sp²-hybridized, each contributing a p orbital to the ring-wide conjugated system.
The overlapping orbitals create delocalized electron density above and below the carbon ring rather than isolated π bonds.

Summary, takeaways, and chapters were generated by AI from the video's transcript and may contain errors. The video belongs to its creator, Brant Kedrowski.